Upgrading a Linear Supply with a Current Meter
I bought an Astron RS-20 on the cheap at the local hamfest for my radio. Only problem was the supply was lacking some basic features such as a panel meter for the load current and a banana jack on the front to make the supply output accessible. No problem, with a 200mV digital panel meter laying around, I knew exactly what I wanted to do...
The meter needed a pseudo floating power supply. I created that by using a 5V zener diode and a dropping resistor from the control voltage which was around 23V to the output voltage which is around 13.7V. The output voltage became the new ground reference. I star connected all of the "ground" signals of my panel meter to the tie point which was used to connect the two output current sense (load balancing really) resistors. I used a dropping resistor of about 1k Ohm which give about 10mA of supply current before the zener diode gets starved for current. The meter has a power consumption of about 2.6mA, so 7.4mA is reserved for spikes in power and variations in load current. I didn't look at the power supply current with a scope, but if I did and it had current spikes in excess of 10mA, then I could have added a capacitor in parallel with the zener diode to provide the peak currents.
See the attached circuit diagram for my circuit modifications. I downloaded the circuit from:
Which is a great site for tips and schematics for all Astron supplies.