Use a multimeter to identify photocouplers
The optocoupler can be identified with a pointer multimeter. The author takes the four-pin photocoupler PC817 as an example to illustrate its discrimination method.
Inside the optocoupler, it contains a light emitting diode and a phototransistor.
1. Determine the pins of the light-emitting diodes. Use the MF30 multimeter R×1kΩ to measure the positive and negative directions of any two of the four pins. If the index of the needle is infinite once, but there is a resistance value of about 30kΩ after the test pen is exchanged, then the black test pen is connected The pin is the positive pole of the light-emitting diode, and the pin connected to the red test pen is the negative pole of the light-emitting diode.
2. Determine the collector and emitter of the phototransistor. The photosensitive transistor in the optocoupler is usually NPN type, which has many similarities with ordinary NPN type silicon transistor. Use the multimeter R×10kΩ block to measure the remaining two feet of PC817. If the resistance is infinite at one time, and there is a resistance value of about 250kΩ after exchanging the test leads, then the pin connected to the black test lead is the emitter of the phototransistor, and the pin connected to the red test lead is the collector of the phototransistor. .
So far, the pin arrangement of the four-pin optocoupler PC817 has been completely determined, as shown in the attached figure. As for the pin arrangement of the multi-pin optocoupler tube, the pins of all light-emitting diodes should be identified first, and then the pins of the corresponding phototransistors should be determined.
Check Transformer Power with Multimeter
It is not enough to use only one multimeter. You can find a few bulbs for motorcycles. According to the output voltage of the transformer, connect the bulbs in series to the output terminal of the transformer. When the voltage drops significantly, stop connecting bulbs in parallel and remember the voltage value. Then use a multimeter to measure the current value at this time, and remember the current value. Voltage value × current value = basic rated power
Advantages and Disadvantages of Multimeters vs. Digital Both analog and digital multimeters have their own advantages and disadvantages.
The pointer multimeter is an average meter. It has an intuitive and vivid reading indication.
(The general reading value is closely related to the swing angle of the pointer, so it is very intuitive).
The digital multimeter is an instantaneous sampling instrument. It takes a sample of 0.3 seconds to display the measurement results. Sometimes the results of each sampling are very similar, not exactly the same. This is not as convenient as the pointer type for reading the results.
Generally, the pointer multimeter does not have an amplifier inside. Therefore, the internal resistance is small. For example, the MF-10 type has a DC voltage sensitivity of 100 kΩ/V. V.
Due to the internal use of the operational amplifier circuit in the digital multimeter, the internal resistance can be made very large. It is often 1M ohms or greater. (That is, higher sensitivity can be obtained). This makes the impact on the circuit under test can be smaller. Measurement Higher precision.
Due to the small internal resistance of the pointer multimeter, and the use of discrete components to form a shunt and voltage divider circuit, the frequency characteristics are uneven (relative to the digital type). The frequency characteristics of the pointer multimeter are relatively better.
The internal structure of the pointer multimeter is simple, so the cost is low, the function is less, the maintenance is simple, and the overcurrent and overvoltage ability is strong.
A variety of oscillation, amplification, frequency division, protection and other circuits are used inside the digital multimeter, so it has many functions, such as measuring temperature, frequency (in a lower range), capacitance, inductance, or as a signal generator, etc.
Because the internal structure is mostly integrated circuits, the overload capacity is poor. (However, some of them can automatically shift gears, automatic protection, etc., but the use is more complicated). After damage, it is generally not easy to repair.
The output voltage of the digital multimeter is low (usually not more than 1 volt). It is inconvenient to test some components with special voltage characteristics (such as thyristors, light-emitting diodes, etc.)
