Diagrams of common models and working principles of multimeters

How a multimeter works
"Multimeter" is the abbreviation of multimeter, it is an indispensable tool in our electronic production. Multimeters can measure current, voltage, resistance, and some can also measure the magnification of the transistor, frequency, capacitance, logic potential, decibel value, etc. There are many types of multimeters, and now there are mechanical pointer and digital multimeters. Each has its advantages and disadvantages; for electronic beginners, it is recommended to use an analog multimeter, because it is very helpful for us to be familiar with some electronic knowledge principles. The following mainly introduces the measurement principle of the mechanical pointer multimeter.

The basic principle of this type of multimeter is to use a sensitive magnetoelectric DC ammeter (microammeter) as the meter head. When a small current passes through the meter, there will be a current indication. However, the meter head cannot pass large currents. Therefore, some resistors in parallel and series must be connected to the meter head to shunt or step down, so as to measure the current, voltage and resistance in the circuit. The following are respectively introduced.

1. The principle of measuring DC current.

As shown in Figure 1a, connect an appropriate resistor (called a shunt resistor) in parallel to the meter head for shunting, and the current range can be expanded. Changing the resistance of the shunt resistor can change the current measurement range.

2. Principle of measuring DC voltage.

As shown in Figure 1b, a suitable resistor (called a multiplier resistor) in series with the meter head can be used to step down the voltage to extend the voltage range. Changing the resistance value of the multiplier resistor can change the measuring range of the voltage.

Measuring resistance: - first put the meter rods together and short-circuit to deflect the pointer to the right, and then adjust the "Ω" zero adjustment knob to make the pointer exactly point to 0. Then touch the two meter rods to the two ends of the measured resistance (or circuit) respectively, read the reading of the pointer on the ohm scale line (line), and multiply it by the number in the scale to obtain the resistance value of the measured resistance. For example, use R*100 gear to measure resistance, and the pointer is at 80, then the measured resistance value is 80*100=8K.

3. Principle of AC voltage measurement.

As shown in Figure 1c, because the meter head is a DC meter, when measuring AC, a parallel and series half-wave rectifier circuit needs to be installed to rectify the AC into DC and then pass the meter head, so that it can be based on the direct current The size is used to measure AC voltage. The method of extending the AC voltage range is similar to that of the DC voltage range.

4. Principle of resistance measurement.

As shown in Figure 1d, connect appropriate resistors in parallel and in series with the meter head, and connect a battery in series to allow current to pass through the resistor under test. The resistance value can be measured according to the magnitude of the current. Changing the resistance value of the shunt resistor can change the range of the resistance.

The use of a multimeter
The dial of the multimeter (taking 105 as an example) is shown on the right. The measurement item and measurement range can be changed by the knob of the switch. The mechanical zero adjustment knob is used to keep the pointer still at the left zero position. The "Ω" zero adjustment knob is used to align the pointer to the right zero position when measuring resistance to ensure the accuracy of the measured value.

Measuring resistance: - first put the meter rods together and short-circuit to deflect the pointer to the right, and then adjust the "Ω" zero adjustment knob to make the pointer exactly point to 0. Then touch the two meter rods to the two ends of the measured resistance (or circuit) respectively, read the reading of the pointer on the ohm scale line (line), and multiply it by the number on the scale to get the resistance value of the measured resistance. For example, use R*100 gear to measure resistance, and the pointer is at 80, then the measured resistance value is 80*100=8K.

The measuring range of the multimeter is as follows:

DC voltage: 5 levels-0-6V; 0-30V; 0-150V; 0-300V; 0-600V.

? AC voltage: 5 levels-0-6V; 0-30V; 0-150V; 0-300V; 0-600V

? DC current: 3 levels-0-3mA; 0-30mA; 0-300mA.

? Resistance: 5 levels-R*1; R*10; R*100; R*1K; R*10K

Measuring resistance: - first put the meter rods together and short-circuit to deflect the pointer to the right, and then adjust the "Ω" zero adjustment knob to make the pointer exactly point to 0. Then touch the two meter rods to the two ends of the measured resistance (or circuit) respectively, read the reading of the pointer on the ohm scale line (line), and multiply it by the number in the scale to obtain the resistance value of the measured resistance. For example, use R*100 to measure resistance, and the pointer is at 80, then the measured resistance value is 80*100=8K.

