Diy Bench power supply Oscillating with Inductive load.

I have been woking on a bench power supply for the past two months or so.
I decided on a discrete regulator design with adjustable voltage and and current limit.
Yesterday I boxed most of it, and decided to test using a couple of parallel connected 12V car bulbs. The bulbs were flickering slightly. Scoped the output and revealed a 16- 25HZ almost square wave/ round edges type oscillation or motor boating.
If i turn down the current limiting it gets even worse.
With my DC load all works as it should.
My electronics knowledge doesn't stretch as far as to fix this. The sense leads come together at the croc clips of the output leads. So there are four wires coming out of the box. The negative output terminal connects directly to neg of main filter caps.
Connecting a 4700uF capacitor across the leads at the load doesn't help much.
Any ideas? See schematic attached.


  • Paul Power supp.pdf
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I suggest you communicate with "Paul" and ask him whether it would be a bad idea to install a 470uF, 100V bypass capacitor from D5 cathode to D15 anode. Similarly would it be a bad idea to install a second 470uF, 100V bypass capacitor from D2 anode to D15 anode? Finally I suggest that it might be a good idea to connect another resistor "R16B", between Q8 emitter and D15 anode.

I suggest you have your lab technician take scope photos of D5 cathode, when (A) the electronic load is connected and no motorboating occurs; (B) the headlamp load is connected and motorboating is present. Scope photos of D2 anode might be revealing as well.

Have you considered the possibility that it is the thermal properties of the headlamps, rather than their inductance, which are giving you difficulty? You could connect a series inductor to see whether the problem gets worse, better, or unchanged.
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Thanks Mark for the reply. Just as matter of interest, I placed a 1uF between D8 Cathode and D15 Anode, and the motor boating was completely gone. Why would this be?
Will try your suggestions too.
If I make C1 and C2 smaller, might this help with the "speed" of the loop faster?
Do please notice that there is a 10k resistor missing from my circuit. It is connected between Q6 emitter and D17 Anode.
I have attached the original schematic from the 70's. Apparently it was a very good supply. "Paul" went and simplified the circuit a bit for his needs, but most of the values are identical. The circuit works great except for the problem I mentioned in my first post!
Have managed to get rid of the oscillation by adding a cap between D8 cathode and D15 Anode. I'm not sure how big this cap should be?
So I guess my next step would be to try and better this circuit slightly as I have already made a pcb for it.
Further suggestions welcome.
Definitely gonna try reducing C2 as mentioned by you guys!


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OK. Just confirmed my cap idea actually doesn’t work!! :confused:
Tried the 100uf across sensor wires but no effect.
It’s just strange that if I short the + sense wire to the main + out on the pcb instead of at the croc clip, everything works great, This confirms that the current board is actually working.
Tried making C2 smaller, this too does nothing.
Something is up here...
Joined 2009
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I doubt "Paul" designed that power supply. It looks like the schematic for an older HP power supply. Those old service manuals are actually a good place to start if you want to learn how to build power supplies. I do suggest reading the parts of the service manual that describe the circuit functionality and tradeoffs if you're going to modify the circuit.

The best way to debug this would be to draw the schematic in your favourite circuit simulator and simulate the stability of the various feedback loops.