Venerable LM723 regullator - Page 1
Forum section: electronics / projects

author: IconicPCB
author_rank: Super Contributor
timestamp: January 02, 2025, 12:53:58 am
content: Has anyone tried to press LM723 into regulating two outputs a +5V and a +11V rail ?If so how were the results?Any gotchas?

author: Konkedout
author_rank: Frequent Contributor
timestamp: January 02, 2025, 03:28:59 am
content: What is your idea on how to use one LM723 to regulate two outputs....unless maybe you will use it to control a switcher? I am not sure that two linear regulated outputs with one uA723 is do-able in a practical way. I say it that way only because I hesitate to say that anything is impossible.The uA723 is cheap enough so that it makes more sense to use another one rather than add too many parts in an effort to get one to do double duty.But like the VW Beetle, it was great in its time however I don't think I would start a design today with one, unless I had a bunch of them. I did have one cheap commercial linear adjustable bench power supply which used a 723. It had some issues (maybe overvoltage on startup? Or maybe oscillation...) Even though I am a competent circuit designer I found it difficult to modify this thing to fix the problems. I ended up giving it away to a buddy.The old linear regulator that I really like is the LM317. I have done some good stuff with those. But the drawback to the LM317 is that Vin-Vout must be at least 2V or 2.5V.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 02, 2025, 03:48:20 am
content: Quote from: IconicPCB on January 02, 2025, 12:53:58 amThe 723 has a single error amplifier so it can regulate just one output.You might use the output stage zener to generate a second ‘somewhat’ regulated output 6.2v lower.Ie 5V may be primary and 11.2v the ancillary, but you waste all 5V current thru that 6v2Not sure what you imagine you are saving here ?

author: Konkedout
author_rank: Frequent Contributor
timestamp: January 02, 2025, 05:33:38 am
content: Not sure what you imagine you are saving here ?GOOD POINT!!

author: DimitriP
author_rank: Super Contributor
timestamp: January 02, 2025, 05:35:53 am
content: Compared to 723 the LM317 is a rock concertLT3042, ADM7154, ADM7154 are even quieter in applications that this matters and for those that care about those sort of things

author: IconicPCB
author_rank: Super Contributor
timestamp: January 02, 2025, 12:22:30 pm
content: Board space is a bit of a concern.Rail noise is definitely an issue. This, noise is primary concern hence the possible choice of 723.LM723 comes in metal can, DIP14 and finaly SOIC14 SMD package.Even though it comes in an SMD package it may be to costly in terms of PCB space.Hence the question one LM723 two rails. Yes there are many fine AD ..sorry LT regulators but the cost may be prohibitive.I guess if you do not ask...

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2025, 12:33:37 pm
content: How many amps?

author: HalFoster
author_rank: Frequent Contributor
timestamp: January 02, 2025, 01:39:15 pm
content: Quote from: IconicPCB on January 02, 2025, 12:22:30 pmOr read the datasheet

author: Kleinstein
author_rank: Super Contributor
timestamp: January 02, 2025, 01:53:57 pm
content: The specs on the zener at the output are quite loose. So it may work to use it to drop some voltage, e.g. as a virtual ground or so but it's likely not worth a try. One of the 2 voltage would be not well regulated.The xx723 are also not all the same: some are low noise and others not so much.If space is an issue, there are plenty of small 5 V regulators (e.g. SOT23) and a LM317 is often also smaller than a 723 plus power transistor.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 02, 2025, 05:41:21 pm
content: at start, it's an only one rail psu chipsearch something else for dual operation(s) even the 3 legs one can be subbed by small smps variants, even LDO's one will be good toohow many amps was asked ??

author: IconicPCB
author_rank: Super Contributor
timestamp: January 02, 2025, 06:29:54 pm
content: A burden of 120mA is to be sustainedYes not all 723s have been designed equal. Some use a band gap reference some use what is probably a buried zener.As for RTFM the opening question should give cause to suspect it had been explored.

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2025, 10:32:26 pm
content: The LM723 only has one error amplifier, as stated on the data sheet, so how can it possibly give two different voltage outputs?What's the input voltage? You might find that heat sinking will limit the minimum PCB footprint.

author: IconicPCB
author_rank: Super Contributor
timestamp: January 02, 2025, 10:46:24 pm
content: An external error amp and series pass transistor may be necessary to provide the higher voltage based on 5V supply.5volt supply ought to provide a stable low noise reference.I think this is the way to go.Thank You all for Your insight.

author: iMo
author_rank: Super Contributor
timestamp: July 19, 2025, 08:16:49 am
content: FYI - here is an older design which was mass produced (even exported - like Conrad's Voltcraft BK127C) by TESLA in ~70-90ties in several variants but always with the same schematics, basically.The 723 is used there as the voltage reference source and CV opamp and pass transistor driver.The 741 creates the CC part driving the 723's current limiter.I own one such a power source variant (30V/1.5A, analog meter), it has original 10t voltage setting potentiometer and it works.Interesting are the details around the 723 - the compensation, and how the 741 drives the 723's CC part.PS: An anecdotal note: "TESLA" was a large conglomerate of Czechoslovak companies (around 50, created also of some internationals nationalized after WWII and the regime change in 1948) producing literally everything related to electronics, microelectronics, and electrotechnics in former Czechoslovakia. It was named after Nicola Tesla in 1946, but in 50ties (because of a political dissonance with the former Yugoslavia's chief Josip Broz Tito - mind Nicola Tesla descended from his country) they started to interpret meaning of "TESLA" officially as "TEchnika SLAboprouda" which means something like "low power electronics"..