Need circuit for Current adjust circuit for LM723 IC - Page 1
Forum section: electronics / beginners

author: techguru
author_rank: Regular Contributor
timestamp: December 31, 2017, 04:46:21 pm
content: Dear all,I want to design Variable adjust voltage and Current adjust function using LM723 Ic for 0-30V@ 5Ampere Regulated power supply.The problem is that i want to have smaller pontentiometer resistor for current control usally given by formulae Rsc=0.7V/5A(0 to .14_Ohm). Having such a small potentiometer is impossible... Then what is the solution.....

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2017, 04:59:33 pm
content: A possible solution is having a separate difference amplifier and variable voltage difference for the current limit. The 723 internal current limit could still be there as an additional fast emergency limit.There are several circuit plans to do this. I remember some relatively good examples from old Phillips power supplies.

author: Ian.M
author_rank: Super Contributor
timestamp: December 31, 2017, 06:05:45 pm
content: You may want to take a look at a LM723 thread from earlier this year:https://www.eevblog.com/forum/projects/how-do-make-a-current-limiting-knob-using-lm723-for-linear-reg-power-supplies/. It includes schematics of a few Philips LM723 based PSUs and I contributed a LTspice simulation of the Philips PE 1535-00 0V - 40V, 0A - 0.5A. single output PSU in reply#29, which has adjustable current limiting, doesn't need any auxiliary supply rails and could be beefed up to 1A or even 2A out at the expense of some beefier transistors, a *big* heatsink and a few bucks on your air-conditioning bill. For a 5A PSU, you'd need to add a tap-changer or a switching preregulator as dumping over 200W into the heatsink with the output shorted in constant current mode is expensive and impractical.

author: David Hess
author_rank: Super Contributor
timestamp: January 01, 2018, 05:02:23 am
content: Check out the long discontinued Motorola MC1466L which essentially implements two 723s in parallel with the voltage and current error amplifier outputs combined. The MC1466L datasheet shows exactly how it works including application circuits.Two 723s could be combined at the frequency compensation, current limit, or collector output pins producing a pseudo MC1466L. Or the current error amplifier can be implemented externally with a differential pair or operational amplifier like Kleinstein suggests.

author: techguru
author_rank: Regular Contributor
timestamp: January 08, 2018, 08:29:48 am
content: Is there any other technique to control current in regulated power supply?

author: David Hess
author_rank: Super Contributor
timestamp: January 08, 2018, 09:09:24 pm
content: Quote from: techguru on January 08, 2018, 08:29:48 amDo you mean other than doubling up on error amplifiers and using them in parallel by combining their outputs? Sure, but the other methods are practically as complicated.A 5 amp variable current limit for a 30 volt power supply is not trivial because of power dissipation considerations so if the Motorola MC1466L example is too complicated, and it does not even cover the power dissipation issue, I suggest reconsidering exactly what you need.

author: hugo
author_rank: Regular Contributor
timestamp: January 11, 2018, 12:15:15 am
content: Quote from: techguru on December 31, 2017, 04:46:21 pmThis is the schematic of a very simple lab power supply 0-20V@1A using two LM723 ICs:http://www.ve2ums.ca/chasse/Serge/Atelier/Projets/Membres/VE2EMM/alimentation_ang.htm

author: techguru
author_rank: Regular Contributor
timestamp: January 11, 2018, 06:07:58 pm
content: greetings to all,What are the techniques to control current in regulated power supplies ?

author: lordvader88
author_rank: Frequent Contributor
timestamp: January 12, 2018, 01:46:08 am
content: Very soon I need to make a little split rail PSU for my hobby stuff. I was trying to find a good circuit with short circuit protection using lm723. I need to start researching for it, a bunch of caps are en-route.

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2018, 11:16:36 pm
content: Please check some of the replies to your other threads.https://www.eevblog.com/forum/beginners/need-guidance-in-current-limiting-variable-design-technique/msg1357870/#msg1357870https://www.eevblog.com/forum/beginners/regulated-power-supply-circuit-design/msg1337210/#msg1337210To avoid instability and surges, it's actually better to regulate the current foremost, then have another loop for the voltage control.

author: David Hess
author_rank: Super Contributor
timestamp: January 12, 2018, 11:59:02 pm
content: Quote from: lordvader88 on January 12, 2018, 01:46:08 amDo you mean with variable voltage tracking and current limiting?The LM723 would not be my first choice but it can certainly be used. Two would normally be needed for a tracking supply and power dissipation limits and higher currents will require external pass transistors.I would use TO-220 regulators for their built in thermal protection which the LM723 lacks or perhaps even drive TO-220 regulators with the LM723s.Quote from: Hero999 on January 12, 2018, 11:16:36 pmI disagree but when two control loops are used, this does require attention to design. The example below clamps the current error amplifier though D2 to the compensation pin of the LM301A. Unfortunately there are not many modern operational amplifiers which can be used this way.

author: lordvader88
author_rank: Frequent Contributor
timestamp: January 14, 2018, 08:10:00 am
content: ^^^Well I own some LM723 's so I'm going to make something now/tomorrow with a 36Vout 1.3A transformer I just scored today from a stereo being thrown out. Todays circuit will be just a variable output PSU with a TIP41C darlington for a pass regulatorNever used either before, so fun times ahead

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2018, 08:37:24 am
content: Quote from: lordvader88 on January 14, 2018, 08:10:00 amBeware that the maximum continious voltage rating for the LM723 is only 40V, yet a 36VAC transformer will have a peak voltage of 51V, which is what you'll measure on the smoothing capacitor.

author: Ian.M
author_rank: Super Contributor
timestamp: January 14, 2018, 09:13:17 am
content: If that's 36V RMS, it will be nearer 50V peak DC after rectification. 1.3A RMS gives you 0.8A continuous DC current, so a single TIP41C isn't going to cut it as its DC SOA line only gives you 0.7A at 50V Vce. If you derate to 0.5A max output it should be OK if its on a big enough heatsink. See datasheet:http://audiolabga.com/pdf/TIP41C.pdfYou'll also run into trouble with the LM723 max voltage rating. You'll need to limit its V+ and Vcc supplies to +40V max, which with a NPN Darlington pass transistor will limit the max output voltage to 37V. While there are ways round this limitation, they are all considerably more complex and are best avoided for a first LM723 experiment.If the 36V is nominal rather than measured RMS at no load, you are likely to get a extra 10%-15% when lightly loaded, and if the AC line is also 10% high, it could push the unregulated DC voltage up as high as 64V. That's even less suitable for a simple linear regulator as pass transistor SOA limitations will be even more of a P.I.T.A.If the 36V secondary has a center tap, it would be better to use two diodes to full wave rectify 18V, giving you 24V peak, and worst case (no load and high line) about 32V, with a 1.3A DC current rating, which would be good for a 0-15V 0-1A PSU.

author: BravoV
author_rank: Super Contributor
timestamp: January 14, 2018, 10:54:01 am
content: Quote from: Ian.M on December 31, 2017, 06:05:45 pmIts the same guy, the one that keep asking and creating new thread every time he saw the circuit that he doesn't like, and then made another request for help again and again .... and again.Also what annoying he loves to create a new thread for every single question he made.Looks like he was ignored at previous nick after for a while after his ignorance, and now again with another new nick.

author: BravoV
author_rank: Super Contributor
timestamp: January 14, 2018, 10:58:14 am
content: Quote from: Hero999 on January 12, 2018, 11:16:36 pmThe same guy here ->https://www.eevblog.com/forum/projects/how-to-calculate-potentiometer-value/msg1237728/#msg1237728