Hi there!
I'm a beginner and having quite a hard time building my Lab PSU (1,25-12V, ?-2A)
What I have so far, is a quite OK working µC Voltage Control of an LM317 from its Vref to 12V using an MCP1541 Voltage Reference on an MCP4922 DAC which then goes to an LM358 (Gain of 10) which then in turn drives the LM317 ADJ pin. The LM358 has a negative supply rail which comes from an TL7660. Also, there is an LM334 for the minimum load.
As some may notice, this is inspired by Daves µSupply project which sadly never has been finished due to some design flaws and featuritis I assume.
I tried to build up the current sensing using the MAX4080T and a 0.1R shunt resistor like in this schematic:
http://www.eevblog.com/files/uSupplyBenchRevB.pdfAttention, Dave swapped the Inputs on U3B accidently. Instead of the TLC272BCP I used LM358 (with neg. supply) and instead of the LT3080 i used an LM317. Also I used an BD139 instead of the 2N3904 (Q2).
BASICALLY that worked, however, current limiting was SWITCHING the ADJ pin on and off, instead of driving it in a linear way. You guess that ripple on the LM317 output. I tried several caps on C19 and C2 (1µ, 3.3µ, 10µ, 22µ, 33µ, 100µ, 220µ), the higher values made the switching kind of go away but the measured current on the MAX4080 obviously settled on a much higher level due to the high capacitance so that I got huuuge error there.
Was the thing switching because of me, simply using different parts than in the schematics (maybe the LM358 are too slow?) or is there another error in the schematics?
I also tried to simulate (didn't build that one up) the diff. amp from schematics Rev. C:
http://www.eevblog.com/files/uSupplyBenchRevC.pdfHowever Dave mentioned somewhere that this circuit won't work cause the LM358 aren't Rail to Rail Opamps. So I tried to go for a higher supply voltage as well as negative voltage supply but the whole simulation behaved very strangely an when going to current limiting it mostly didn't even solve (or I'd have had to wait much longer for it to get there). I also tried to swap the LM358 with LT1007 (and others) but I had a hard time choosing Opamps due to my little knowledge.
There's also another circuit which actually basically works: Two LM317 in series, one for current limiting and one for voltage control. However my concern was, if the current limiting LM317 lowers the input voltage below the dropout voltage of the voltage controlling LM317, wouldn't that thing become horribly unstable? I can't imagine that this would be a practical circuit, or am I missing something? Would the current limiting LM317 normally be AFTER the voltage controlling LM317?
Attached you'll find the LTSpice Simulation of the Diff. Amp. from Daves Schematics Rev. C
You'll need to rename the LM358.txt to LM358.lib and have it in the same directory as the ASC file.
Basically as I said, there is a voltage control circuit which works ok for me (looks ok on the oscilloscope as well) and I'd like to add something which I can control from a DAC (4.096V reference voltage, 12 Bit) which either pulls the ADJ pin to ground in a linear way or something like an NPN transistor (darlington?) which comes after(?) the voltage regulating LM317 and pulls the output to ground if there is overcurrent (also in a linear way). However for the transistor approach, I'd need far more help as I have absolutely(!) no idea which transistors to choose to make them work in a good operating area (read something about amplification and saturation.. NOO IDEA).
Many thanks for your help!