I killed my LM317 :( - Page 1
Forum section: electronics / beginners

author: augustol
author_rank: Contributor
timestamp: July 03, 2021, 02:33:18 am
content: So, I'm testing and old flash from a friend to learn a little about the electonics involved in making a flash and to see if I could revive it. The old batteries are completelly dead, so I got an LM317 from a local store to try to power it up. Not sure how much current it draws, but I figured 1.5A would be enough. Maybe it wasn't.So, here's what happened: I assembled the circuit in a breadboard and tested if I was getting the right voltages. I supplied 12v on the input of the LM317 from an old PSU from a computer. Hooked up the voltage divider circuit just like in the datasheet with a 200R resistor and a 5K trimpot. On the output I hooked up an Led so that I could see if everything was ok. Also, 100uF electrolitic cap on the output to ground. I measured all voltages, 12V on the input and adjusted the pot until I got 5.85V on the output. I left it powered up for a couple of minutes with my multimeter hooked up on the output so that could see if anything changed. everything seemed ok.So I decided to hook up the contacts of the flash so that I could power it up. And so I did. Nothing exploded, but the led got dimmer the moment I turned on the flash circuit. Measuring voltages again, my output immediatelly went down to 3.2-ish volts when I powered the other circuit on. I thought: hm, it looks like the LM is limiting the current because it might be drawing too much current. I let the LM cool down a bit and powered it again, now measuring the current: 400mA.So, I measured everything again and couldn't find anything to give me a clue of what was happening. One thing a did notice was the voltage on the adjust pin wich was 4.3V to ground. The datasheet says max 1.3V between pins 1(adjust) and 3 (input). But only with the LED hooked up I was getting constant 5.85V with no problems. Eventually the LM317 got extremelly hot and stopped responding, as I moved the pot it only outputs 0.5V or 10.5V.Was it overcurrent from the flash circuit? Maybe I should have used a higher than 200R value on the resistor on the adjust pin? What do you guys think? I honestly don't know if it was my mistake or if I just didn't pick a regulator with the right output current capabilities (which, if so, is also a mistake of mine).

author: sleemanj
author_rank: Super Contributor
timestamp: July 03, 2021, 02:49:05 am
content: No significant current passes through the resistor unless you wired it wrong.The current rating of such a regulator is dependant on adequate cooling to dump the power away.If we take your 1.5A as a given and assume your 12v supply did not sag, then 12v - 5.85v = 6.15v * 1.5A = 9.2 Watts, you will have trouble getting an LM317 to handle that I expect, an unheatsinked LM317 or similar can do somwhere in the region of 1WFor such a large drop in voltage at such a large current you should use a buck converter (aka step-down converter).

author: george.b
author_rank: Frequent Contributor
timestamp: July 03, 2021, 04:51:52 am
content: Once upon a time, I was putting together a simple adjustable power supply with a brand new LM317 (supposedly) made by ST I had bought locally. It worked fine the first time. The second time it acted up and actually cooked the 10-turn pot I was using with it.I replaced the LM317 and the pot - same thing; two burnt potentiometers.I then replaced the LM317 with an older, known good one, (supposedly) same manufacturer, from my junk pile, as well as the adjustment pot for an identical one - problem solved.Most components on the Brazilian market are counterfeit. I'm pretty sure that's what happened then, and unless you wired it wrong, I suspect that's what happened to you now. It'snotunheardof.

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2021, 11:15:25 am
content: Quote from: augustol on July 03, 2021, 02:33:18 amThe LM317 is internally protected against over-temperature/current, so it should be fine, unless the input voltage was too high, or the wrong polarity.Oh and the voltage between the input and adjust pin, can be as high as 40V, without damaging it. I think you mean the regulator keeps the voltage between the output and adjust pin to 1.3V maximum. The typical value is 1.25V, 1.2V minimum.

author: StillTrying
author_rank: Super Contributor
timestamp: July 03, 2021, 12:24:40 pm
content: Flashes and digital cameras are difficult to supply because of very large start up currents.Is there any way you've been shorting the 12V input while the 100uF on the output is charged.Protection diode(s) on page11.https://www.ti.com/lit/ds/symlink/lm317.pdfDoes the old PSU from a computer have a good 5V DC output, that should be close enough to a 4 X 1.5V battery supply for a flash.

