TL431 Reverse Formula - Page 1
Forum section: electronics / projects

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: January 20, 2024, 10:03:16 pm
content: After my near death experience in 2022, it is a year later and I still have issues with what be very easy to most of you.I'm trying to create a TL431 Reverse Formula formula in a spreadsheet and I went in desperation to ChatGPT V3 and asked it to reverse the formula Vout = (1+(R1/R2)) *2.5the standard formula.My goal is to achieve solve for R2 or R3 if either is missing.The Chat GPT answers are too ridiculous to list here.TI's spreadsheet all locked down frequently throws errors and refuses to calculatehttps://www.ti.com/tool/TL431CALC#downloadsI deem that it is unreliable and furthermore you must enable macros!I've simulated it in TINA and the voltage divider current is 700+ µA and the sense current comes out to about 2-3 µA.The total IC should only pass between 1-100 mA.I usually keep it at 20mA.I've lost a few brain cells I'm sure from that NDE (3+ months in hospital and a long recovery).Can someone help do the math to solve for either R2 or R3 if one or the other is missing?Thanks.Edited for the proper resistor numbers, grammar.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 20, 2024, 10:14:57 pm
content: R1/R2 = -1 + Vout/2.5Update: The above was the answer to the question originally posed. The OP subsequently edited the first post.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: January 20, 2024, 10:21:35 pm
content: I'm trying to solve for either R2 or R3 when only 1 of them is known.The equation you provided does not compute.At least to me...

author: wasedadoc
author_rank: Super Contributor
timestamp: January 20, 2024, 10:22:44 pm
content: Quote from: Quarlo Klobrigney on January 20, 2024, 10:21:35 pmYou should be ashamed of yourself.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 20, 2024, 10:26:28 pm
content: Quote from: Quarlo Klobrigney on January 20, 2024, 10:03:16 pmDo you really mean R1 or R2 ?R2/R3 has a voltage divider solution, but R1 is merely a source resistor, so there is no divider solution for that.You do not show any load current, something must be using this 3.3V ?R1 needs to be small enough to deliver a max load current plus the rated min knee current of your chosen 431 plus some chosen margin.It needs to not deliver so much current you exceed the 431 rating.You can make R2 easier by being lazy, eg pick R3 so 498uA flows, giving 500uA (4k7+330R is 0.19%) needed thru R2, which sits on 2.5V.taking 3.3V R3 = (3.3-2.5)/500u = 1600, or 1500+100Rresistor pairs calculator is herehttps://www.qsl.net/in3otd/parallr.html#

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: January 20, 2024, 10:31:28 pm
content: Things like that happen all the time still.I know how to calculate the max through current.I meant R2 or R3 per that photo.Thanks for the correction.I was working on a PCB and I had R1 stuck in my head

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: January 20, 2024, 10:57:07 pm
content: 1609.6000 Ω to be exact Captain.I will try to make further sense of your example.I do however have a stuck bit where one would have to guess calculate the lower bit.I still need to solve either resistor at a glance via the spreadsheet.If you could elaborate further it would be appreciated greatly.Edited for the fact that my fingers type p for b etc. Still re-training the brain

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 21, 2024, 05:09:45 am
content: Do you mean figuring out R2 or R3 when the other resistor is known, but also the output voltage?Code:Vo = (1 + (R2/R3)) * 2.5R2 / R3 = (Vo / 2.5) - 1R2 = R3 * ((Vo / 2.5) - 1)R3 = R2 / ((Vo / 2.5) - 1)

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: January 21, 2024, 06:45:23 am
content: Thanks very much for your help.Here is what you see in the attached photo converted to the xls coordinates:=(1+D184/E184)*2.5=(B184/2.5)-1=D184/((3.3/2.5)-1)=((B184/2.5)-1)*E184The yellow areas in the right fields are your formulas provided.The pink areas (do not touch) in the right fields are the computed results from the above formulas.The left green areas are inputs for R1, R2, R3, Vin, Vo.Yellow = Field description.Green = Input your data.Pink = xls formula and Result. Do not input data.Again I couldn't have realized this in my current state.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 21, 2024, 08:20:34 am
content: No problem! You're likely suffering some kind of PTSD?I don't want to be that person saying "Are you down? Just be OK!" but, just relax and don't get very anxious at recovering, it's a slow process!Try to find happiness insmall things, go away, meet nice people... this is the first step for your mental health.Before you even realize, the signal in your brain will make tik-tok again.Forcing yourself probably will only make it even worse!I also had some unfateful encounter in 2019, 2 months in the hospital, tons of antibiotics, 3 surgeries, 15KG weight loss with already very low fat, so all muscle flew away and I came out extremely thin, 69Kg 1.86m, it took 2 months to be able to walk again without fainting.Then 1.5 years in bed, wakign up every 30 minutes due pain/discomfort, waiting forever for another surgery (Because "Ahh covid everywhere we can't fix you up") to restore my life back.So things piled up and my muscles and mental health went crap.Wouldn't have been so bad if lots of people I called friends started moking at me after 3 years on sick leave, slowly recovering "You're faking it up", "Snowflake", "You're letting us down"...However I eventually rose out of the hole by myself, so I can say, it's slow but it's possible!And yeah, I couldn't never think the same way again, priorities changed, not wasting my energy so easily with ungrateful people, nor wanting to satisfy everyone anymore.Same at work. David could you....? No. You're being paid 50% more while talking and taking coffe all day.Do your own homework. I no longer "eat it" trying to make everyone happy.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: January 24, 2024, 01:42:19 pm
content: I am sorry for you and all others who have to endure all of the medical nonsense of today.Some places can get away with killing you and you can't file a lawsuit.Well no PTSD, just ANGER!I walked In to do an outpatient procedure that would be about 2 hours at most and then walk out.I would have been back to my physical and perfectly normal life, but never made it out. Instead due to utter incompetence I ended staying 3 months.I was also abused by a certain small percentage of unqualified staff who were hired only because they took the "shot" where others did not and were fired because of it.Over a year later I'm almost back but who will compensate me for the year of hell?You do in fact find who your real friends are in such a trial.I no longer suffer fools.Thanks again for your response and help.