how to calculate Potentiometer value? - Page 1
Forum section: electronics / projects

author: Gurumurthy
author_rank: Regular Contributor
timestamp: June 20, 2017, 07:37:56 am
content: hi to all,This schematics has been taken fromhttp://www.ti.com/lit/ds/symlink/lm723.pdf,figure28.My doubt is how to calculate P1?What we have to assume for R1 and R2?

author: BravoV
author_rank: Super Contributor
timestamp: June 20, 2017, 07:52:01 am
content: Why you have to create a new thread on every question on your quest to build the 723 based power supply ?

author: Bendba
author_rank: Regular Contributor
timestamp: June 20, 2017, 07:52:20 am
content: Your link was a bit faulty. Here it is, fixedhttp://www.ti.com/lit/ds/symlink/lm723.pdf,figure28

author: Nusa
author_rank: Super Contributor
timestamp: June 20, 2017, 08:17:41 am
content: Quote from: Gurumurthy on June 20, 2017, 07:37:56 amLook at Table 1 in the same datasheet.

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2017, 08:19:10 am
content: Quote from: BravoV on June 20, 2017, 07:52:01 amIndeed.There's no need for so many freaking threads. It's counter-productive. Please keep it to one thread per project!

author: Gurumurthy
author_rank: Regular Contributor
timestamp: June 20, 2017, 08:31:34 am
content: Dear all,Yes the table1 gives value for P1 but how to they calculate that value?no where in the data sheet they have said the technique or formula for calculating P1. Thank you.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 20, 2017, 10:42:35 am
content: Quote from: Gurumurthy on June 20, 2017, 08:31:34 amEither you aren't asking what we think you are asking, or else you are as dumb as a box of rocks, because Table 2 -which is right underneath Table 1- gives the formulas for calculating the divider resistor values.If you insert a potentiometer in between R1 and R2 then you can make the range adjustable from "some non-zero value" to "some higher non-zero value"; the width of the adjustment range being proportional to the ratio of the resistance of the pot vs. the total divider resistance (including the pot).But all of this is irrelevant because the 50* year old '723 is a poor choice to make a 0-30V, 0-5A CV/CC lab supply. It only has one error amplifier hard-wired to an internal pass transistor and a rather mediocre voltage reference. In another one of your many many threads on this same topic someone referenced (the former) National Semiconductor document LB-28 which shows the classic two op-amp circuit for making a variable CC/CV power supply. You would be well advised to study that document rather than waste more time on the '723.* - designed in 1967 by the infamous Bob Widlar.

author: Gurumurthy
author_rank: Regular Contributor
timestamp: June 20, 2017, 03:52:27 pm
content: Hi Sir,Sure Sir. I will try to analyse the application circuit of National semiconductor.

author: Vtile
author_rank: Super Contributor
timestamp: June 20, 2017, 04:00:37 pm
content: With voltage division and ohm's law.More precisely with addition, subtraction, multiplication and division.https://en.wikipedia.org/wiki/Voltage_divider

author: ivan747
author_rank: Super Contributor
timestamp: June 21, 2017, 06:01:16 pm
content: Quote from: BravoV on June 20, 2017, 07:52:01 amI want a link to that power supply designSent from my iPhone using Tapatalk

author: debininja
author_rank: Contributor
timestamp: June 21, 2017, 11:01:40 pm
content: Quote from: ivan747 on June 21, 2017, 06:01:16 pmHere you go:http://www.ti.com/lit/an/snoa857/snoa857.pdfInteresting project I'm working on right now is to replace many of those components with modern, cheap parts, as well as replacing the LM395s with discrete components (TIP120s + driving NPN). So far, this is what I have (as you can see, I replaced the reference voltage zeners with TL431s because they are so popular and cheap, as well as really accurate). Note, this is not complete, nor is it accurate!:Oh, and another thing I'd like to add. I also would not recommend making a LM723 based PSU. The current fluctuates all over the place. Here's what my LM723 based PCB + pass transistor looks like: