Production of 0-30V voltage regulator tester

Usually, when we measure the voltage value of a voltage regulator, we need to set up a circuit and adjust the resistance of the current limiting resistor. The measurement is troublesome and the measured value is not accurate enough. The tester introduced in this article can measure the voltage regulator of 0.5W and 1W with a voltage value within 30V. The measured value is relatively accurate, and the digital meter display is more intuitive.

How it works

The stable currents tested for different power Zener diodes are also different. The stable current of 0.5W is generally smaller, so 5mA is used here. The stable current of 1W is relatively large, so 20mA is used here.

The following figure is the working principle diagram of the tester. The 220V mains electricity is transformed through transformer 1 and generates two voltages, 18V and 9V respectively. The 18V voltage is rectified by capacitors C1 and C4 and diodes VD2 and VD3 to double the voltage, generating a voltage of about 40V on capacitor C4. The junction field effect transistor VTI or VT2 is a constant current source circuit. When the voltage regulation value of a 0.5W voltage regulator is to be measured, the switch S is closed to contact 1, and the constant current source outputs a current of about 5mA. When the voltage regulation value of a 1W voltage regulator is to be measured, the switch S is closed to contact 2, and the constant current source outputs a current of about 20mA. In this way, the constant current passes through the voltage regulator tube to be tested on the socket, and the voltage regulation value is obtained on the voltage regulator tube, and the voltage is measured by the digital voltmeter connected to both ends of the voltage regulator tube.

The circuit composed of VT3, VD4 and other components is used to make the digital meter read 0 when no measurement is made. When measuring the voltage regulator tube, the voltage of the voltage regulator tube on the socket is less than 30V, the voltage regulator tube VD4 does not break down, the transistor VT3 is in the cut-off state, and the stable voltage is added to the digital meter through R3 (since the resistance value of R3 is much smaller than the internal resistance value of the digital meter, it does not affect the meter reading). When the voltage regulator tube is not measured, the voltage of the voltage regulator tube on the socket reaches 40V, the voltage regulator tube VD4 breaks down, the transistor VT3 is in a saturated state, the 40V voltage is all added to R3, and the digital meter displays 0.

At the same time, another 9V voltage is added to the voltage regulator integrated circuit LM7805 after rectification by diode VDI and filtering by capacitor C2, and outputs a stable voltage of 5V as the power supply of the digital meter.

Component Selection

Transformer T is 3W. Capacitors C1 and C4 are 50V/220μF electrolytic capacitors. All diodes are lN4007. Field effect tubes VT1 and VT2 are 2SK363, of which VT1's Idss>5mA. VT2's Idss>20mA. Voltage regulator IC is LM7805 or AN7805. Voltage regulator tube VD4 is 38V/0.5W. Transistor is 9011, with β between 100 and 150. Voltage meter is LED digital voltage meter made of ICL7107 chip. Switch S is 1x2 toggle switch. The measurement socket is replaced by the lead socket of old TV.

Production method and debugging

First, the transformer must be modified. The method is to disassemble the transformer core, use 0.23mm enameled wire to wind 190 turns on the low-voltage coil frame to form a 9V coil, and then reinstall the core.

Then design and make a circuit board according to Figure 1. The pin arrangement of the field effect tube and the voltage regulator integrated circuit is shown in the figure below. After the components are soldered correctly, connect the wires (do not connect the voltage meter for the time being). Measure whether the voltage of pins 2 and 3 of the IC is 5V. If it is not 5V, check whether there is any error in the pin soldering. Then measure the voltage across the capacitor C4, which should be between 40V and 50V. Turn the switch S to contact 1, and then replace the resistor R1 with a 100Ω potentiometer. Without connecting the test voltage regulator, adjust and measure the current on the socket to 5mA. After determining the resistance value of the potentiometer, weld it with a fixed resistor. In the same way, adjust the other current to 20mA, and then measure the collector-emitter voltage of VT3 to 0V when the test voltage regulator is not connected. When connecting the test voltage regulator, the collector-emitter voltage should be the same as the voltage of the test voltage regulator (measured with a digital meter). After it is normal, connect the digital meter.

Finally, find a plastic shell to fix the circuit board and transformer on the bottom plate inside the box, and fix the switch, test socket, and meter head on the panel. The tester is now complete.