Diode failures are mainly manifested in open circuits, short circuits, and unstable regulators. Among the three kinds of faults, the former one shows an increase in the power supply voltage; the latter one shows that the power supply voltage drops to zero volts or the output is unstable.

Judgment of common diode faults:

By testing the voltage across the zener diode when the circuit is energized, the fault nature of the zener diode can be judged.

Open road

After the diode is open, the voltage across it will increase, so it is not difficult to make a judgment based on this.

2. Short circuit

The diode has a short circuit fault, and the voltage across it will drop to zero volts.

3. The voltage regulation value is unstable

Poor quality of the diode will cause unstable output voltage. When using the multimeter's DC voltage block to measure the voltage across the zener diode, it will be found that the voltage is suddenly high or low.

How to use a multimeter to test the quality of ordinary diodes:

1. Identify the positive and negative poles of the diode. One end of the white wire is the negative pole and the other end is the positive pole.

2. Turn the knob on the multimeter to the on / off position, and insert the red and black test pens in the correct position of the multimeter.

3. Connect the red test lead to the positive pole of the diode and the black test lead to the negative pole. Then observe the reading and if it overflows (ie, shows 1), the diode is broken. If there is a reading, exchange the test leads. If there is a reading without overflowing, the diode is broken.

4. If it is a light emitting diode, if the diode is normal, you can see a faint light, and the long foot is positive.

First, the detection of zener diode

  1. Discrimination of positive and negative electrodes

From the appearance, the positive end of the metal-encapsulated zener diode body is flat, and the negative end is semicircular. One end of the plastic voltage regulator diode body printed with a color mark is a negative electrode, and the other end is a positive electrode. For zener diodes with unclear marks, you can also use a multimeter to determine the polarity. The measurement method is the same as that of ordinary diodes. That is, use a multimeter R & TImes; After the results, adjust the two test leads for measurement. In the two measurement results, the smaller the resistance value, the black test lead is connected to the anode of the zener diode, and the red test lead is connected to the anode of the zener diode.

If the forward and reverse resistances of the zener diode are measured to be small or infinite, it means that the diode has broken down or is open-circuit damaged.

  2. Measurement of regulation value

Use 0 ~ 30V continuously adjustable DC power supply. For zener diodes below 13V, the output voltage of the stabilized power supply can be adjusted to 15V, and the positive pole of the power supply is connected in series with a 1.5kΩ current limiting resistor. The negative pole is connected, the negative pole of the power supply is connected to the positive pole of the zener diode, and then the multimeter is used to measure the voltage value across the zener diode. The measured reading is the zener diode's zener value. If the voltage stabilizing value of the zener diode is higher than 15V, the regulated power supply should be adjusted to more than 20V.

You can also use a megohmmeter below 1000V to provide test power for the Zener diode. The method is:

Connect the positive end of the megohmmeter to the negative pole of the zener diode. After the negative end of the megohmmeter is connected to the positive pole of the zener diode, shake the handle of the megohmmeter at a uniform speed while monitoring the voltage across the zener diode with a multimeter. Value (the voltage range of the multimeter should be determined by the stable voltage value). When the multimeter's indicated voltage indicates stability, this voltage value is the stable voltage value of the zener diode.

If the stable voltage value of the zener diode is measured, it means that the diode is unstable.