lm723 cc - Page 1
Forum section: electronics / projects

author: p.larner
author_rank: Frequent Contributor
timestamp: October 18, 2023, 02:28:04 am
content: has anyone any opamp addon schematics to give the lm723 better cc control?

author: Konkedout
author_rank: Frequent Contributor
timestamp: October 18, 2023, 03:55:37 am
content: I have been designing power for about 45 years. I had a cheap linear bench power supply which used an LM723. I think it was oscillating, and I had more difficulty than it was worth trying to fix it.Of course the LM723 was probably the very first linear regulator IC and was a wonderful thing in its day. Today....I think...not so much.If I wanted to do a linear constant current power supply I think I like the following ingredients (not all in the same design). I would not use an LM723Reference such as TL431 or LM4040LM317 is great within its power limitations.An op amp such as maybe LM324 (for good cheap.)For the pass element, I would use a "linear" type FET (from Infineon or there are a few others.) Or see if you can find rugged bipolars. I still use an e-load that I built 25 years ago which uses a "Fetlington" configuration of MTP3055E MOSFET + four bipolars which may be TIP41 with individual emitter resistors.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 18, 2023, 10:09:06 am
content: you have plenty of schematics on the web, based on standard op amps etc ... even some major brand leaked ones ...i have lm723 based psu's, even my own design, never had oscillations with them, your design is not ok if it oscillate

author: p.larner
author_rank: Frequent Contributor
timestamp: October 18, 2023, 10:13:45 am
content: i just find the current foldback poor,as in you cant set max current from say1ma at 3volts to say 20a at 30v,not without extra opamps,dont see any schematics to get that kind of range tho!.

author: Ian.M
author_rank: Super Contributor
timestamp: October 18, 2023, 10:29:25 am
content: Very few PSUs offer adjustable current limiting that's effective and accurate over four orders of magnitude. That's high-end SMU territory, with a four or five digit price tag expected!Most '723 circuits aren't any better than a Philips PE 1535-00 bench PSU, which has fairly crude variable limiting, (See:https://www.eevblog.com/forum/projects/how-do-make-a-current-limiting-knob-using-lm723-for-linear-reg-power-supplies/msg1212923/#msg1212923for a LTspice sim of its '723 based circuit),and even commercial PSUs are rarely an order of magnitude better than that.TLDR: if you need an accurate 1mA or even 10mA limit, a bench PSU is the wrong tool for the job!

author: coromonadalix
author_rank: Super Contributor
timestamp: October 18, 2023, 11:50:24 am
content: the @OP need to provide more information

author: Zero999
author_rank: Super Contributor
timestamp: October 20, 2023, 08:37:37 am
content: Perhaps the MP26085 would be more suitable?More information is required, as mentioned above.

author: Dr.Krieger
author_rank: Contributor
timestamp: October 20, 2023, 11:45:47 pm
content: Just use a classic Op Amp CC and pull down LM723 output or pin2.This was going to be a 20A variant of the Vellman K7200 0-30V/0-10A PSU (never finished).It uses the K7200's CV stage which has the output voltage "floating on Vrs" (so the ~0-500mV voltage drop on the shunt does not affect Vout regulation), but instead of a 2nd LM723 for the CC, it uses a LM358 Op Amp (cheap and does not require a negative supply) .In my case i was sending voltage to LM's pin2 to make its internal transistor turn on, but you can also pull-down its output (like T3 is doing in my circuit).The Vref can be made by a cheap TL431, or you canmaybeborrow the LM723's on-board ~7.15V Vref if the grounding of the two is the same.

author: p.larner
author_rank: Frequent Contributor
timestamp: October 21, 2023, 06:48:30 pm
content: what is the max v+ just the normal 37v? 60 would be better.

author: Dr.Krieger
author_rank: Contributor
timestamp: October 22, 2023, 01:31:29 am
content: Quote from: p.larner on October 21, 2023, 06:48:30 pmAre you talking about the Op Amp's max operating voltage?Why must it handle 60V? Does your circuit use high-side current sensing? Otherwise 5V Vcc should be more than enough for a 2.5Vref and a <1V shunt voltage drop.If you provide more information about what you want to do, you will get better help....

author: coromonadalix
author_rank: Super Contributor
timestamp: October 22, 2023, 01:40:01 am
content: and the schematic is using what we call float voltage for the 723 circuit, this provide more stabilityeven this circuit can be optimized to go down at zero volts, the 723 by default is not going down to zero voltson the xformer you have 2 secondaries, one for the power, one for the controli think you are asking us to build your psu ??the velleman k7200 psu is a good designhttps://cdn.velleman.eu/downloads/0/manual_k7200.pdfbut nothing is perfect .... if you need perfection, buy already made ones of good reputation(s)personally i hate any psu design who has the current sense in the negative / gound line ...the best one, is before the collector of the high power transistors, any voltage drop is not on the output side / emitter side if they are npn

author: Dr.Krieger
author_rank: Contributor
timestamp: October 22, 2023, 05:09:17 pm
content: Elektor's "Precision PSU" is also a good tried-and-true design that many people used for decades as their Lab PSU.Add to it Elektor's Dissipation Limiter (Nov 1991) since it doesn't switch trafo taps like the K7200, and you can go to 50V+ max while able to output its max current at 5-10V with very low dissipation!

author: iMo
author_rank: Super Contributor
timestamp: October 22, 2023, 05:25:21 pm
content: I am a big fan of the 723 for 4 decades. I like it and I've been collecting themBut, when targeting a precise CC, or the construction as the above, it has absolutely no sense to mess with the 723 anymore, imho.723 is hard to get part today and perhaps expensive. It was an allrounder 40-50y ago (I did as teen with it) when used as-is with its coarse CC.There is zillion CC/CV precision designs, even here on the eevblog people spent hundreds of hours elaborating various designs..Inserting the 723 into today's schematics as the Vref only is a nonsense..

author: p.larner
author_rank: Frequent Contributor
timestamp: October 22, 2023, 06:15:21 pm
content: shame the linked elector manual is in german,i cant understand a word!.

author: Dr.Krieger
author_rank: Contributor
timestamp: October 22, 2023, 08:22:52 pm
content: Quote from: iMo on October 22, 2023, 05:25:21 pmTrue, LM723 its not what it used to be in the 70s/80s, today something like a LT3086 or a good Op Amp design with good Vref chips will beat it in everything for sure...Quote from: p.larner on October 22, 2023, 06:15:21 pmIt starts in German but it also includes English, starting in page 23....If you search on the web you will find everything you need about this PSU. Don't expect others to do it for you...Here is another pdf for it: