Voltage source with three ranges 0.2/2/10V

This voltage source provides a stable and adjustable voltage over three commonly used ranges. The stable ouput voltage can be use for checking and setting the gain of operational amplifiers and testing trip points setting.

Instead of using selector switch with fixed resistors for setting output voltage, a 10-turn variable resistor is used for output voltage adjusting and a 3-1/2 digit DVM module is fitted to provide continuous monitoring of the output voltage. In this way, no dial marking is required for the VR.

AC240V input is stepped down via a small 240/20x2 transformer and rectified to give peak voltages of +/- 28V. This is regulated down to +/- 15V via the 7815 and 7915 three terminal regulators.

Opamp UA1 and 5.8V zener diode D2 forms the stable voltage circuit where the zener diode is fed constant current from the stablised -10V it itself generated. The variable voltage is selected from the wiper of a 5Kohm ten-turn potentiometer with its two ends connected to this -10V and a germanium diode fed from positive 15V supply. The germanium diode is to provide a 0.2V offset so that output voltage adjustment can be changed right through zero and a little beyond.

A three positions center-off switch is used for controlling the output and selecting the correct polarity from the unit. The DVM module is powered from a 6V lithium battery inside the case and switched on automatically when the main unit powered up, using a photo-coupler.

Range selection switch is a 4 pole three position center-off switch. One pole is used to switch the buffer op-amp into different gains with the same 0 to -10V input from the 5K ten-turn pot. The remaining poles are used to switch the range and decimal point of the DVM module.