Digital multimeter Tutorial

How do you test with a multimeter?

Voltage measurement

1.Measurement of DC voltage , such as battery, Walkman power supply, etc. First, insert the black test lead into the "com" hole and the red test lead into the "V Ω". Select the knob to a range larger than the estimated value (Note: the values on the dial are all the maximum range, "V-" indicates the DC voltage range, "V~" indicates the AC voltage range, and "A" is the current range), then connect the test leads to the power supply or both ends of the battery; keep the contact stable. The value can be read directly from the display. If it shows "1.", it means that the range is too small, so you need to increase the range before measuring industrial electrical appliances.
If "-" appears on the left side of the value, it means that the polarity of the test lead is opposite to the actual power supply polarity, and the red test lead is connected to the negative pole.

2.Measurement of AC voltage . The probe jack is the same as the DC voltage measurement, but the knob should be turned to the AC range "V~". There is no positive or negative difference in AC voltage , and the measurement method is the same as before.
Whether measuring AC or DC voltage, pay attention to personal safety and do not touch the metal part of the probe with your hands.

Measurement of current
1. Measurement of DC current . First insert the black test lead into the "COM" hole. If the current is greater than 200mA, insert the red test lead into the "10A" jack and turn the knob to the DC "10A" position; if the current is less than 200mA, insert the red test lead into the "200mA" jack and turn the knob to the appropriate range within DC 200mA. After adjustment, you can measure. Connect the multimeter in series to the circuit, keep it stable, and read the value. If it shows "1.", then increase the range; if "-" appears on the left side of the value, it means that the current flows from the black test lead into the multimeter.
2. Measurement of AC current . The measurement method is the same as 1, but the gear should be set to the AC gear. After the current measurement is completed, the red pen should be inserted back into the "VΩ" hole. If you forget this step and measure the voltage directly, haha! Your meter or power supply will "go up to the sky in a wisp of green smoke" - scrapped!
3. Resistance measurement
: Insert the test leads into the "COM" and "VΩ" holes, turn the knob to the required range in "Ω", and connect the test leads to the metal parts at both ends of the resistor. You can touch the resistor with your hands during measurement , but do not touch both ends of the resistor with your hands at the same time, as this will affect the measurement accuracy - the human body is a conductor with large but finite resistance. When reading, keep good contact between the test leads and the resistor; pay attention to the unit: the unit is "Ω" at the "200" level, "KΩ" at the " 2K " to "200K" level, and "MΩ" above "2M".
4. Diode measurement:
The digital multimeter can measure light-emitting diodes, rectifier diodes ... When measuring, the position of the test leads is the same as for voltage measurement, turn the knob to " " (as shown in the figure) ; connect the red test lead to the positive pole of the diode and the black test lead to the negative pole. The forward voltage drop of the diode will be displayed . The voltage drop of Schottky diode is about 0.2V, the voltage drop of ordinary silicon rectifier tube (1N4000, 1N5400 series, etc.) is about 0.7V, and the light-emitting diode is about 1.8~2.3V. If the display shows "1." when the test leads are replaced, it is normal because the reverse resistance of the diode is very large, otherwise the tube has been broken down.

5.The test lead insertion position of the transistor
is the same as above; its principle is the same as that of the diode. First, assume that pin A is the base, connect the black test lead to this pin, and the red test lead to the other two pins respectively; if the two readings are both around 0.7V, then connect the red pen to pin A and the black pen to the other two pins. If both display "1", pin A is the base, otherwise it needs to be re-measured, and this tube is a PNP tube. So how to judge the collector and emitter? Digital meters cannot use the pointer swing to judge like pointer meters, so what should we do? We can use the "hFE" gear to judge: first set the gear to the "hFE" gear, and you can see a row of small jacks next to the gear, which are divided into PNP and NPN tube measurements. ( Copyright http://www.diangon.com/ ) The tube type has been determined before. Insert the base into the corresponding tube type "b" hole, and the other two pins into the "c" and "e" holes respectively. At this time, you can read the value, that is, the β value; then fix the base and swap the other two pins; compare the two readings, the pin position with the larger reading corresponds to the surface "c" and "e".
Tips : The above method can only directly measure small tubes such as the 9000 series. If you want to measure large tubes, you can use the wiring method, that is, use a small wire to lead out the three pins. This is much more convenient.

