HELP WITH CURRENT LIMIT INDICATOR ON POWER SUPPLY

Thread Starter

Craps1956

Joined Aug 21, 2015
35
Hi all members

I came across the following power supply circuits from different sources on the net. All look similar with minor differences. Thought I would try putting one together.
Fig 1.jpgFig 2.jpgFig 3.jpg
https://320volt.com/en/lm350-0-30v-0-3a-laboratory-power-supply/
https://www.hobbyelektro.eu/laboratorni-zdroj-0-30v-0-3a/

After assembly, it could not be adjusted down to 0v and the maximum volts out was a little over 33v
I solved these problems with the following modifications (FIG 4 My mods).
Now all is good with an adjustable voltage of 0v – 30.1v
Fig 4 My Mods.png

But now I need help in solving the following problem with the overload led D9 (which I think should always be off if there is no load at the psu output terminals) However with no load on the output and all pots turned fully anti clockwise D9 comes on and when turning the current pot a tiny bit clockwise (volt pots still at 0) D9 turns off. Other then this everything else works as expected.

So how do I fix the led coming on too soon?

Should I increase the value of R9? Or I am thinking the problem may be with T2 turning on too soon and or the values of R4, R5, R6, R7 or R8 Don’t know which.

Any help is much appreciated
 

AlbertHall

Joined Jun 4, 2014
12,023
I think this means that the forward voltage of D8 is slightly greater than the base emitter voltage needed to turn on T1.
This is a pretty standard effect on similar variable bench supplies. It simply means that the current limit is adjustable down to zero. Why would you not want that?
 

sparky 1

Joined Nov 3, 2018
717
There is a kit available called Hiland power supply. Like a Howland current pump.
This video the author reverse engineers the supply and shows how the precision current control indicator light comes on.
Smart guy, like his quick reviews and tear downs. Watch from about 5:30
 
Last edited:

Thread Starter

Craps1956

Joined Aug 21, 2015
35
Thank you all for your responses and interest in my problem.



I have made some measurements as per attached.

Hope this will be helpful towards assisting me in solving the ‘naughty’ led.
M01.pngM02.png
 
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