Bizarre LM317 issue -  no set point adjustment possible - Page 1
Forum section: electronics / projects

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 17, 2025, 08:55:44 pm
content: I'm completely baffled.Built an LM317 regulator on a breadboard, designed for 1.4V output with 6V DC in. R1 and R2 220R and 27R, or thereabouts, 200mA load. Small cap across output, and one across R2.Fed by regulated PSU at 6V.All I get is 1.2V, and changing the resistors makes absolutely no difference to the output! Remove caps, same.I tried two different regulated PSUs - one linear, one SMPS. No change. OCP set at 1A, which it never gets near.Tried 5 different LM317T (although probably same batch). Same.So I bought a bag of 5 LM317T PCB built regulator boards - nothing to do with my breadboard now. They have a 10-turn pot to vary voltage. All I get is around 1.2V again. Varying input voltage up to ca. 20V, there's no variation and it's around 1.2V. Pot does absolutely nothing.I take the load off of the PCB one and output simply tracks input less about 1V. Adjuster pot does nothing. Add 120R load (as I know LM317 does not like zero load) and again Vout tracks Vin less a volt or so.Tried another PCB - same result.I've never seen anything like it - completely confounded. Used different DMMs to check.I must be doing something very stupid.Plug a 7805 in to the breadboard (and make the obvious adjustments to circuit as pinout is different, and don't need R1/R2) and I get 4.99V reliably, and all tracks and regulates perfectly.Any thoughts?

author: Terry Bites
author_rank: Super Contributor
timestamp: February 18, 2025, 08:29:59 am
content: I think you have miscalulated, misread the datasheet or used some crappy online calculator.An LM317 needs to have an input voltage at least 2-3V above the set output voltage.So 5+3=8V is needed at the input. See "dropout voltage" spec. in the data sheet.I cant read your mind so I dont know which resistor is R1 and which is R2. What have you got against pictures?So the upper resistor needs to be 220R and the lower one 680R. Not quite 5V.A better choice is R1=120 and R2=360. That meets the minimum load requirement of 10mA.To get 5V from a 6V supply you'd want an LDO, eg TPS7A7001

author: magic
author_rank: Super Contributor
timestamp: February 18, 2025, 08:37:39 am
content: Quote from: 6SN7WGTB on February 17, 2025, 08:55:44 pmSo am I.Are you sure you are measuring between OUT and ground and not between OUT and ADJ?

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2025, 08:57:08 am
content: Sounds like somehow the adjust pin is connected directly to GND, 1.2V is the difference between Adjust and Output.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 18, 2025, 12:58:27 pm
content: For the avoidance of doubt I’ve tried them with plenty of drop in to out as I understand the minimum delta requirements.I need to post a picture of this!

author: Fryguy
author_rank: Regular Contributor
timestamp: February 18, 2025, 01:22:15 pm
content: He needs 1,4V output - not 5V !The LM317 should be able to do it - it is above the internal reference of 1,25V and the 6V input is high enough . . .What are we missing here ?

author: bigendian
author_rank: Regular Contributor
timestamp: February 18, 2025, 01:30:38 pm
content: Can you change the source input current/voltage, giving unlimited current ? Maybe issue is there.Maybe resistor tolerance is important here ?

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 18, 2025, 05:44:28 pm
content: Here's an example set up.It doesn't matter what PSU or DMM I use, or load resistor. Just did this to get it all in the picture.This is the bought-in PSU board. The pot does virtually nothing, a few tens of mV.What am I doing wrong???Yes, it's mysterious that I'm getting the internal reference voltage.

author: floobydust
author_rank: Super Contributor
timestamp: February 18, 2025, 06:08:12 pm
content: Uh these cheap chinese boards can be junk - bad silicon, wrong parts, put in backwards etc. Think of all the money saved buying a 10 pack lol. Even the trimpot can be labelled wrong and the value way off.I would measure resistance from the ADJ terminal to GND, in case there is a short-circuit. The OUT-ADJ resistor could be open or high value. edit: in pictures, R1 is typically 200 or 220 ohms for these.For sanity I would confirm the multimeter is reading OK as well.

author: bigendian
author_rank: Regular Contributor
timestamp: February 18, 2025, 06:11:00 pm
content: you can use this calculator, with R1 to 240. Mount it yourself and add a good 220uF capacitor a in the output.https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator

author: Ian.M
author_rank: Super Contributor
timestamp: February 18, 2025, 06:16:35 pm
content: 240 ohms for R1 is out of spec, unless you can guarantee the external load is greater than 5mA!Seehttps://www.eevblog.com/forum/beginners/lm317-divider-values-and-minimum-load-current/

author: bigendian
author_rank: Regular Contributor
timestamp: February 18, 2025, 06:32:09 pm
content: Ok sorry, just read was a suggested starting value.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 18, 2025, 08:06:17 pm
content: As I've said I breadboarded it with completely separate chips which I've had for ages.Hence why I bought in completely unrelated ready-made PCBs.And I said I've tried two flavours of PSU, and three dMMs.Added all manner of caps, diodes (per IC data sheet).Anyway, again as I said, with a 7805 all this stuff (breadboard, PSU, DMM....) worked fine!How can they all be going wrong?!

author: bigendian
author_rank: Regular Contributor
timestamp: February 18, 2025, 08:42:18 pm
content: Leave 7805 in peace that is not of any use now.10mA is the minimum for a regulated output. Try to apply a load to draw a current > 10mA and see if something change.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 18, 2025, 08:46:21 pm
content: Simple check to do (on an unpowered circuit): ohm out from the ADJ pin of the LM317 to ground. You should have resistance there when the trimpot somewhere in the middle. Then ohm out from OUT to ADJ. Make sure things aren't shorted on the boards.

author: bigendian
author_rank: Regular Contributor
timestamp: February 18, 2025, 08:55:21 pm
content: This thread is very interesting.Also, what producer is your LM317 ? Looks like TI and ST are both producers, and would check exact datasheet.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 18, 2025, 09:19:21 pm
content: Unf******believable.I dig out an old LM317 and hey presto, it works.So, unbelievably I have got a bag of 5 useless LM317T, and FIVE useless PCBs, utterly unrelated aside all being chinese shyte.They're eBay and Amazon purchases versus the decent one I found which will be ex-Mouser, RS or Farnell.They can't even ****ing copy a 20-year old chip.Sorry for a complete wild goose chase, and total waste of everybody's time.I know not to use this cheap shyte for other than prototyping but have been well and truly caught out by TWO different sources here a year apart.

author: bigendian
author_rank: Regular Contributor
timestamp: February 18, 2025, 09:22:45 pm
content: ohoo , good to know !

author: magic
author_rank: Super Contributor
timestamp: February 18, 2025, 09:29:29 pm
content: Eh, no way. There is no IC which outputs constant 1.2V regardless of input voltage and regardless of "third pin" voltage, whatever the "third pin" might be.You screwed something up in your tests. Most likely measuring between ADJ and OUT instead of GND and OUT or shorted ADJ to GND.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 19, 2025, 07:46:12 am
content: Quote from: magic on February 18, 2025, 09:29:29 pmYou screwed up reading and/or understanding.If you'd read what I said - I bought ready-made PCBs. There is no opportunity to 'measure' wrongly as they have screw terminal VIN, GND, VOUT and GND.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 19, 2025, 07:57:44 am
content: Running some tests on the PCB ready-made ones (which BTW have a heat sink):Using my electronic load (so I'm not accused of 'screwing up') - very interesting:They 'work' with load up to around 100mA, then lose ability to regulate, with output falling to low volt level. Two so far have a sharp cut off (assume 'protection') at around 180mA.Wind the load back down and the output comes back up as foldback, er, unfolds.So my initial testing, which was at my desired current of around 200mA (with a so-called 1.5A LM317T using a fixed resistor load), is on the boundary of them cutting out. When I halved my load I unfortunately was on the boundary of regulation so, again, was getting failure.Set to a 50mA load, they run broadly to specification regarding regulation, drop out, and so on.These are 100mA chips in 1.5A bodies. FFS.Discuss...

author: ftg
author_rank: Frequent Contributor
timestamp: February 19, 2025, 08:23:22 am
content: Quote from: 6SN7WGTB on February 19, 2025, 07:57:44 amThis has been encountered by others as well with ebay/aliexpress LM317's.Fine until one goes above around 100 to 200mA and then they fail.Luckily yours seem to fail to a low voltage and not to an input to output short.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 19, 2025, 09:17:55 am
content: Counterfeit from 100mA to 150mA, and real one (IMG2894) at 200mA (apols- order of pics may be wrong but it's clear the outcome)

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2025, 09:28:44 am
content: Sadly I can't say I'm surprised. It could be the LM317L or similar which is only rated to 100mA and is much cheaper than the LM317.https://www.ti.com/lit/ds/symlink/lm317l.pdf

author: bigendian
author_rank: Regular Contributor
timestamp: February 19, 2025, 10:36:05 am
content: Likely, the L is different case, not to220 that was under discussion.Btw, very good to know. This thread can avoid unsuseful time spend on those crappy counterfeit stuff.Thanks @6SN7WGTB for posting results.

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2025, 11:27:43 am
content: Quote from: bigendian on February 19, 2025, 10:36:05 amThat doesn't mean anything. You do know they repackage parts. It can easily be a lower power regulator in a TO-220 package.

author: bigendian
author_rank: Regular Contributor
timestamp: February 19, 2025, 11:49:27 am
content: ah, ok

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 19, 2025, 03:14:20 pm
content: Quote from: 6SN7WGTB on February 17, 2025, 08:55:44 pmAre you sure your voltmeter/DMM is correct?You want 1.4 VDC but get 1.2 VDC. That's pretty close, so a 0.2 VDC measuring error can totally confuse.Or there is a lack of capacitors at the input or output of LM317.Another reason for the wrong voltage is some PCB pinout mistake (user error).

author: Fryguy
author_rank: Regular Contributor
timestamp: February 19, 2025, 03:48:38 pm
content: What i don't understand is , why people buy obey/ali & co cheap fakes in the first place - hell , the original LM317 is a cheap part .Well - at least for me it is . And i fail to see , why i should invite headaches for a couple of cents .But based on 6SN7WGBTs experience i'll check all my LM317s with a simple bread board setup @500mA !I will be honestly surprised , if i find a single bad one . Maybe in the "desoldered" box . . .

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: February 19, 2025, 04:12:08 pm
content: Bought simply to do some prototyping from eBay as didn't want to pay a large postage cost for a couple of bucks of parts.However, yes, you're right, they were predictably shit.I never use other than proper distributors for build parts, but the nature the fault caught me out!

author: Fryguy
author_rank: Regular Contributor
timestamp: February 19, 2025, 04:43:26 pm
content: Quote from: 6SN7WGTB on February 19, 2025, 04:12:08 pmThat's exactly , why i wait with orders for low value prototyping parts until i need something more expensive or a lot of them .On the other hand - LM317 is a very useful and often needed part - i always keep between 20 and 50 pieces in stock !

author: bigendian
author_rank: Regular Contributor
timestamp: February 19, 2025, 05:02:23 pm
content: Are TI and ST the only high level producer ?

author: moffy
author_rank: Super Contributor
timestamp: February 19, 2025, 08:49:28 pm
content: Quote from: bigendian on February 19, 2025, 05:02:23 pmOnsemi as well.

author: floobydust
author_rank: Super Contributor
timestamp: February 19, 2025, 09:23:35 pm
content: Beware the "single gauge" thin tab TO-220 package. You want the "double gauge" lol.TI: KCS vs. KCT suffix.OnSemi: KA317TU vs. LM317T and I saw a note 6: "SINGLE GAUGE DESIGN WILL BE SHIPPED AFTER FPCN EXPIRATION IN OCTOBER 2011."ST: DG vs. T suffix.Fairchild: (all are single gauge)Another thing to consider is if the flea fart tiny heatsink is going to work for you. It's a trap, you won't get 2W out of it as a wild guess.