Repair of a multimeter m 830b does not show an ohmmeter. Multimeter circuits. Schematic diagram of a multimeter

MULTIMETER DIAGRAMS

There are currently three main models in productiondigital multimeters, these are dt830, dt838, dt9208 and m932. The first model on our markets appeared dt830.

Digital multimeter dt830

Constant pressure:
Limit: 200mV, Resolution: 100μV, Accuracy: ± 0.25% ± 2
Limit: 2V, Resolution: 1mV, Accuracy: ± 0.5% ± 2
Limit: 20V, Resolution: 10mV, Accuracy: ± 0.5% ± 2
Limit: 200V, Resolution: 100mV, Accuracy: ± 0.5% ± 2
Limit: 1000V / 600V, Resolution: 1V, Accuracy: ± 0.5% ± 2

AC voltage:
Limit: 200V, Resolution: 100mV, Accuracy: ± 1.2% ± 10
Limit: 750V / 600V, Resolution: 1V, Accuracy: ± 1.2% ± 10
Frequency range from 45Hz to 450Hz.

D.C:
Limit: 200μA, Resolution: 100nA, Accuracy: ± 1.0% ± 2
Limit: 2000μA, Resolution: 1μA, Accuracy: ± 1.0% ± 2
Limit: 20mA, Resolution: 10μA, Accuracy: ± 1.0% ± 2
Limit: 200mA, Resolution: 100μA, Accuracy: ± 1.2% ± 2
Limit: 10A, Resolution: 10mA, Accuracy: ± 2.0% ± 2

Resistance:
Limit: 200Ω, Resolution: 0.1Ω, Accuracy: ± 0.8% ± 2
Limit: 2kΩ, Resolution: 1Ω, Accuracy: ± 0.8% ± 2
Limit: 20kΩ, Resolution: 10Ω, Accuracy: ± 0.8% ± 2
Limit: 200kΩ, Resolution: 100Ω, Accuracy: ± 0.8% ± 2
Limit: 2000KΩ, Resolution: 1KΩ, Accuracy: ± 1.0% ± 2
Output voltage on ranges: 2.8V

HFE transistor test:
I, constant: 10mkA, Uc-e: 2.8V ± 0.4V, hFE measurement range: 0-1000

Diode test
Test current 1.0mA ± 0.6mA, U test 3.2V max.

Polarity: automatic, Overload indication: "1" or "-1" on the display, Measurement rate: 3 meas. per second, Power supply: 9V.The price is about $ 3.

More perfect and multifunctional modeldigital multimeter, becamedt838. Along with the usual features, here they added toBuilt-in 1 kHz sine wave generator.

Digital multimeter dt838

Measurements per second: 2

Constant voltage U = 0.1mV - 1000V

AC voltage U ~ 0.1V - 750V

DC current I = 2mA - 10A

AC frequency range current 40 - 400Hz

Resistance R 0.1 Ohm - 2 MΩ

Input resistance R 1 MΩ

Gain - h21 to 1000 - transistors

Continuity mode< 1 кОм

Power supply 9V, Krona VTs

The price is about 5 ye.

Internal and external filling is almost identical to the dt830 model. A similar feature is the low reliability of movable contacts.

Currently, one of the most advanced models isdigital multimeter m932 ... Features: automatic range selection and non-contact detection of static electricity.

Digital multimeter m932

Digital Multimeter Specifications m932:
CONSTANT VOLTAGE Measurement limits 600 mV; 6; 60; 600; 1000 V
Accuracy ± (0.5% + 2 digits)
Max. resolution 0.1 mV
In. resistance 7.8 MΩ
Input protection 1000V
ALTERNATING VOLTAGE Measurement limits 6; 60; 600; 1000 V

Max. resolution 1 mV
Frequency band 50 - 60 Hz

In. impedance 7.8 MΩ
Input protection 1000V
DC CURRENT Measurement limits 6; 10 A
Accuracy ± (2.5% + 5 digits)
Max. resolution 1 mA

AC CURRENT Measurement limits 6; 10 A

Max. resolution 1 mA
Frequency band 50 - 60 Hz
RMS Measurement - 50 - 60 Hz
Input protection Fuse 10 A
RESISTANCE Measurement limits 600 Ohm; 6; 60; 600 kΩ; 6; 60 MOhm
Accuracy ± (1% + 2 digits)
Max. resolution 0.1 ohm
600V input protection
CAPACITY Measurement limits 40; 400 nF; 4; 40; 400; 4000 uF
Accuracy ± (3% + 5 digits)
Max. resolution 10 pF
600V input protection
FREQUENCY Measurement limits 10; 100; 1000 Hz; ten; 100; 1000 kHz; 10 MHz
Accuracy ± (1.2% + 3 digits)
Max. resolution 0.001 Hz
600V input protection
COEF. PULSE FILLING Measuring range 0.1 - 99.9%
Accuracy ± (1.2% + 2 digits)
Max. resolution 0.1%
TEMPERATURE Measuring range - -20 ° C - 760 ° C (-4 ° F - 1400 ° F)
Accuracy ± 5 ° C / 9 ° F)
Max. resolution 1 ° С; 1 ° F
600V input protection
TEST P-N Max. test current 0.3 mA
Test voltage 1 mV
600V input protection
CIRCUIT RING Threshold< 100 Ом
Test current< 0.3 мА
600V input protection
GENERAL DATA Max. displayed number 6000
Linear scale 61 segments
Measurement speed 2 per second
Auto power off after 15 minutes
Power supply 9 V "Krona" type
Operating conditions 0 ° С - 50 ° С; rel. humidity: no more than 70%
Storage conditions -20 ° С - 60 ° С; rel. humidity: no more than 80%
Overall dimensions 150 x 70 x 48 mm

The DT-830B multimeter is a Chinese-made device that is used by many. Those who constantly deal with electronics cannot do without such technology. This article explains what the DT-830B multimeter is. An instruction with a detailed description of the device allows even beginners to use it.