Measuring resistance: - first put the meter rods together and short-circuit to deflect the pointer to the right, and then adjust the "Ω" zero adjustment knob to make the pointer point to 0. Then touch the two meter rods to the two ends of the measured resistance (or circuit) respectively, read the reading of the pointer on the ohm scale line (line), and multiply it by the number in the scale to obtain the resistance value of the measured resistance. For example, use R*100 to measure resistance, and the pointer is at 80, then the measured resistance value is 80*100=8K. Since the reading on the left part of the "Ω" scale line is dense and difficult to see, an appropriate ohm range should be selected when measuring. Keep the pointer in the middle or right of the scale line, so that the reading is clearer and more accurate. Every time you change gears, you should short-circuit the two gauge rods again and adjust the pointer to the zero position to measure the accuracy.

Measure the DC voltage:-first estimate the magnitude of the voltage to be measured, then turn the switch to the appropriate V range, connect the positive meter rod to the "+" terminal of the measured voltage, and the negative meter rod to the "-" terminal of the measured voltage . Then read the measured voltage according to the number of the range and the number pointed to by the pointer on the scale line (second line) marked with the DC symbol "DC-". If you use V300 to measure, you can directly read the indicated value of 0-300. If you use the V30 volt range to measure, you only need to remove a "0" from the number 300 on the scale line, and treat it as 30, and then treat the numbers 200, 100 as 20, 10 in turn, and you can directly read the value indicated by the pointer. For example, use the V6 volt range to measure the DC voltage, and the pointer is at 15, and the measured voltage is 1.5 volts.

Measuring resistance: - first put the meter rods together and short-circuit to deflect the pointer to the right, and then adjust the "Ω" zero adjustment knob to make the pointer exactly point to 0. Then touch the two meter rods to the two ends of the measured resistance (or circuit) respectively, read the reading of the pointer on the ohm scale line (line), and multiply it by the number in the scale to obtain the resistance value of the measured resistance. For example, use R*100 to measure resistance, and the pointer is at 80, then the measured resistance value is 80*100=8K.

Measure the DC current:-first estimate the magnitude of the current to be measured, then turn the switch to the appropriate mA range, and then connect the multimeter in series in the circuit, as shown in the figure. At the same time, observe the scale line marked with the DC symbol "DC". For example, if the current range is selected in the 3mA range, at this time, the number 300 on the surface scale line should be removed, and the two "0"s should be removed as 3, and then 200, 100 is regarded as 2, 1, so that the measured current value can be read out. For example, use the DC 3mA gear to measure the DC current, and the pointer is at 100, the current is 1mA.

Measuring resistance: - first put the meter rods together and short-circuit to deflect the pointer to the right, and then adjust the "Ω" zero adjustment knob to make the pointer exactly point to 0. Then touch the two meter rods to the two ends of the measured resistance (or circuit) respectively, read the reading of the pointer on the ohm scale line (line), and multiply it by the number in the scale to obtain the resistance value of the measured resistance. For example, use R*100 to measure resistance, and the pointer is at 80, then the measured resistance value is 80*100=8K.

Measuring AC voltage:-The method of measuring AC voltage is similar to measuring DC voltage. The difference is that there is no difference between positive and negative for AC, so when measuring AC, the meter stick does not need to be divided into positive and negative. The reading method is the same as the reading method for measuring DC voltage above, except that the numbers should be read at the position of the pointer on the scale line marked with the AC symbol "AC". 3. Precautions for using the multimeter

Multimeter is a more precise instrument. If used improperly, it will not only cause inaccurate measurement but also extremely easy to damage. However, as long as we master the use methods and precautions of the multimeter, and proceed with caution, the multimeter can be durable. Note the following when using a multimeter:

1) The measuring current and voltage cannot be rotated in the wrong gear. If the resistance gear or current gear is mistakenly used to measure the voltage, it is very easy to burn the meter. When the multimeter is not in use, turn the gear to the AC voltage gear to avoid damage due to improper use.

2) When measuring DC voltage and DC current, pay attention to the polarity of "+" and "-", and do not connect them wrongly. If you find that the pointer is turned on and reversed, you should immediately change the meter rod to avoid damage to the pointer and meter head.

