Working principle: The measuring circuit is shown in Figure 1. After the 6V of the transformer T is D5 rectified and C3 filtered, the base voltage of the adjustment tube Q is provided. Adjust W to control the degree of conduction of Q. That is, the control collector A point changes the voltage between 4V and 1400V. The 500V voltage output from the high voltage winding is rectified by a voltage doubler rectifier circuit composed of C1, D1-D4, and the C2 filter output is 1400V high voltage. During the test, the device under test is connected between A and B. When the resistance value of W is small, the base potential of Q decreases, and the potential of point A rises. Until the milliampere meter indicates the rated reverse current of the tube under test (measured small power tube "0.1mA, high power tube "1mA is appropriate"). At this time, the voltage value indicated by the voltmeter is the back pressure at the reverse current. When measuring the resistance value, the resistance is connected between A and B, and the voltmeter reading is divided by the milliampere meter reading. Unit: Ω=V/A. After each run out, set W to maximum and return the output voltage to the low 4V.
The device can conveniently measure the back pressure of the triode, the diode, the Zener tube, the thyristor, etc.; the capacitor withstand voltage; the resistance is greater than 10 MΩ. The output voltage is continuously adjustable up to 1400V.
Component selection
Q requires that when Ic=1A, β≥5, BVcbo≥1500V, reverse leakage current “0.01uA. It can be selected from high pressure pipe such as C1942, C2027, BU208, and heat sink. T is a tube radio power transformer, available in the flea market.
The internal resistance of the voltmeter is 10kΩ/V or more. It is best to have: 1500V, 500V, 250V, 50V, 10V several files. The milliampere meter should have two steps of 0.5mA and 5mA. Can be replaced by a multimeter. Pay attention to the insulation between the circuits when making.
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