Many models are produced, differing in quality, precision and functionality.

The device is intended for the following basic measurements:

electric current values;
voltage between 2 points in an electrical circuit;
resistance.

In addition, the DT-830B multimeter and other related models can perform many additional operations:

ring the circuit with a resistance below 50 ohms with an audible alarm;
test the semiconductor diode for integrity and determine its forward voltage;
check the semiconductor transistor;
measure electrical capacitance and inductance;
using a thermocouple;
determine the frequency of the harmonic signal.

How does a multimeter work?

The dial shows the measured values as numbers on a plastic or glass display.
The switch allows changing the functions of the device, as well as switching ranges. When inoperative, it is set to the "Off" position.
Sockets (connectors) in the housing for installing probes. The main thing, with the COM inscription and negative polarity, has a general purpose. A probe with a black wire is inserted into it. The next one, marked VΩmA, has positive polarity with a red lead.
Test flexible leads in red and black with pliers.
Transistor control panel.

Multimeter DT-830B: instruction with a detailed description of the measurement modes

Not everyone understands how to measure the required parameters with a device. When the DT-830B multimeter is used, the instruction manual must be followed exactly. Otherwise, the device may burn out.

1. Resistance measurement

The function is necessary when you need to conduct electrical wiring in the apartment or find a break in the home network. Not everyone knows how to use a multimeter in this case, but you just need to set the switch in the resistance measurement sector to the appropriate measurement range. The device has an audible signal that the circuit is closed. If there is no signal, this means that there is a break somewhere or the resistance value of the circuit is higher than 50 ohms.

The range of minimum resistances (up to 200 ohms) is called a short circuit. If you connect the red and black probe to each other, the device should show a value close to zero.

The Chinese-made DT-830B multimeter has the following features when measuring electrical resistance:

High error of readings.
When measuring small resistances, the value obtained at the contact of the probes should be subtracted from the readings. To do this, they are pre-closed. In other ranges of the sector, the error decreases.

2. How to measure DC voltage

The device switches to the DCV sector, divided into 5 ranges. The switch is set in a deliberately larger range of values. When measuring voltage with power from a 3 V or 12 V battery, you can set the sector to the "20" position. You should not put on a large value, since the error of readings will increase, and if less, the device may burn out. For rough measurements, if you need an accuracy of only 1 V, the multimeter can be immediately set to the "500" position. The same is done when the measured voltage is unknown in magnitude. After that, you can gradually switch the range to lower values. The highest measurement level is signaled by the "HV" warning, which lights up in the upper left corner. Large voltages require caution in working with the device, although, like a voltmeter from a DT-830B multimeter, it is more reliable than an ammeter or ohmmeter.

Observing the polarity of the probes for a digital instrument is optional. If it does not match, this will not affect the value of the readings, and the "-" sign lights up on the left of the screen.

3. How to measure AC voltage

Installation in the ACV sector is the same as in DCV. 220-380 V can lead to failure of the device if connected incorrectly.

4. Measurement of DC current

Low currents for electronic circuits are measured in the DCA sector. Voltage measurement is not permitted in these switch positions. In this case, a short circuit will occur.

To measure the current value up to 10 A, the third socket is used, into which the red probe should be rearranged. The readings can be taken in just a few seconds. Usually, an ammeter is used to measure the current of electrical appliances. In this case, the device should be used with caution and when measurements are really necessary.

5. Monitoring the health of diodes

In the opposite direction on the diode, the device should show infinity (one on the left). In the forward direction, the voltage across the junction is 400-700 mV.

In this sector, you can also check the health of the transistor. If we imagine it as two oppositely connected diodes, each transition must be checked for breakdown. To do this, find out where the base is. For the pnp type, it is necessary to find a terminal (base) with a positive probe so that the negative probe shows infinity on the other two (emitter and collector). If the transistor is of the npn type, the base is at the negative probe. To find the emitter, you need to measure the resistance of its junction, which is always greater than the collector one. For a serviceable element, it should be in the range of 500-1200 ohms.

By ringing the transitions with a multimeter in the forward and reverse directions, you can determine whether the transistor is working or not.

6. Sector hFE

The device can determine the current gain of the h21 transistor. To do this, it is enough to insert its 3 pins into the corresponding sockets of the socket. The display immediately shows the value "h21". To obtain correct results, it is necessary to distinguish between the types pnp (right side of the socket) and npn (left side).

7. Opportunities for improving the device

For the DT-830B multimeter, the instruction provides a certain number of functions. The models differ slightly from each other, and if desired, you can improve any, for example, add the measurement of the capacitance of a capacitor, temperature and all other additional functions listed earlier.

