I am in the process of revese-engineering a custom precision mA power supply that quit fuctioning last year. It is similar to an Op-Amp based linear regulator but deviates in a few areas. I am a novice at describing this thing, but I can e-mail a schematic and/or pics to anyone interested. (Doesn't seem like I can add images to these posts). I bought all new components and have set up a breadboard for testing.
I uses a OPA234U op-amp controlling a 2N3055 Transistor. It has a
7809 and an NTE960 regulator in series to step the input/reference voltage down from 12 VDC to 9 then to 5 VDC. The 9 volts signal is not used and I am not sure why it has two regulators unless it is more precise that way. The +5 volt signal is connected to a 22k ohm res and then to the CCW of the 2k ohm pot. The CW is connected to the-12VDC and also to a 1 ohm 25w power resistor and then to the main - VDC output.
The op-amp has +12V to V+, -12V to V-. The output goes to a 47 ohm resistor then to the base of the transistor. The +In is connected to the wiper of a 2k ohm 2w pot. The -In is connected to a 10k ohm resistor then to the main -VDC output.
The transistor emitter is connected to +12VDC and the collector to a switch then a 2A fuse and then to the main +VDC output.
There is a 4700 uf capicitor across the 12 VDC input and a 1000 uf cap across the 5VDC NTE960 output. Both to the common ground or -12VDC input.
If this all sounds confusing, like it does to me, you can e-mail me at snipped-for-privacy@yahoo.com and I will provide schematic and pics that I have so far. This unit worked very well and had less than +/- 0.1 mA drift to a 35 ohm load torque motor and would put out over 300 mA. My potential problems have to be either the pot wired wrong, transistor wired wrong or damaged op-amp while soldering to its connector.
Thank you for your time and effort.
DanH