Two detection methods of varactor diode

Varactor diode is a special kind of diode, also called variable reactance diode. It is a kind of diode made by the dependence relationship and principle of PN junction capacitance and its reverse bias voltage Vr. It can be used for tuning circuits. The function of the varactor diode is to use the principle of variable capacitance between PN junctions to make semiconductor devices, which are used as variable capacitors in circuits such as high-frequency tuning and communication. Let's introduce two detection methods of varactor diodes.
       Detecting method with pointer multimeter
       The detection method is shown in Figure 3-42 and Figure 3-43. Set the measuring switch of the multimeter to the RxlOk block, connect the black test lead to the positive pole of the varactor under test, and the red test lead to the negative pole. The resulting resistance should be a few thousand ohms to 200kΩ (this value is the forward resistance of the tube under test, and varies with The tube model varies). Change the meter pen to measure the reverse resistance of the tube. Its resistance value should be ∞. If the pointer deflects slightly, it means that the tube is leaking in the reverse direction or has been damaged. If the measured forward and reverse resistance values are both 0 or ∞, it means that the tube under test has been broken down or has been damaged by an open circuit.

Experience has proved that the dispersion of the forward resistance of different types of varactor diodes is very large. For example, the forward resistance value of the 2CC2 varactor diode is 180kΩ, and the value in this example is 10kΩ. In addition, for some varactor diodes with relatively small forward resistance, the Rxlk block can also be used for detection. But in the test practice, it is found that some types of varactor diodes, when tested with Rxlk gear, whether it is a forward measurement or a reverse measurement, the resistance value obtained is infinite (the pointer does not swing). In this case, do not easily determine that the tube under test is a bad tube, you should use the Rx10k block to perform the necessary retest, and then make the corresponding conclusion.
use digital multimeter to detect
(1) The diode block of the digital multimeter can be used to detect the unidirectional conductivity of the PN junction of the varactor diode, and then judge whether it is good or bad. The detection method is shown in Figure 3-44. Set the digital multimeter to the diode block, first connect the red and black test leads to the two pins of the tube to be tested, and then exchange the test leads to retest. In these two measurements, if some of the readings are about 0.58~0.65V, the value shown in Figure 3-44(a) is 0.566V. At this time, the test leads are in forward connection, and the red test lead is connected to the positive terminal. , The pin connected to the black test lead is the negative electrode, and the voltage value obtained is the forward voltage drop value of the tested varactor diode; in another measurement, as shown in Figure 3-44 (b), the meter displays the overflow symbol "1" "At this time, the test leads are in reverse connection, the pin connected to the red test lead is the negative pole of the tube under test, and the pin connected to the black test lead is the positive pole.

(2) The junction capacitance of varactor diodes can be measured by the capacitance block of a digital multimeter. The detection method is shown in Figure 3-45. First turn off the power, turn the digital multimeter to the capacitor stop, insert the negative pole of the varactor under test into the hole on the left, and insert the positive pole into the hole on the right to ensure that the reverse bias is applied. Then turn on the power and observe that the number appearing on the multimeter should decrease rapidly from a certain value and stabilize at a certain value. This value is the junction capacitance of the varactor diode under test. If the overflow symbol "1" is displayed, it means that the PN junction is short-circuited; if it displays "000", it means that the tube is open.

It should be pointed out that before measuring the junction capacitance, the varactor diode should be short-circuited and discharged, and then the power supply should be turned on, otherwise the above charging process will not be observed. In addition, the nominal junction capacitance of the varactor diode is also related to the ambient temperature and is tested at room temperature of 25°C.

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