6. Measurement of MOS field effect tubes
N-channel has domestic 3D01, 4D01, and Japanese 3SK series. Determination of G pole (gate): Use the diode range of the multimeter. If the positive and negative voltage drops between a certain pin and the other two pins are both greater than 2V, that is, "1" is displayed, and this pin is the gate G. Then exchange the test leads to measure the other two pins. In the case of the smaller voltage drop, the black test lead is connected to the D pole (drain) and the red test lead is connected to the S pole (source).
1. Voltage range:
When testing or making, it can be used to measure the voltage of each foot of the device, and compare it with the normal voltage to determine whether it is damaged. It can also be used to detect the voltage value of the voltage-stabilizing diode with a smaller voltage-stabilizing value. The principle is as shown in the figure: R is 1K, and the voltage at the power supply end depends on the nominal voltage-stabilizing value of the voltage-stabilizing tube. It is generally 3V or more than the nominal voltage, but not more than 15V. Then use a multimeter to detect the voltage value at both ends of the D tube. This value is the actual voltage-stabilizing value of the D tube.
2. Current range:
Connect the meter in series to the circuit to measure and monitor the current. If the current deviates far from the normal value (based on experience or the original normal parameters), the circuit can be adjusted or repaired if necessary. The 20A range of the meter can also be used to measure the short-circuit current of the battery, that is, connect the two test pens directly to the two ends of the battery. Remember that the time must not exceed 1 second! Note: This method is only applicable to dry batteries, No. 5 and No. 7 rechargeable batteries, and beginners must be guided by personnel familiar with maintenance, and must not operate on their own! The performance of the battery can be judged by the short-circuit current. In the case of fully charged batteries of the same type, the larger the short-circuit current, the better.
3. Resistance range:
One of the methods that can be used to judge the quality of resistors, diodes, and transistors. For resistors, if the actual resistance value deviates too much from the nominal value, it is damaged. For diodes and triodes, if the resistance between any two pins is not a large value (more than a few hundred K), it can be considered that the performance has declined or has been broken down. Note that this triode has no resistance. This method can also be used for integrated blocks. It should be noted that the measurement of integrated blocks can only be compared with the parameters when normal.

7. Structure of pointer multimeter (500 type)
A multimeter consists of three main parts: meter head, measuring circuit and conversion switch.
(1) Meter head
It is a highly sensitive magnetoelectric DC ammeter. The main performance indicators of the multimeter basically depend on the performance of the meter. The sensitivity of the meter refers to the DC current value flowing through the meter when the meter pointer is fully deflected. The smaller this value is, the higher the sensitivity of the meter. The larger the internal resistance when measuring voltage, the better its performance. There are four scale lines on the meter, and their functions are as follows: The first line (from top to bottom) is marked with R or Ω, indicating the resistance value. When the conversion switch is in the ohm position, read this scale line. The second line is marked with ∽ and VA, indicating the AC, DC voltage and DC current values. When the conversion switch is in the AC, DC voltage or DC current position, and the range is in a position other than AC 10V, read this scale line. The third line is marked with 10V, indicating the AC voltage value of 10V. When the conversion switch is in the AC, DC voltage position, and the range is AC 10V, read this scale line. The fourth line is marked with dB, indicating the audio level.
(2) Measurement circuit
The measurement circuit is a circuit used to convert various measured quantities into a small DC current suitable for the meter to measure. It is composed of resistors, semiconductor components and batteries. It can convert various measured quantities (such as current, voltage, resistance, etc.) and different ranges into a small DC current of a certain limit through a series of processing (such as rectification, shunting, voltage division, etc.) and send it to the meter for measurement.
(3) Conversion switch
Its function is to select various measurement circuits to meet the measurement requirements of different types and ranges. There are generally two conversion switches, each marked with different gears and ranges.

8.Symbols 
(1) ∽ indicates AC and DC
(2) V-2.5KV 4000Ω/V indicates that for AC voltage and 2.5KV DC voltage, its sensitivity is 4000Ω/V
(3) A-V-Ω indicates that it can measure current, voltage and resistance
(4) 45-65-1000Hz indicates that the operating frequency range is below 1000 Hz, and the standard operating frequency range is 45-65Hz
(5) 2000Ω/V DC indicates that the sensitivity of the DC range is 2000Ω/V
The symbols on the clamp meter and the shaking dial are similar to the above symbols (the others cannot be written because the symbol format is incorrect. 『Indicates a magnetoelectric rectifier with mechanical reaction force instrument』Indicates three-level protection against external magnetic fields』Indicates horizontal placement))