The basis of the multimeter is

DT-830B multimeter: circuit and repair

For an inexpensive small-sized device, the ICL7106 microcircuit is most often used.

When measuring the voltage, the signal comes from the switch through the resistor R17 to the input 31 of the microcircuit. When the AC voltage is measured, it is rectified through the diode D1, after which the signal also passes through the chain to pin 32 of the microcircuit.

The measured direct current creates on the resistors, after which the signal is also fed to input 32. The microcircuit is protected by a 0.2 A fuse installed at the input.

The device often fails if contacts are lost and if turned on incorrectly. First of all, the fuse is checked and changed.

The device works reliably when measuring voltage, since it is well protected at the input from overloads. Failures can occur when measuring resistance or current.

Burnt resistors can be identified visually, and diodes and transistors can be checked using the methods given earlier. A check is made for the absence of breaks and the reliability of the contacts.

When repairing the device, the power supply is checked first. Then the operability of the microcircuit is checked. It should be operational if the voltage at pin 30 is 3 V, and there is no breakdown between the power supply and the common pin of the microcircuit.

When disassembling, do not lose the balls of the switch, without which there will be no reliable fixation.

When to change the battery?

The power supply of the device changes when the numbers on the display disappear and the measurement results deviate from the approximate known values. An image of a battery appears on the screen. To replace it, you need to remove the back cover, remove the old one and install a new element.

Using the DT-830B multimeter is very convenient: the battery is changed easily and very rarely. You just need to work with him very carefully. The device can be easily burned if used incorrectly.

Radio amateurs periodically encounter the problem of a multimeter breakdown. Most often, the problem is that the multimeter was soldered using acid and the contacts simply oxidize. In this case, it is very easy to fix the problem, but there is a more serious problem, for example (as in my case), forgetting to discharge the capacitor, they put it in a digital multimeter and want to measure the capacity, after which the tester refuses to measure anything at all.

Having opened the multimeter, we obviously will not see anything, since the microcircuit was killed by static. The microcircuit itself will most likely be 324, as in the photo. Principled DT9205A schematic can.

But since the multimeter is made in China, then most likely we will not find any data on this microcircuit. So at first I did not find anything, but then I decided to search, having entered not all the elements of the microcircuit inscription, but only numbers. And the result made me happy - the microcircuit turned out to be lm324, or rather a Chinese copy, only with different letters. It is possible to change it to some other op-amp. If you have a radio store in the city, then you can quickly go there and buy this microcircuit, but if there is no such store (as in my case) or it is far away, and the capacitance meter is very necessary, then we change it to any existing microcircuit that contains in itself 4 operational amplifiers. If there are no quadruples, just put two microcircuits that contain 2 op amps, as I did at first.

True, it later turned out that with them the multimeter gives an error. This was due to the fact that the gain of my op amps was different from the gain of lm324. But there was nowhere to go, since I already said earlier we do not have radio stores, and ordering on the Internet is also not the best option - it will take a long time for the order to arrive, and I decided to put others. Just a couple of days before the repair of the DT9205A multimeter, an order of five TL074s arrived.

True, I had them in a DIP case and so that it did not interfere with the closing of the lid DT9205A- I soldered it with wires.

Perhaps when you change the op-amp, even if it is lm324, then the multimeter will show a mute not correctly. In this case, if the deviation is not very large, then this error is removed by a trimming resistor next to the microcircuit (shown by the red arrow), but since there may be deviations in the capacitor rating, it is better to measure its capacitance on another multimeter and adjust yours to the same reading.

And finally, a couple of pictures of the work after the renovation.

Enough time has passed since then - and the multimeter works without problems. I wish you all creative success! Article author: 13265

Discuss the article REPAIR OF THE DT9205 MULTIMETER

SKF flux

In any case, no matter how you dismantle this resistor from the board, bumps of old solder will remain on the board, we need to remove it using a dismantling braid, dipping it in an alcohol-rosin flux. We put the tip of the braid directly on the solder and press it, warming it up with the tip of the soldering iron until all the solder from the contacts is absorbed into the braid.

Dismantling braid

Well, then it's a matter of technology: we take the resistor we bought in the radio store, put it on the contact pads that we freed from the solder, press it down with a screwdriver from above and touch the pads and leads located at the edges of the resistor with the tip of a 25-watt soldering iron, solder it in place.

Solder Braid - Applications

The first time, it will probably turn out crooked, but the most important thing is that the device will be restored. On the forums, opinions on such repairs were divided, some argued that due to the cheapness of multimeters, it makes no sense to repair them at all, they say they threw it out and went to buy a new one, others were even ready to go to the end and re-solder the ADC). But as this case shows, sometimes repairing a multimeter is a fairly simple and cost-effective business, and any home craftsman can easily handle such repairs. Everyone! AKV.

