Diy bench power supply oscillating when I connect 12V car lamps

[diydidi]

Hi
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.

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....
Any ideas? See schematic attached.

 

[JohnRoberts]

My head hurts a little from looking at the schematic.

If you are getting an instability in the current limiting that is generally caused by lag, while the car lamps may be a weird load when cold.

Lamps are often used as current limiters by themselves because they are lower impedance when cold, then rise as they heat up.

JR 

 

[Evertide]

If i turn down the current limiting it gets even worse.

So if you turn it up it should be better...

The curent it to small for the load...


Tried making C2 smaller, this too does nothing.

What if you make it mutch bigger?


And what happens if you leave it on for half an hour?


Succes with experementing.