EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: electronwaster on March 02, 2012, 11:16:48 pm
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I'm designing and building a single linear power supply with controllable current and voltage, mostly to learn how these things work (i.e. how they regulate), and also as a way of furthering my basic understanding of electronics. I did post about this (at length!) in the beginners forum, but at least 60% of the discussion was about my chosen method of learning rather than discussing the power supply itself - perhaps that was my fault. I would rather discuss and improve the project itself, and I don't like to keep resuscitating an old thread, hence this new thread in projects.
I've been watching Dave's PSU videos and learning lots, I think his overall design is focussed heavily on precision, low power, and microcontroller integration, not a perfect match for my needs, but I did pinch the idea about using a 1 ohm series resistor and some opamps to convert current to voltage, and then comparing that to a 0-1V pot for 0-1A and power the base of an NPN that controls output.
The circuit that I ended up with at the end of the previous thread I made up onto stripboard and it's good, but not great. There is more voltage sag when I apply a load than the simulator simulated (surprise surprise), so I guess the internal resistance of the transformer is greater than I modelled, or some other departure from real life.
Anyhow, that's done, and now I want to build one with a constant current mode too.
My rough specs are as follows:
0-15V output
0-1A limit
single input voltage
single output voltage
If it works out, I'll just build another, and use them together as a dual supply with a nice toroidal transformer I have with two 25V secondary windings.
I've attached a pic of the current circuit, it's not on the breadboard yet, before you ask. Can anyone suggest any improvements, or anywhere it might not work in real life?
Thanks,
electronwaster
Notes:
R15 is the load resistor
R3+R4 is a facsmile of the current-control potentiometer, to turn the knob, I adjust the two values
R5+R6 do the same for voltage.
U1 spits out the voltage difference on either side of the 1 ohm sense resistor, R2
U2 compares the 0-1V output of U1 with the 0-1V output of the current pot.
Q1 can short the base of Q2 when in current limit mode, reducing the output
U3 is a buffer/voltage-follower, and needs to be rail-to-rail, to minimise load on the voltage pot, and drive the base of the pass transistor, Q2
Please ask if anything is unclear.
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Don't forget caps. Also, you'll need a resistor on the output of U3 to limit current when Q1 turns on.
See attached. So far as the values go, they might need some tweaking.
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Hi Jack,
Yes - I hadn't added a single cap yet, thanks for doing that for me. I'l add those to the circuit and reply to the thread with the updated version.
Thanks,
electronwaster