I'm having some newly discovered issues (thanks to my new scope) with my old power supply. I've attached a photo of the analog part of it. It uses a TL084 quad opamp for the voltage and current control loops. The reference voltages are created by 2x 8-bit DACs (not shown), which themselves are controlled by a microcontroller. It takes care of the user-interaction.
![](http://farm8.staticflickr.com/7085/7143253023_f1f5f7fee7_n.jpg)
Issues discovered so far:
* about 10% voltage overshoot when the output is activated (via relay) - this would kill 3.3V devices.
* massive current overshoot, the current control loop seems deliberately slow.
I've played with the relevant capacitor a bit (C26, top right opamp). If it is removed altogether, the thing never gets out of over-current mode once entered. Replacing it with 22pF makes it unstable. I will have to get a 5pF-ish trimmer cap. to experiment further.
The way the over-current limiting is implemented here seems somewhat flawed to me. The time it takes to kick in is way to long and it also depends on the set-current. For low values (say 20mA) it takes quite a long time on a human timescale until current is finally limited.
See the attached traces. The colors match the dots in the schematic.
![](http://farm9.staticflickr.com/8008/7143252837_d30ed158fb_s.jpg)
I'm not actually thinking about fixing or redoing the control circuitry at all, I'm just playing with it. Maybe there are some simple things to improve its behaviour. I thought about adding an external module to compensate for its flaws, but getting an all new power supply sounds more attractive. This things is about 20 years old btw.