Help with current limiting device

[Rob11966]

Hi Folks,

I am usually found around the tubes forum but I am currently building a benchtop power supply and I am struggling with a problem at the moment.

The power supply is a variable 0-28 V supply capable of 6A at 12V. The power supply works fine, no problems but I am trying to add on a varible current limiting device. This is the one that I have chosen

Variable (Adjustable) Current Limiter Circuit | Free Circuit Diagram

Trouble is that it does not seem to work. When the current limiting 'module' is place in the circuit, all that happens is that the output voltage seems to get reduced and is only variable between 0 and about 3v (using a 10 ohm/10w resistor as load). I think that the schematic may be wrong. The authors refer to shorting the base of Q4 to ground but there is no ground on the original schematic?

I have attached an original schematic and also my idea of what it should like. Is anyone able to comment?

Thanks in advance,

Rob

 

[sawreyrw]

The circuit on the left should work fine. The one on the right will not work, because the sense transistor will always be on.

 

[Rob11966]

The circuit on the left should work fine. The one on the right will not work, because the sense transistor will always be on.

Thanks - The one on the right was the one that I have bread boarded. Armed with your information, I replaced all of the parts sequentially. It was a bad BD139.

It's now working fine.

Thanks again,

Rob