9. Use of multimeter 
(5) Measuring voltage: When measuring voltage (or current), you must select a good range. If you use a small range to measure a large voltage, there is a risk of burning the meter; if you use a large range to measure a small voltage, the pointer deflection is too small and the reading cannot be read. The range should be selected so that the pointer deflects to about 2/3 of the full scale. If you do not know the magnitude of the voltage to be measured in advance, you should first select the highest range and then gradually reduce it to the appropriate range.
a Measurement of AC voltage: Set one of the multimeter's conversion switches to the AC/DC voltage range, and the other conversion switch to the appropriate range of AC voltage. Connect the two probes of the multimeter in parallel with the circuit or load to be measured.
b Measurement of DC voltage: Set one of the multimeter's conversion switches to the AC/DC voltage range, and the other conversion switch to the appropriate range of DC voltage. Connect the "+" probe (red probe) to the high potential and the "-" probe (black probe) to the low potential, that is, let the current flow in from the "+" probe and out from the "-" probe. If the test leads are connected in reverse, the meter pointer will deflect in the opposite direction, which can easily bend the pointer.
(6) Measuring current: When measuring DC current, set one of the multimeter's switches to the DC current range and the other switch to the appropriate range of 50uA to 500mA. The current range selection and reading method are the same as those for voltage. When measuring, you must first disconnect the circuit, and then connect the multimeter in series to the circuit being measured in the direction of the current from "+" to "-", that is, the current flows in from the red test lead and flows out from the black test lead. If the multimeter is mistakenly connected in parallel with the load, the internal resistance of the meter head is very small, which will cause a short circuit and burn the instrument. The reading method is as follows:
Actual value = indicated value × range/full deviation
(7) Measuring resistance: When measuring resistance with a multimeter, the following method should be used:
a. Select the appropriate multiplier. The scale lines of the multimeter's ohm range are uneven, so the multiplier should be selected so that the pointer stays on the part with fewer scale lines. The closer the pointer is to the middle of the scale, the more accurate the reading. In general, the pointer should point to 1/3~2/3 of the scale.
b. Ohm zero adjustment. Before measuring resistance, short-circuit the two test leads and adjust the "Ohm (electrical) zero adjustment knob" at the same time so that the pointer just points to the zero position on the right side of the ohm scale line. If the pointer cannot be adjusted to zero, it means that the battery voltage is insufficient or there is a problem inside the meter. And every time the multiplier is changed, the ohm zero adjustment should be performed again to ensure accurate measurement.
c. Reading: The reading on the meter head multiplied by the multiplier is the resistance value of the measured resistor.
(8) Precautions
a. When measuring current and voltage, the range cannot be changed with power on.
b. When selecting the range, select the larger one first and the smaller one later, and try to make the measured value close to the range.
c. When measuring resistance, do not measure with power on. Because when measuring resistance, the multimeter is powered by the internal battery. If it is measured with power on, it is equivalent to connecting an additional power supply, which may damage the meter head.
d. After use, the conversion switch should be placed in the maximum AC voltage position or neutral position.

10. Digital multimeter 
Nowadays, digital measuring instruments have become the mainstream and tend to replace analog instruments. Compared with analog instruments, digital instruments have high sensitivity, high accuracy, clear display, strong overload capacity, easy to carry and simpler to use. The following takes the VC9802 digital multimeter as an example to briefly introduce its usage and precautions.
(1) Usage method
a Before use, you should carefully read the relevant instruction manual and be familiar with the functions of the power switch, range switch, jacks, and special sockets.
b Set the power switch to the ON position.
c Measurement of AC and DC voltage: Turn the range switch to the appropriate range of DCV (direct current) or ACV (alternating current) as needed, insert the red test lead into the V/Ω hole, the black test lead into the COM hole, and connect the test lead in parallel with the circuit to be measured, and the reading will be displayed.
d. Measurement of AC and DC current: Set the range switch to the appropriate range of DCA (direct current) or ACA (alternating current), insert the red test lead into the mA hole (when <200mA) or 10A hole (when >200mA), the black test lead into the COM hole, and connect the multimeter in series with the circuit to be measured. When measuring DC, the digital multimeter can automatically display the polarity.
e. Measurement of resistance: Set the range switch to the appropriate range of Ω, insert the red test lead into the V/Ω hole, and the black test lead into the COM hole. If the measured resistance value exceeds the maximum value of the selected range, the multimeter will display "1", and a higher range should be selected. When measuring resistance, the red test lead is the positive pole and the black test lead is the negative pole, which is exactly the opposite of the pointer multimeter. Therefore, when measuring polar components such as transistors and electrolytic capacitors, you must pay attention to the polarity of the test leads.
(2) Precautions for use
a If the voltage or current to be measured cannot be estimated in advance, the meter should be switched to the highest range and measured once, and then the range should be gradually reduced to an appropriate position depending on the situation. After the measurement, the range switch should be switched to the highest voltage range and the power should be turned off.
b When the range is full, the meter only displays the number "1" in the highest position, and the other positions disappear. At this time, a higher range should be selected.
c When measuring voltage, the digital multimeter should be connected in parallel with the circuit to be measured. When measuring current, it should be connected in series with the circuit to be measured. When measuring DC current, the positive and negative polarities do not need to be considered.
d When the AC voltage range is used to measure DC voltage by mistake, or the DC voltage range is used to measure AC voltage by mistake, the display will display "000" or the numbers on the low position will jump.
e It is forbidden to change the range when measuring high voltage (above 220V) or large current (above 0.5A) to prevent arcing and burning of switch contacts. When " ", "BATT" or "LOW BAT" is displayed, it means that the battery voltage is lower than the working voltage.
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