Active Load

[janengelbrecht]

Inspired by Daves Active Load design:

Suggestions and comments will be appreciated 

Its seems to work...but i need another FET and rail-to-rail OpAmp.
The expensive 10 turns pots is not ideal too.

My problem really is that i think the opamp that drive the MOSFET ought to have an output voltage at 4.5V...but it is nearly 1 Volt higher....hmm..... maybe I ought to go back to my uni and start all over Not that it in any way influences the function of the circuit Its just....it shouldnt really work as i see it ....but it does  

(The OpAmp U1:A has a resistor with a really high value : Gate-source resistance in MOSFET as its feedback resistance so it ought to have an amplification near its Open Loop Av. So one would assume it acts like a comparator ? Then it should settle at half supply when there is 0 volt difference between its inputs. Well if such things as offset errors didnt exist - Tried to remove R2 - no difference! What is it i am missing ? God damn...now i can feel i only has worked with software the last 19 years )

[Richard Head]

Why two pots and not just a resistor in series to limit the range?
I would add a bit of hysteresis on the current limit so the current limit LED either goes ON or OFF and not gradually switches on.
I suggest some bypass caps across the power rails and Zener also.

[janengelbrecht]

Hmm two pots for coarse and fine adjust
Your other advice i will implement at once - thx....still i have some troubles understanding why it actually works
In my next project i will not use this trial/error approach in my design proces but actually calculate it all first

Current limiting LED: It really doesnt gradually turn on/off..... Its either ON or OFF

[Seekonk]

I would expect to find the fine pot to be in series with the course pot source voltage source OR something like a 20K resistor in the lower leg of the fine pot.  As it is the course pot sets the max voltage and the fine pot is 0-100% of that.