Currently, a huge variety of digital measuring instruments of varying degrees of complexity, reliability and quality are produced. The basis of all modern digital multimeters is an integrated analog-to-digital voltage converter (ADC). One of the first such ADCs suitable for the construction of inexpensive portable measuring devices was a converter based on the ICL71O6 microcircuit manufactured by MAXIM. As a result, several successful low-cost models of 830 series digital multimeters were developed, such as М830В, М830, М832, М838. Instead of the letter M, there can be DT. This series of instruments is currently the most widespread and most repeatable in the world. Its basic capabilities: measurement of direct and alternating voltages up to 1000 V (input resistance 1 MΩ), measuring direct currents up to 10 A, measuring resistances up to 2 MΩ, testing diodes and transistors. In addition, in some models there is a mode of sound continuity of connections, temperature measurement with and without a thermocouple, generation of a meander with a frequency of 50 ... 60 Hz or 1 kHz. The main manufacturer of this series of multimeters is Precision Mastech Enterprises (Hong Kong).

Scheme and operation of the device

The basis of the multimeter is the ADC IC1 of the 7106 type (the closest domestic analogue is the 572PV5 microcircuit). Its structural diagram is shown in Fig. 1, and the pinout for the version in the DIP-40 package is shown in Fig. 2. The 7106 core can be preceded by different prefixes depending on the manufacturer: ICL7106, ТС7106, etc. Recently, more and more often used are chipless microcircuits (DIE chips), the crystal of which is soldered directly to the printed circuit board.

Consider the circuit of the Mastech M832 multimeter (Fig. 3). Pin 1 of IC1 supplies a positive 9V battery supply voltage, and Pin 26 supplies a negative battery supply. Inside the ADC there is a 3 V stabilized voltage source, its input is connected to pin 1 of IC1, and the output is connected to pin 32. Pin 32 is connected to the common pin of the multimeter and is galvanically connected to the COM input of the device. The voltage difference between pins 1 and 32 is approximately 3 V in a wide range of supply voltages - from nominal to 6.5 V. This stabilized voltage is supplied to an adjustable divider R11, VR1, R13, ac of its output - to the input of microcircuit 36 (in measurement mode currents and voltages). The divider sets the potential U er at pin 36, equal to 100 mV. Resistors R12, R25 and R26 have protective functions. Transistor Q102 and resistors R109, R110nR111 are responsible for indicating the discharge of the battery. Capacitors C7, C8 and resistors R19, R20 are responsible for displaying the decimal points of the display.

Rice. 3. Schematic diagram of the M832 multimeter

Voltage measurement

A simplified circuit of the multimeter in the voltage measurement mode is shown in Fig. 4. When measuring DC voltage, the input signal is fed to R1… R6, from the output of which through a switch (according to the scheme 1-8 / 1… 1-8 / 2) it is fed to the protective resistor R17. This resistor, in addition, when measuring alternating voltage, together with the capacitor C3, forms a low-pass filter. Then the signal goes to the direct input of the ADC microcircuit, pin 31. The potential of the common pin, generated by the 3 V stabilized voltage source, pin 32, is fed to the inverse input of the microcircuit.

When measuring AC voltage, it is rectified by a half-wave rectifier on diode D1. Resistors R1 and R2 are selected so that when measuring sinusoidal voltage, the device shows the correct value. ADC protection is provided by R1… R6 divider and R17 resistor.

Current measurement

A simplified circuit of the multimeter in the current measurement mode is shown in Fig. 5. In the mode of measuring direct current, the latter flows through the resistors RO, R8, R7 and R6, which are switched depending on the measuring range. The voltage drop across these resistors through R17 is fed to the ADC input, and the result is displayed. ADC protection is provided by diodes D2, D3 (some models may not be installed) and fuse F.

Resistance measurement

A simplified circuit of the multimeter in the resistance measurement mode is shown in Fig. 6. In the resistance measurement mode, the dependence expressed by the formula (2) is used. The diagram shows that the same current from the voltage source + LJ flows through the reference resistor Ron and the measured resistor Rx (the currents of inputs 35, 36, 30 and 31 are negligible) and the ratio of UBX and Uon is equal to the ratio of the resistances of the resistors Rx and Ron. R1… .R6 are used as reference resistors, R10 and R103 are used as current setting resistors. ADC protection is provided by thermistor R18 [in some cheap models, ordinary resistors with a nominal value of 1 ... 2 kOhm are used), transistor Q1 in zener diode mode (not always installed) and resistors R35, R16 and R17 at inputs 36, 35 and 31 of the ADC.

Continuity mode

The dialing circuit uses IC2 (LM358), which contains two operational amplifiers. A sound generator is assembled on one amplifier, and a comparator on the other. When the voltage at the input of the comparator (pin 6) is less than the threshold, at its output (pin 7) it is set that opens the key on the transistor Q101, as a result of which a sound signal is emitted. The threshold is determined by the divider R103, R104. Protection is provided by resistor R106 at the comparator input.

Defects of multimeters

Factory defects of M832 multimeters

Defect manifestation	Possible reason	Defect elimination
		Check elements C1 and R15













		Open the connector pins



When measuring alternating voltage, the readings of the device "float", for example, instead of 220 V, they change from 200 V to 240 V


		Solder IC2 pins


		To restore reliable contact you need: Correct the conductive rubber bands; Wipe the corresponding contact pads on the PCB with alcohol; Tinker these contacts on the board

The correctness of the LCD display can be checked using an alternating voltage source with a frequency of 50 ... 60 Hz and an amplitude of several volts. As such a source of alternating voltage, you can take the M832 multimeter, which has a meander generation mode. To check the display, put it on a flat surface with the display up, connect one probe of the M832 multimeter to the common output of the indicator (bottom row, left output), and apply the other probe of the multimeter alternately to the rest of the display outputs. If it is possible to get the ignition of all segments of the display, then it is serviceable.

