Transistor reverse breakdown voltage tester-----Transistor tester

Here we introduce a small instrument that can measure the reverse breakdown voltage of transistors using an ordinary multimeter. Its measurement range is 0~1000V, which can meet general measurement requirements.
1. Circuit Principle

The circuit of the transistor reverse breakdown voltage tester is shown in the figure below.

The circuit can be regarded as a high-voltage constant current source feeding power to the tube to be tested, and the reverse breakdown voltage of the tube can be directly read with a multimeter. The circuit is essentially a feedback DC converter, which converts 3V DC voltage into a main high-voltage DC power of about 1200V. After the circuit is powered on, due to the coupling feedback between the windings N1 and N2 of the transformer T, the circuit forms a strong oscillation, the oscillating voltage is boosted by the winding N3, and charged to the capacitor C2 after rectification by VD2, so that about 1200V DC high voltage can be obtained at both ends of C2. This high voltage is applied to the tube to be tested through the current limiting of the high-value resistor R2, causing the tube to break down, and the multimeter connected to both ends of the tube to be tested can directly read the reverse breakdown voltage of the tube. Since the resistance value of R2 is very large, it acts as a constant current source, and the current is very small during breakdown, so the tube to be tested will not be damaged.

　　
2. Component selection and production

VT uses silicon NPN transistors with V(BR)ceo≥60V, Icm≥0.8A, such as 2SCl008, and β≥50. VD1 can use ordinary 1N4148 silicon switching diodes, and VD2 requires a fast recovery diode with a reverse voltage greater than 1500V, such as FRl08.

RP uses a WSW organic solid core trimmer potentiometer, R1 is an RTX-1kΩ/0.125w carbon film resistor, R2 uses an RJ-1MΩ/1w metal film resistor, C1 is a CD11-100μ/16V electrolytic capacitor, and C2 uses a 1000pF/2kV high voltage disc capacitor.

Transformer T needs to be made by yourself: use EE20 type magnetic core with initial magnetic permeability of 1500 or 2000, and use matching skeleton to wind, first wind N1, N2, and then wind N3. N1 and N2 are wound with φ0.35mm enameled wire for 10 turns respectively, with center tap; N3 is wound with φ0.17mm high-strength enameled wire for 260 turns. Insulating tape must be added between layers to prevent internal voltage breakdown and sparking.

Since the circuit operating current is small and the number of coil turns is small, there is no magnetic saturation problem, so no air gap is required when assembling the magnetic core. When winding and wiring, pay attention to the same-name end of the winding, that is, the beginning of the coil (the end marked with "・" in the figure), and the same-name ends cannot be connected in reverse when wiring. After the magnetic core is wound, wrap it with two turns of tape to fix it.

x1~x4 are 4 terminals, x1 and x2 are used to connect the test tube, and x3 and x4 are used to connect the multimeter.

The printed circuit board of the tester is shown in the figure below. The size of the printed circuit board is 50mm×30mm. The power supply G uses two No. 5 power supplies, and the housing of the machine box is preferably made of organic glass. During debugging, connect the DC 2500V range of the 500-type multimeter at x3 and x4, close the power switch s, and use a small screwdriver to fine-tune RP so that the reading of the multimeter is around 1200v. At this time, the collector current of the transistor VT is about 200mA. If the voltage is only about 100V, it means that the same-name terminals of the coil are connected in reverse, just swap the two ends of the winding N3.

When measuring, just connect the tube to be tested to x1 and x2, and connect the multimeter to the x3 and x4 terminals. The meter reading is the reverse breakdown voltage of the tube. This device can be used to measure the reverse breakdown voltage of crystal diodes and the reverse breakdown voltage of crystal triodes. For example: V(BR)ceo, V(BR)ceo, etc.