Bench PSU

[ante]

Hallo all.

I found this schematic on the net some time ago. I like your opinion on a mod of the circuit. Is it possible to use a couple of op-amps instead of the resistors R1, R2 to get continuous adjustable voltage and current control? I think L2 could serve as dc-shunt for the current and with a fixed voltage as a reference for the op-amps one could feed the switch-IC (Max 724) 2.1 Volts at the feedback input. Perhaps output voltage could be adjusted down to 0V, or at least close. If it works as I think it ought to, it can serve as a bench PSU putting out 0 to 35 V @ 0 to 12 amps. Not bad! Am I totally of the tracks here?? :roll:

 

[Nigel Goodwin]

Not bad! Am I totally of the tracks here?? :roll:

I'm afraid so :lol:

You can't replace R2 with an op-amp, to make the current limiting fully variable would require different techniques. I don't think R1 has anything to do with setting the voltage, and it's difficult to get a PSU to work down to zero volts - the usual way is to have a negative voltage in the PSU, thus the power supply thinks it's outputing 2-3V, but the external voltage is actually zero.

 

[panic mode]

You do have adjustable voltage with P1 and P2

 

[ante]

Error!

Sorry, It should be R4, R5 and P1 , P2

 

[Noggin]

Not bad! Am I totally of the tracks here?? :roll:

I'm afraid so :lol:

You can't replace R2 with an op-amp, to make the current limiting fully variable would require different techniques. I don't think R1 has anything to do with setting the voltage, and it's difficult to get a PSU to work down to zero volts - the usual way is to have a negative voltage in the PSU, thus the power supply thinks it's outputing 2-3V, but the external voltage is actually zero.

How about an LED? That'll drop the voltage... Get one of those yellow/green/blue leds that change color depending on the drop.... then again I guess you'd have to have a high current LED for that to work. Then again, you've got to worry about high currents through the LED as well as very low currents....

nevermind, forget the diode

 

[ante]

Well, here is an addition to the circuit. Is this the begining of something I can use? If so, would you please help me fill in the blanks? Maybe this will come out a nice project for everyone in need of a PSU. :roll:

 

[Sebi]

I mean You need only one opamp for current limiting.

 

[ante]

Thanks Sebi

A couple of questions on this. The diode, what will happens with the voltagedrop? And second, shouldn`t R5 be moved to the output (neg). The shunt voltagedrop must influence the output voltage? Maybe even P1 should move to the (pos) output? Is there someone who have used the MAX 724 (or similar) switch regulator in bench PSU:s? Every bit helps.

 

[Sebi]

To avoid the output voltage drop on shunt move the voltage divider low point (R5) to output negative (after the shunt).No problem with the voltage drop on diode: when the current sense amp out go higher, just give additional voltage to voltage sense pin, so the IC decrease the output voltage.

 

[ante]

Thanks Sebi !
I´l try this one out.
Will return on subjekt if fails. :shock:
by for now.