3) If you do not know the magnitude of the measured voltage or current, you should use the gear first, and then select the appropriate gear to test, so as to avoid excessive deflection of the needle and damage the meter head. The closer the selected gear is to the measured value, the more accurate the measured value will be.

4) When measuring resistance, do not touch the naked ends of the component (or the metal part of the two meter rods) with your hands, so as to prevent the resistance of the human body from being connected in parallel with the resistance to be measured, which will make the measurement result inaccurate.

5) When measuring resistance, if the two meter rods are short-circuited and the "zero ohm" knob is adjusted to the value, the pointer still does not reach 0 point. This phenomenon is usually caused by insufficient battery voltage in the meter, and a new battery should be replaced. Can only measure accurately.

6) When the multimeter is not in use, do not turn it on the resistance position, because there is a battery inside. If you are not careful, it is easy to short-circuit the two meter rods, which will not only consume the battery, but even damage the meter head in serious cases.

The model and purpose of the multimeter:
1. Multimeter is an indispensable measuring instrument in power electronics and other sectors, generally for the purpose of measuring voltage, current and resistance.

2. The multimeter is divided into pointer multimeter and digital multimeter according to the display mode.

3. The multimeter can be divided into three types according to the range conversion: manual range (MAN RANGZ), automatic range (AUTO RANGZ), manual range (AUTO/MANRANGZ)

4. According to function, use and price, it can be divided into 9 categories, low, medium and high gear digital multimeter, digital/analog hybrid meter, digital/mixed graph dual display meter, universal oscilloscope, etc.

          Tips for using a multimeter
1. Selection of pointer table and digital table:

1. The reading accuracy of the pointer meter is poor, but the pointer swinging process is relatively intuitive, and the swing speed range can sometimes objectively reflect the size of the measured (such as measuring the slightness of the TV data bus (SDL) when transmitting data). Jitter); the digital meter reads intuitively, but the process of digital change looks messy and not easy to watch.

2. There are generally two batteries in an analog watch, one is a low-voltage 1.5V and the other is a high-voltage 9V or 15V. The black test lead is positive compared to the red one. For digital watches, a 6V or 9V battery is commonly used. In the resistance mode, the output current of the pointer meter is much larger than that of the digital meter. The R&TImes;1Ω file can make the speaker emit a loud "click", and the R&TImes;10kΩ file can even light up the light-emitting diode (LED).

3. In the voltage range, the internal resistance of the pointer meter is relatively small compared to the digital meter, and the measurement accuracy is relatively poor. In some high-voltage and low-current situations, it is even impossible to measure accurately, because its internal resistance will affect the circuit under test (for example, the measured value will be much lower than the actual value when measuring the accelerating voltage of the television picture tube). The internal resistance of the voltage file of the digital meter is very large, at least in the megohm level, which has little effect on the circuit under test. However, the extremely high output impedance makes it susceptible to the influence of induced voltage, and the measured data may be false in some occasions with relatively strong electromagnetic interference.

4. In short, pointer meters are suitable for measuring relatively large current and high voltage analog circuits, such as televisions and audio amplifiers. Digital meters are suitable for measuring low voltage and small current digital circuits, such as BP machines, mobile phones, etc. No, pointer table and digital table can be selected according to the situation.

2, measurement techniques (if not explained, it means the pointer table is used):

 

Tips for using a multimeter



Tips for using a multimeter

1. Measuring speakers, earphones, dynamic microphones: Use R&TImes; 1Ω file, connect either one of the test leads to one end, and the other test lead to touch the other end, and it will emit a crisp “click” sound when it is normal. If it does not sound, the coil is broken. If the sound is small and sharp, there is a rubbing problem and it cannot be used.

2. Capacitance measurement: Use the resistance file to select the appropriate range according to the capacitance of the capacitor, and pay attention to the positive electrode of the capacitor for the black test pen of the electrolytic capacitor when measuring. ① Estimating the size of the microwave method-level capacitor: It can be judged based on experience or by referring to standard capacitors of the same capacity, based on the amplitude of the pointer swing. The referenced capacitor does not have to have the same withstand voltage value, as long as the capacity is the same. For example, a 100μF/250V capacitor can be estimated with a 100μF/25V capacitor as a reference. As long as their pointers have the same swing amplitude, it can be concluded that the capacity is the same. ② Estimate the capacitance of picofarad capacitors: R&TImes; 10kΩ range is required, but only capacitors above 1000pF can be measured. For capacitors of 1000pF or a little larger, as long as the needle swings slightly, the capacity is considered sufficient. ③. Measure whether the capacitor is leaking: For capacitors above 1,000 microfarads, you can first use R×10Ω to quickly charge them, and initially estimate the capacity of the capacitor, then change to R×1kΩ and continue testing for a while, then the pointer is not Should return, but should stop at or very close to ∞, otherwise there will be leakage. For some timing or oscillating capacitors below tens of microfarads (such as the oscillating capacitor of a color TV switching power supply), the leakage characteristics are very high, as long as there is a slight leakage, it cannot be used. At this time, it can be charged in the R×1kΩ range after charging. Then switch to the R×10kΩ gear to continue the measurement, and the needle should stop at ∞ instead of returning.

3. Check the quality of diodes, transistors, and voltage regulators online: because in actual circuits, the bias resistors of the transistors or the peripheral resistances of the diodes and voltage regulators are generally relatively large, mostly above hundreds of thousands of ohms. In this way, We can use the R×10Ω or R×1Ω gear of the multimeter to measure the quality of the PN junction on the road. When measuring on the road, use R×10Ω to measure the PN junction should have obvious positive and negative characteristics (if the difference between the forward and reverse resistance is not obvious, you can use R×1Ω to measure), generally the forward resistance is at R When measuring in ×10Ω mode, the needle should indicate around 200Ω, and when measuring in R×1Ω mode, the gauge needle should indicate around 30Ω (there may be slight discrepancies according to different phenotypes). If the measured value of the forward resistance is too large or the reverse resistance is too small, it means that there is a problem with the PN junction, and there is a problem with the tube. This method is particularly effective for repairs, and it can quickly find out the broken tubes, and even detect the tubes that have not been completely broken but whose characteristics have deteriorated. For example, when you use a small resistance file to measure the forward resistance of a PN junction that is too large, if you weld it down and use the commonly used R×1kΩ file to test again, it may still be normal. In fact, the characteristics of this tube have deteriorated. It does not work properly or is unstable.

Tips for using a multimeter

4. Resistance measurement: It is important to select a good range. When the pointer indicates 1/3 to 2/3 of the full range, the measurement accuracy is accurate and the reading is accurate. It should be noted that when measuring megaohm-level large resistance with R×10k resistance file, do not pinch your fingers on both ends of the resistance, so that the resistance of the human body will make the measurement result too small.

5. Measuring Zener diode: what we usually use
The voltage regulation value of the Zener tube is generally greater than 1.5V, and the resistance range of the pointer meter below R×1k is powered by the 1.5V battery in the meter. In this way, the voltage regulator tube is measured with a resistance range below R×1k. Like the test diode, it has complete unidirectional conductivity. But the R×10k gear of the pointer meter is powered by a 9V or 15V battery. When using R×10k to measure a voltage regulator tube with a voltage lower than 9V or 15V, the reverse resistance value will not be ∞, but there will be a certain The resistance value, but this resistance value is still much higher than the forward resistance value of the Zener tube. In this way, we can initially estimate the quality of the regulator tube. However, a good voltage regulator tube must have an accurate voltage regulation value. How can this voltage regulation value be estimated under amateur conditions? It's not difficult, just find another pointer watch. The method is: first place a watch in the R×10k gear, and connect the black and red test leads to the cathode and anode of the regulator tube, then simulate the actual working state of the regulator tube, and then take another watch and place it in At the voltage file V×10V or V×50V (according to the voltage regulation value), connect the red and black test leads to the black and red test leads of the watch just now. The voltage value measured at this time is basically this The voltage stabilization value of the zener tube. Say "basically" because the bias current of the block meter to the Zener tube is slightly smaller than the bias current in normal use, so the measured voltage regulation value will be slightly larger, but the difference is basically the same. This method can only estimate the voltage regulator tube whose voltage regulation value is less than the voltage of the pointer meter's high-voltage battery. If the voltage regulation value of the Zener tube is too high, it can only be measured with an external power supply (so it seems that when we choose a pointer meter, it is more suitable to choose a high-voltage battery voltage of 15V than 9V).

Tips for using a multimeter

6. Test transistor: usually we use R×1kΩ file, whether it is NPN tube or PNP tube, whether it is low-power, medium-power, or high-power tube, the be junction and cb junction of the measurement should show exactly the same unidirectional direction as the diode. Electricity, reverse resistance is infinite, and its forward resistance is about 10K. In order to further estimate the quality of the tube characteristics, if necessary, the resistance gear should be changed to perform multiple measurements. The method is: set the R×10Ω gear to measure the forward conduction resistance of the PN junction at about 200Ω; set the R×1Ω gear to measure The forward conduction resistance of the PN junction is about 30Ω. (The above is the data measured by the 47-type meter, and the other models are probably slightly different. You can test a few more good tubes to sum up, and be aware of it) If the reading is too large Too much, it can be concluded that the characteristics of the pipe are not good. You can also set the meter to R×10kΩ and test again. For tubes with low withstand voltage (basically, the withstand voltage of the triode is above 30V), the cb junction reverse resistance should also be ∞, but the reverse resistance of the be junction There may be some, the needle will be slightly deflected (generally it will not exceed 1/3 of the full scale, depending on the pressure of the tube). Similarly, when measuring the resistance between ec (for NPN tube) or between ce (for PNP tube) with the R×10kΩ gear, the needle may be slightly deflected, but this does not mean that the tube is broken. But when measuring the resistance between ce or ec with the range below R×1kΩ, the meter head should indicate infinity, otherwise the tube will have a problem. It should be noted that the above measurement is for silicon tubes and is not applicable to germanium tubes. However, germanium tubes are rare now. In addition, the "reverse" refers to the PN junction, and the direction of the NPN tube and the PNP tube are actually different.

Nowadays, most of the common transistors are plastic-encapsulated. How to accurately determine which of the three pins of the transistor is b, c, e? The b pole of the triode is easy to measure, but how to determine which is c and which is e? Three methods are recommended here:
One method: For the pointer meter with the hFE jack of the triode, after measuring the b pole, insert the triode into the jack at will (of course, the b pole can be inserted accurately), measure the hFE value, and then connect the tube Measure it upside down, and if the hFE value is relatively large, the insertion position of each pin is correct.

Tips for using a multimeter

The second method: For meters without hFE measurement jacks, or if the tube is too large to be inserted into the jack, you can use this method: For NPN tubes, first measure the b pole (whether the tube is NPN or PNP and its b pin It is easy to measure, right?), put the meter in the R×1kΩ file, connect the red test lead to the hypothetical e pole (note that the hand holding the red test pen does not touch the tip or pin of the test pen), and connect the black test pen to the hypothetical C pole, while holding the tip of the meter pen and this pin with your fingers, pick up the tube, lick the pole b with your tongue, and see that the pointer of the meter head should have a certain deflection. If you connect the test pens correctly, the pointer will deflect. Larger, if the connection is wrong, the pointer deflection will be smaller, the difference is very obvious. From this, the c and e poles of the tube can be determined. For the PNP tube, connect the black test lead to the hypothetical e pole (do not touch the pen tip or pin), and the red test lead to the hypothetical c pole. At the same time, pinch the tip of the test pen and this pin with your fingers, and then lick b with the tip of your tongue. Extremely, if the test leads are connected correctly, the pointer of the meter head will deflect more. Of course, the test leads need to be exchanged twice during measurement, and the judgment can be made after comparing the readings. This method is suitable for all types of transistors, which is convenient and practical. According to the deflection amplitude of the hands, the magnification ability of the tube can also be estimated. Of course, this is based on experience.

The third method: first determine the NPN or PNP type of the tube and its b pole, then set the meter to the R×10kΩ gear. For NPN tubes, connect the black test lead to the e pole and the red test lead to the c pole, and the needle may have a certain amount Deflection, for PNP tubes, when the black test lead is connected to the c pole and the red test lead is connected to the e pole, there may be a certain amount of deflection of the watch needle, and vice versa. From this, the c and e poles of the triode can also be determined. But for high-pressure pipes, this method is not applicable.

For the common imported high-power plastic tube, the c pole is basically in the middle. Some b of medium and small power tubes may be in the middle. For example, the commonly used 9014 triode and its series of other types of triodes, 2SC1815, 2N5401, 2N5551 and other triodes, some of the b poles are in the middle. Of course, they also have c poles in the middle. Therefore, when repairing and replacing the transistors, especially these low-power transistors, do not use them and install them directly as they are. Be sure to test them first.