In the current measurement mode when using the V, Ω and mA inputs, despite the presence of a fuse, there may be cases when the fuse blows out later than the safety diodes D2 or D3 have time to break through. If a fuse is installed in the multimeter that does not meet the requirements of the instructions, then in this case the resistances R5 ... R8 may burn out, and this may not appear visually on the resistances. In the first case, when only the diode breaks through, the defect appears only in the current measurement mode: the current flows through the device, but the display shows zeros. In case of burnout of resistors R5 or R6 in the voltage measurement mode, the device will overestimate the readings or show an overload. When one or both resistors are completely burned out, the device is not reset in voltage measurement mode, but when the inputs are closed, the display is set to zero. When the resistors R7 or R8 burn out on the current measuring ranges of 20 mA and 200 mA, the device will show an overload, and in the 10 A range - only zeros.

When a very high voltage is applied to the input of the device in the voltage measurement mode, a breakdown may occur on the elements (resistors) and on the printed circuit board; in the case of the voltage measurement mode, the circuit is protected by a divider on the resistances R1 ... R6.

A 3 V stabilized voltage source in an ADC for cheap Chinese models can in practice give a voltage of 2.6 ... 3.4 V, and for some devices it stops working even at a supply voltage of 8.5 V.

Often, in DT multimeters, when the probes are open in the resistance measurement mode, the device approaches the overload value for a very long time (“1” on the display) or does not set at all. It is possible to “cure” a low-quality ADC microcircuit by reducing the value of the resistance R14 from 300 to 100 kOhm.

When measuring resistances in the upper part of the range, the device "flushes" the readings, for example, when measuring a resistor with a resistance of 19.8 kOhm, it shows 19.3 kOhm. It is "treated" by replacing the capacitor C4 with a capacitor of 0.22 ... 0.27 μF.

With DT series devices, it sometimes happens that the alternating voltage is measured with a minus sign. This indicates an incorrect installation of D1, usually due to incorrect marking on the diode body.

It happens that manufacturers of cheap multimeters put low-quality operational amplifiers in the sound generator circuit, and then when the device is turned on, a buzzing buzzer is heard. This defect is eliminated by soldering a 5 μF electrolytic capacitor parallel to the power supply circuit. If this does not ensure the stable operation of the sound generator, then it is necessary to replace the operational amplifier with the LM358P.

Often there is such a nuisance as battery leakage. Small drops of electrolyte can be wiped off with alcohol, but if the board is heavily flooded, then good results can be obtained by washing it with hot water and laundry soap. After removing the indicator and unsoldering the buzzer, using a brush, for example, a toothbrush, you need to thoroughly soap the board on both sides and rinse it under running water from the tap. After repeating the washing 2 ... 3 times, the board is dried and installed in the case.

Most of the devices produced recently use DIE chips ADCs. The crystal is installed directly on the PCB and is filled with resin. Unfortunately, this significantly reduces the maintainability of the devices, because when the ADC fails, which is quite common, it is difficult to replace it. Unpackaged ADCs are sometimes sensitive to bright light. For example, if you work near a table lamp, the measurement error may increase. The fact is that the indicator and the board of the device have some transparency, and light, penetrating through them, enters the ADC crystal, causing a photoelectric effect. To eliminate this drawback, you need to remove the board and, after removing the indicator, glue the location of the ADC crystal (it is clearly visible through the board) with thick paper.

Schemes М830 ... The difference is not big DT830 or М830 ...

Exceptionally everyone needs to be able to use measuring instruments.
A voltammeter is a universal device (short for "tester", from the word "test"). There are a lot of varieties. We will not consider all of them. We will take the most easily available Chinese-made multimeter DT-830B.

MULTIMETER DT-830B consists of:
-display w / c
-switch multiposition
- sockets for connecting probes
-panel for testing transistors
- back cover (will be needed to replace the battery of the device, a 9 volt Krona cell)
The switch positions are divided into sectors:
OFF / on - power switch of the device
DCV - DC voltage measurement (voltmeter)
ACV- overcurrent voltage measurement (voltmeter)
hFe - transistor measurement enable sector
1.5v-9v- battery check.
DCA - DC current measurement (ammeter).
10A - sector of the ammeter for measuring large values of direct current (according to the instructions
measurements are taken within a few seconds).
Diode - a sector for testing diodes.
Ohm - resistance measurement sector.

DCV sector
On this device, the sector is divided into 5 ranges. Measurements are taken from 0 to 500 volts. A large DC voltage will be encountered only when repairing a TV. This device must be operated with extreme caution at high voltages.
When switched to the "500" volt position, the HV warning lights up on the screen in the upper left corner. the fact that the uppermost level of measurement is turned on and when large values appear, you need to be extremely careful.

Typically, voltage measurement is carried out by switching high positions of the range to lower ones, if you do not know the value of the measured voltage. For example, before measuring the voltage on the battery of a cell phone or car, on which the maximum voltage of 3 or 12 volts is written, then we safely put the sector in the "20" volt position. If we set it to a lower value, for example, "2000" millivolts, the device may fail. If we put on a large, the readings of the device will be less accurate.
When you do not know the value of the measured voltage (of course, within the framework of household electrical equipment, where it does not exceed the values of the device), then set the "500" volts to the upper position and take a measurement. In general, it is possible to measure roughly, with an accuracy of one volt, at the "500" volt position.
If greater accuracy is required, switch to the down position only so that the measured voltage does not exceed the value at the switch position of the instrument. This device is convenient for measuring DC voltage in that it does not require mandatory polarity. If the polarity of the probes ("+" - red, "-" - black) does not coincide with the polarity of the measured voltage / th, a "-" sign will appear on the left side of the screen, and the value will correspond to the measured one.

ACV sector
The sector has 2 positions on this type of device - "500" and "200" volts.
Handle 220-380 volt measurements with great care.
The procedure for measuring and setting positions is the same as in the DCV sector.
DCA sector.
It is a DC milliammeter and is used to measure small currents, mainly in electronic circuits. We won't be of any use yet.
To avoid damage to the device, do not put the switch on this sector, if you forget and start measuring the voltage, the device will fail.

Sector Diode.
One position for checking diodes for breakdown (for a small
resistance) and open circuit (infinite resistance). The measurement principles are based on the operation of the Ohmmeter. As well as hFE.
HFE sector
For measuring transistors, there is a socket indicating which socket to place which leg of the transistor. The transistors of both p - p - p and p - p - p conductivities are checked for breakdown, breakage and for a greater deviation from the standard resistances of the transitions.

Digital multimeter М832. Electrical diagram, description, characteristics

It is impossible to imagine a repairman's workbench without a handy, inexpensive digital multimeter. This article discusses the device of the 830 series digital multimeters, the most common malfunctions and how to fix them.

Currently, a huge variety of digital measuring instruments of varying degrees of complexity, reliability and quality are produced. The basis of all modern digital multimeters is an integrated analog-to-digital voltage converter (ADC). One of the first such ADCs suitable for the construction of inexpensive portable measuring devices was a converter based on the ICL71O6 microcircuit manufactured by MAXIM. As a result, several successful low-cost models of 830 series digital multimeters were developed, such as М830В, М830, М832, М838. Instead of the letter M, there can be DT. This series of instruments is currently the most widespread and most repeatable in the world. Its basic capabilities: measurement of direct and alternating voltages up to 1000 V (input resistance 1 MΩ), measuring direct currents up to 10 A, measuring resistances up to 2 MΩ, testing diodes and transistors. In addition, in some models there is a mode of sound continuity of connections, temperature measurement with and without a thermocouple, generation of a square wave with a frequency of 50 ... 60 Hz or 1 kHz. The main manufacturer of this series of multimeters is Precision Mastech Enterprises (Hong Kong).

Scheme and operation of the device

Rice. 1. Block diagram of the ADC 7106

Rice. 2. Pinout of the 7106 ADC in the DIP-40 package

The voltage difference between pins 1 and 32 is approximately 3 V in a wide range of supply voltages - from nominal to 6.5 V. This stabilized voltage is supplied to an adjustable divider R11, VR1, R13, ac of its output - to the input of the microcircuit 36 (in the measurement mode currents and voltages).

The divider sets the potential U er at pin 36, equal to 100 mV. Resistors R12, R25 and R26 have protective functions. Transistor Q102 and resistors R109, R110nR111 are responsible for indicating the discharge of the battery. Capacitors C7, C8 and resistors R19, R20 are responsible for displaying the decimal points of the display.

Rice. 3. Schematic diagram of the M832 multimeter

The range of operating input voltages Umax directly depends on the level of the adjustable reference voltage at pins 36 and 35 and is:

The stability and accuracy of the display is dependent on the stability of this reference voltage. The display N readings depend on the input voltage UBX and are expressed as a number:

Let's consider the operation of the device in basic modes.

Voltage measurement

A simplified circuit of the multimeter in the voltage measurement mode is shown in Fig. 4. When measuring DC voltage, the input signal is fed to R1 ... R6, from the output of which through a switch (according to the scheme 1-8 / 1 ... 1-8 / 2) it is fed to the protective resistor R17. This resistor, in addition, when measuring alternating voltage, together with the capacitor C3, forms a low-pass filter. Then the signal goes to the direct input of the ADC microcircuit, pin 31. The potential of the common pin, generated by the 3 V stabilized voltage source, pin 32, is fed to the inverse input of the microcircuit.

Rice. 4. Simplified circuit of the multimeter in voltage measurement mode

Current measurement

Rice. 5. Simplified circuit of the multimeter in the current measurement mode

Resistance measurement

Rice. 6. Simplified circuit of the multimeter in resistance measurement mode

A simplified circuit of the multimeter in the resistance measurement mode is shown in Fig. 6. In the resistance measurement mode, the dependence expressed by the formula (2) is used. The diagram shows that the same current from the voltage source + LJ flows through the reference resistor Ron and the measured resistor Rx (the currents of inputs 35, 36, 30 and 31 are negligible) and the ratio of UBX and Uon is equal to the ratio of the resistances of the resistors Rx and Ron. R1 .... R6 are used as reference resistors, R10 and R103 are used as current setting resistors. ADC protection is provided by thermistor R18 [in some cheap models, ordinary resistors with a nominal value of 1 ... 2 kOhm are used), transistor Q1 in zener diode mode (not always installed) and resistors R35, R16 and R17 at inputs 36, 35 and 31 of the ADC.

