psu using lm723+opamp for current limiting - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: December 04, 2020, 11:50:35 am
content: Hi all,ive built a modded version of the 0-30v psu (red pcb from china) in cc mode it oscillates badly so as i have a few 723 ic's and 741 opamps thought i would give this a try( see attached) firstly can the 741 be used in place of the ca 3140 also is the schematic viable ?,cheers all m3vuv.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 04, 2020, 12:09:38 pm
content: The Transistor Q4 adds additional gain to the CC control loop. The 723 is already operating in the floating configuration and in this form does not need additional gain or an extra high slow rate in the CC mode. So it is not such a surprise to have the circuit oscillate. I would more like use the OP with swapped inputs and without the extra gain from Q4. This may need also a negative voltage from the floating axillary supply. With a 741 one may need this anyway because of the input common mode range.

author: Ian.M
author_rank: Super Contributor
timestamp: December 04, 2020, 12:24:33 pm
content: Its dodgy. If the mains supply is at its upper legal limit, C1 will charge to close to 50V. If the current limit is set to 5A, and the output is then shorted, transiently the 2N3055 transistors will each have to pass 2.5A with 47V across them before the voltage on C1 drops significantly, which is well outside the DC SOA line and very close to the 1ms pulse SOA line. Unfortunately at 5A the voltage drops less than 2.3V/ms so odds are they'll be outside the DC SOA long enough to blow. See fig. 2It may be worth looking at circuits that use two LM723 regulators tied together by their frequency compensation pin. That gives you two error amplifiers so you can use one for the voltage loop and the other for the adjustable current loop. Use one '723 to drive the pass transistor and the other for the reference voltage for both control loops to reduce the effect of the driver dissipation on the reference temperature stability.

author: m3vuv
author_rank: Super Contributor
timestamp: December 04, 2020, 01:19:30 pm
content: can you recomend a good schematic for that?,im after some way to use the 723 with an opamp to incease/amplifie the volt drop over the cs resistor so i can use a low value current sense resistor so can have cc mode down to a few milli amps.

author: jonpaul
author_rank: Super Contributor
timestamp: December 04, 2020, 02:19:16 pm
content: LM723 is ancient part, find a modern reg better specs and easier applications.Jon

author: m3vuv
author_rank: Super Contributor
timestamp: December 04, 2020, 02:27:06 pm
content: your quite right but the 723 is what i have to hand,otherwise it means ordering and waiting!.

author: m3vuv
author_rank: Super Contributor
timestamp: December 04, 2020, 02:31:15 pm
content: its not this circuit that oscillates,read my original post againe,its the ebay chinese psu that has the oscillation issues.

author: PKTKS
author_rank: Super Contributor
timestamp: December 04, 2020, 02:45:42 pm
content: Quote from: Kleinstein on December 04, 2020, 12:09:38 pm2x on that.I've used a lot 723 on 80sua723 is **STILL** under production..I like it very much for simple LINEAR PSUs (nothing compares)PIN 13 is specific for FREQ. COMPENSATIONusually very little is needed ... so if you increasethe loop gain bad things will happen under different loads.Simple test ... remove the loop gain and use simplecapacitor feedback on pin 13 (example of datasheet)nothing will ever oscillate like that unless a toasted chipI have never ever needed to compensate 723 beyondthat small capand for current limiting the provided resistor schemais enough to enter CA with foldback if neededPaul

author: m3vuv
author_rank: Super Contributor
timestamp: December 04, 2020, 02:58:58 pm
content: FFS!!!! cant you guys read,its not the circuit i attatched that oscillates!,its what i was thinking of building,its my other chineese 0-30v ebay psu that oscillates not the schematic i posted,just to clear things up ok.

author: PKTKS
author_rank: Super Contributor
timestamp: December 04, 2020, 03:03:50 pm
content: I guess we all assumed your oscillating power supplyis using a typical 723 ... with some sort of that feedback...without further schema all things plane still apply.That crappy feedback is likely to oscillate unlessproperly compensated with at least a TYPE 3 networkPaul

author: David Hess
author_rank: Super Contributor
timestamp: December 04, 2020, 05:25:36 pm
content: The operational amplifier requires an input common mode range which extends to the negative supply so a 741 is not suitable without circuit changes.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 04, 2020, 05:30:48 pm
content: Quote from: m3vuv on December 04, 2020, 02:58:58 pmThe circuit shown will likely oscillate too.The other problem already noted by Ian, too much power for the 2N3055, likely violating the SOA (e.g. a chance to low a transistor). The small transistor to drive the base current may save the day, as the supply may never get to 5 A, or the small transistor my blow first.There is the other problem of sensing the current from only one transistor.Using 1.2 Ohms at the emitters would normally be too much for 2.5 A - way to high in power. For a variable current limit those resistors are a tricky balance: small enough to avoid too much heat and large enough to also allow at least a little fine regulation. 1.2 Ohm would more like something for maybe 0.5 A per transistor - also OK for the 2N3055.It could pay to have good accuracy OP for the current regulator, as the shunt resistor than can be smaller / lower power.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 04, 2020, 11:25:52 pm
content: you want a more sensitive curren sensing, add an power rectifier diode in serie with the sense resistor add .6v to the resistor calculations

author: m3vuv
author_rank: Super Contributor
timestamp: December 04, 2020, 11:57:22 pm
content: surly adding a diode will just drop the voltage over the current sense resistor by 0.6v,shifting the amount of current from the output upwards?,its the total oposit of what i need/want,i want it to current limit/foldback at a few milli amps,hence the idea of using an opamp!.

author: m3vuv
author_rank: Super Contributor
timestamp: December 05, 2020, 12:40:38 am
content: ive just found this,does it look viable?

author: David Hess
author_rank: Super Contributor
timestamp: December 05, 2020, 03:14:44 am
content: I might consider a design using one 723 for voltage control and another 723 for current control:http://electronics-diy.com/electronic_schematic.php?id=964

author: Kleinstein
author_rank: Super Contributor
timestamp: December 05, 2020, 02:44:37 pm
content: Quote from: m3vuv on December 05, 2020, 12:40:38 amThe circuit is viable: essentially the standard lab supply type with 2 OPs. The LM723 is only used as reference and auxiliary regulator. Looks odd, but is not that bad.It is still the old way of adjusting the feedback divider for the set voltage - this works but results in a loop gain that depends on the voltage setting: the higher the set voltage with slower the regulation. Often a bit sluggish at the upper end and possible ringing at low voltage as a compromise. The newer way keeps the FB divider fixed and adjust the reference voltage side. This comes naturally with digital control via DAC.

author: David Hess
author_rank: Super Contributor
timestamp: December 06, 2020, 02:54:18 am
content: Quote from: Kleinstein on December 05, 2020, 02:44:37 pmA surprising number of old power supply designs work that way without issues. It may be more common in tracking supplies where it simplifies the design because no buffer is required between the tracking and individual adjustments.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 06, 2020, 03:57:49 am
content: Quote from: m3vuv on December 05, 2020, 12:40:38 amthis one has current and voltage crossover if i recallyou can build psu with op amps all the way if you want ??

author: Pawelr98
author_rank: Regular Contributor
timestamp: December 06, 2020, 04:41:35 am
content: Try this circuit.It's a rough sketch but it will work just fine.Only single winding, adjustable from 0V up to 30-ish V, Vdrop is pretty much down to Vcesat of the 2n3055 (or whatever you put there) pass transistor.Extra "boost" voltage covers the Vbe voltage drops, while the negative voltage allows the 723 to go down to 0V.Also this circuit maintains 0V minimum at the output at all times, without any adjustment needed, save for maybe 2-3mV of offset voltage.Internal 723 reference is not used, as the -2.5V 431 works as the reference.If anything, I would maybe move the extra current limit transistor to the BD139 emitter or add another extra one there.However the current circuit also works fine, as I use something very similar in my current bench PSU.And switchable transformer taps are a good idea at high power levels, you don't want those 2n3055 to cook do you ?

author: xavier60
author_rank: Super Contributor
timestamp: December 06, 2020, 04:42:04 am
content: AC feedback is mainly via C6 and R7, so the setting of P1 doesn't affect CV loop gain by much. I still don't like seeing Pots used as VRs and it can not be made fail safe. If P1 goes open, output goes high voltage. But it's difficult to make most designs totally fail safe anyway.