Continuity mode

The dialing circuit uses IC2 (LM358), which contains two operational amplifiers. A sound generator is assembled on one amplifier, and a comparator on the other. When the voltage at the input of the comparator (pin 6) is less than the threshold, a low voltage is set at its output (pin 7), which opens the switch on the transistor Q101, as a result of which a sound signal is emitted. The threshold is determined by the divider R103, R104. Protection is provided by resistor R106 at the comparator input.

Defects of multimeters

All faults can be divided into factory defects (and this happens) and damage caused by erroneous actions of the operator.

Since multimeters use tight wiring, shorts of elements, poor soldering and breakage of the leads of elements are possible, especially those located at the edges of the board. Repair of a faulty device should begin with a visual inspection of the printed circuit board. The most common factory defects of M832 multimeters are shown in the table.

Factory defects of M832 multimeters

Defect manifestation	Possible reason	Defect elimination
When the device is turned on, the display lights up and then fades out	Malfunction of the master oscillator of the ADC microcircuit, the signal from which is fed to the LCD substrate	Check elements C1 and R15
When the device is turned on, the display lights up and then fades out. When the back cover is removed, the device works normally	When the rear cover of the device is closed, the helical contact spring rests on the resistor R15 and closes the master oscillator circuit	Bend or slightly shorten the spring
When the device is switched on in the voltage measurement mode, the display readings change from 0 to 1	The integrator circuits are faulty or poorly soldered: capacitors C4, C5 and C2 and resistor R14	Solder or replace C2, C4, C5, R14
The device takes a long time to zero the readings	Poor quality of the capacitor SZ at the input of the ADC (pin 31)	Replace SZ with a condenser with a low absorption coefficient
When measuring resistances, the display takes a long time to set	Poor quality of the capacitor C5 (automatic zero correction circuit)	Replace C5 with a low absorption capacitor
The device does not work properly in all modes, IC1 is overheating.	The long pins of the connector are closed among themselves to test the transistors	Open the connector pins
When measuring alternating voltage, the readings of the device "float", for example, instead of 220 V, they change from 200 V to 240 V	Loss of capacitance of the capacitor SZ. Possible poor soldering of its terminals or simply the absence of this capacitor	Replace SZ with a working capacitor with a low absorption coefficient
When turned on, the multimeter either constantly beeps, or vice versa, is silent in the dialing mode	Poor soldering of the pins of the Yu2 microcircuit	Solder IC2 pins
Segments on the display disappear and appear	Poor contact between LCD and multimeter board contacts through conductive rubber inserts	To restore reliable contact you need: fix conductive rubber bands; wipe the corresponding contact pads on the printed circuit board with alcohol; irradiate these contacts on the board

The above malfunctions may also appear during operation. It should be noted that in the DC voltage measurement mode, the device rarely fails, because well protected from input overloads. The main problems arise when measuring current or resistance.

Repair of a faulty device should begin with checking the supply voltage and ADC operability: stabilization voltage of 3 V and no breakdown between the power pins and the common ADC output.

In resistance measurement mode, faults usually occur in the 200 ohm and 2000 ohm ranges. In this case, when the voltage is applied to the input, the resistors R5, R6, R10, R18, the transistor Q1 may burn out and the capacitor Sb may break through. If the transistor Q1 is completely punctured, then when measuring the resistance, the device will show zeros. In case of incomplete breakdown of the transistor, the multimeter with open probes will show the resistance of this transistor. In voltage and current measurement modes, the transistor is short-circuited by a switch and does not affect the multimeter readings. With a breakdown of capacitor C6, the multimeter will not measure voltage in the ranges of 20 V, 200 V and 1000 V or significantly underestimate the readings in these ranges.

If there is no indication on the display, when there is power to the ADC, or there is a visually noticeable burnout of a large number of circuit elements, there is a high probability of damage to the ADC. The ADC's serviceability is checked by monitoring the voltage of the 3 V stabilized voltage source.In practice, the ADC burns out only when a high voltage is applied to the input, much higher than 220 V. Very often, cracks appear in the compound of the open-frame ADC, the current consumption of the microcircuit increases, which leads to its noticeable heating ...

When a very high voltage is applied to the input of the device in the voltage measurement mode, a breakdown may occur in the elements (resistors) and on the printed circuit board; in the case of the voltage measurement mode, the circuit is protected by a divider on the resistances R1 ... R6.

For cheap DT series models, long parts leads can be shorted to the screen located on the back cover of the device, disrupting the operation of the circuit. Mastech does not have such defects.

A stabilized voltage source of 3 V in an ADC for cheap Chinese models can in practice give a voltage of 2.6 ... 3.4 V, and for some devices it stops working already at a voltage of a supply battery of 8.5 V.

The DT models use low quality ADCs, they are very sensitive to the values of the integrator chain C4 and R14. High-quality ADCs in Mastech multimeters allow the use of elements of close denominations.

Often, in DT multimeters, when the probes are open in the resistance measurement mode, the device approaches the overload value for a very long time ("1" on the display) or is not set at all. It is possible to "cure" a low-quality ADC microcircuit by reducing the resistance R14 from 300 to 100 kOhm.

Since cheap Chinese firms use low-quality unpackaged ADCs, there are frequent cases of broken pins, while it is very difficult to determine the cause of the malfunction and it can manifest itself in different ways, depending on the broken pin. For example, one of the indicator leads is off. Since multimeters use displays with static indication, then to determine the cause of the malfunction, it is necessary to check the voltage at the corresponding pin of the ADC microcircuit, it should be about 0.5 V relative to the common pin. If it is zero, then the ADC is faulty.

An effective way to find the cause of a malfunction is to dial the pins of the analog-to-digital converter chip as follows. Another, of course, serviceable, digital multimeter is used. It turns on in the diode test mode. The black probe, as usual, is inserted into the COM jack, and the red one into the VQmA jack. The red probe of the device is connected to pin 26 [minus power supply), and the black one alternately touches each leg of the ADC microcircuit. Since at the inputs of the analog-to-digital converter, protective diodes are installed in reverse connection, then with such a connection they should open, which will be reflected on the display as a voltage drop across an open diode. The real value of this voltage on the display will be slightly higher, because resistors are included in the circuit. In the same way, all ADC pins are checked when the black probe is connected to pin 1 [ADC power supply plus) and alternately touching the remaining pins of the microcircuit. Instrument readings should be similar. But if you change the polarity of switching on during these checks to the opposite, then the device should always show an open circuit, because the input impedance of a good microcircuit is very high. Thus, the conclusions that show the final resistance for any polarity of connection to the microcircuit can be considered faulty. If the device shows an open circuit at any connection of the investigated output, then ninety percent of this indicates an internal open circuit. The specified test method is quite versatile and can be used to test various digital and analog microcircuits.

There are malfunctions associated with poor-quality contacts on the cradle switch, the device works only when the cradle is pressed. Firms that make cheap multimeters rarely coat the tracks under the rocker switch with grease, which is why they quickly oxidize. Often the tracks are dirty. It is repaired as follows: the printed circuit board is removed from the case, and the switch tracks are wiped with alcohol. Then a thin layer of technical petroleum jelly is applied. Everything, the device is repaired.

Often there is such a nuisance as battery leakage. Small drops of electrolyte can be wiped off with alcohol, but if the board is heavily flooded, then good results can be obtained by washing it with hot water and laundry soap. After removing the indicator and unsoldering the buzzer, using a brush, for example, a toothbrush, you need to thoroughly soap the board on both sides and rinse it under running water from the tap. After repeating the wash 2 ... 3 times, the board is dried and installed in the case.

When buying DT multimeters, you should pay attention to the quality of the switch mechanics, be sure to rotate the multimeter's rocker switch several times to make sure that the switching occurs clearly and without jamming: plastic defects cannot be repaired.

Publication: www.cxem.net

See other articles section.

A multimeter is one of the inexpensive measuring instruments used by both professionals and amateurs who repair home wiring and electrical appliances. Without it, any electrician feels like no hands. Previously, three different instruments were required to measure voltage, current, and resistance. Now all this can be measured using one universal device. Using a digital multimeter is very easy.

There are two basic rules to remember:

⚡ where to connect the measuring probes correctly
⚡ in which position to set the switch for measuring different quantities

Multimeter appearance and connectors

On the front of the tester, all the inscriptions are in English, and even with the use of an abbreviation.

What do these labels mean:

OFF - the device is turned off (so that the batteries of the device are not discharged, set the switch to this position after measurements)
ACV - measurement of variable U
DCV - constant U measurement
DCA - DC current measurement
Ω - resistance measurement
hFE - measurement of the characteristics of transistors
diode icon - continuity or diode test

Modes are switched using a central rotary switch. At the very beginning of using the digital multimeter, it is recommended to immediately mark the pointer mark on the switch with contrasting paint. For example like this:

Most of the failure of the device is just due to the wrong choice of the switch position.

Power is supplied from a krone battery. By the way, by the connector for connecting the crown, one can indirectly judge whether the tester is assembled at the factory or somewhere in Chinese "cooperatives". With a high-quality assembly, the connection takes place through special connectors designed for the crown. In lower quality versions, conventional springs are used.

The multimeter has several test leads and only two test leads. Therefore, it is important to correctly connect the probes for measuring certain quantities, otherwise you can easily burn the device.

Probes are usually of different colors - red and black. The black probe is connected to the connector labeled COM (translated as "common"). Red probe into the other two connectors. The 10ADC connector is used when it is necessary to measure current from 200mA to 10A. The VΩmA connector is used for all other measurements - voltage, current up to 200mA, resistance, continuity.

The main criticism is caused by the factory probes that come with the device. Almost every second multimeter owner recommends replacing them with better ones. However, their cost can be comparable to the cost of the tester itself. As a last resort, they can be improved by reinforcing where the wires are bent and insulating the tips of the probes.

If you want yourself high-quality silicone probes with a bunch of tips, then you can order them with free shipping on Aliexpress.

Arrow testers were also widely used in the past. Some electricians even prefer them, considering them to be more reliable. However, it is less convenient for ordinary consumers to use them due to the large error of the measurement scale. In addition, when working with a pointer multimeter, it is imperative to guess the polarity of the contacts. For digital, if not correctly connected to the poles, the readings will simply be displayed with a minus sign. This is a normal operating mode that will not damage the multimeter.