author: augustol
author_rank: Contributor
timestamp: July 03, 2021, 03:24:24 pm
content: Quote from: StillTrying on July 03, 2021, 12:24:40 pmI did expect the high startup current because of the big caps in there. 2x 500uF 330V. Trying to limit this startup current makes any sense? Even with the lower voltage, the caps did got charged a little bit. They got charged to 8V, actually, which is higher than the supply, wich makes me think the flash is working ok. But that's another topic haha.Well, I didn't spot any shorts, but it's a possibility. I got two more LM317 and I'll try to reassemble the circuit to see if it works ok. I'll add a protection diode just to be safe.About the input: yes, the PSU does have a 5V out, but the battery for the flash is an old lead-acid battery rated for 6VDC. I tried to recharge it, but both batteries are dead.Just out of curiosity, the flash I'm working with here is similar to this one:https://http2.mlstatic.com/flash-frata-f140-D_NQ_NP_406301-MLB20318488977_062015-F.jpgQuote from: sleemanj on July 03, 2021, 02:49:05 amI guess the dissipation is most likely to be the factor here. With the current at 400mA (at least it's what I measured) is still 2.46Watts and the IC did get really hot while I was taking measurements and trying to troubleshoot it.Quote from: george.b on July 03, 2021, 04:51:52 amNow, this worries me a bit. How the hell will I know if the IC is or isn't a counterfeit?Do you have any trusted suppliers to share? hahaQuote from: Zero999 on July 03, 2021, 11:15:25 amYeah, I guess I wasn't interpreting the datasheet information correctly when it comes to the adjust pin voltage. Your explanation actually makes a lot of sense. As far as I know there was no wrong polarities involved. I actually checked it. But, at this point I'm considering all possibilities. I shouldn't have disassembled the circuit so that I could take a look at it afterwards with a clear mind. But, anyway...

author: george.b
author_rank: Frequent Contributor
timestamp: July 03, 2021, 04:29:45 pm
content: Quote from: augustol on July 03, 2021, 03:24:24 pmThat's the million dollar question, isn't it? Sometimes there's something odd with the markings. Sometimes, however, they look identical. One way to find out is like what the guy in the last video I linked above did: using a circuit to validate the parts - either whether they work or not under some condition, or to validate their actual specs. Another way involves decapping and a microscope. It's not necessarily trivial.QuoteAlas, I do not, unless you want to buy stuff from abroad (Arrow, Mouser, Digi-Key...). We used to have Farnell-Newark/element14, and it was my go-to store when I needed to be sure I was getting the real deal, but they ended their operations in Brazil in 2015. Now all we have is small stores that often get their stock from the cheapest sources, which aren't necessarily reputable. The situation here is not good.

author: StillTrying
author_rank: Super Contributor
timestamp: July 03, 2021, 04:35:13 pm
content: From the 1st post"Eventually the LM317 got extremely hot and stopped responding, as I moved the pot it only outputs 0.5V or 10.5V."That sounds just as likely the pot is broken as the LM317.Connect the LM317's ADJ pin directly to the 0V, if it's output is now fixed at 1.25V with just the 200R as a minimum load the LM317's probably OK.If the LM317 is still OK it's possible its going into current limiting mode in the seconds before the flash gets passed its power up surge.There are ways to increase the current of a simple regulator with a PNP power transistor on the input, or an NPN on the output as an emitter follower.https://electronics.stackexchange.com/questions/562785/the-transistor-for-increasing-current-output-of-linear-voltage-regulatorI think I'd put a 10A fuse on the 5V output and try that myself.

author: augustol
author_rank: Contributor
timestamp: July 03, 2021, 04:56:40 pm
content: Oh, I didn't notice there were three links. I got to the forum post and read through it and thought that was it. I'm gonna watch the videos.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: July 03, 2021, 05:01:40 pm
content: Quote from: StillTrying on July 03, 2021, 04:35:13 pmIt's also possible it was thermally throttling.I've had a few counterfeit regulators and a good way to tell the fakeness is they have horrible thermal performance, ie they throttle really easy. I've tested genuine ST regulators on a breadboard and can get about 1.7w heat dissipation before it throttles(consistent with what you'd expect from the datasheet). Another thing I noticed with the fake ones is that the markings don't survive the heat from soldering and/or they come off with IPA.

author: augustol
author_rank: Contributor
timestamp: July 03, 2021, 06:09:07 pm
content: Quote from: StillTrying on July 03, 2021, 04:35:13 pmThe pot is testing ok for resistances with my multimeter. And it didn't overheat or anything. I'll do the testing of the LM317 with 200R resistor with no pot. It's actually good that I measure if it will work ok even after overheating.And I'll check the link, sounds like a possible solution.

author: augustol
author_rank: Contributor
timestamp: July 03, 2021, 07:48:51 pm
content: So, updates on the LM317 case:The IC did not die. I got it back working again. I made a few changes, actually. The suggestion of wiring the adj pin to ground sort of worked. Instead of 1.25V, I got 2.5V.First I made a circuit with a 555 to drive a mosfet and step down the voltage a bit to 8-ish volts and filtered it to get stable voltage, not the square wave shape. Now 8V in / 6V out. Still a lot to dissipate, I know, but better than before. And I got myself a heatsink for this IC.Everything was working, so I decided to hookup the flash to the circuit: same result. But I noticed something new. The moment I turn the flash on, the voltage drops to 3.2-ish volts like before, but it was slowly getting back to 6V. I didn't let it get back to 6V because I was afraid to overheat the regulator again, so I stopped. The heatsink did get a tiny bit warm, though.I haven't tried the emitter follower circuit yet. I want to do a little bit more research on that first to get it right.

author: george.b
author_rank: Frequent Contributor
timestamp: July 03, 2021, 10:00:25 pm
content: Quote from: augustol on July 03, 2021, 07:48:51 pmWell, totally dead or not, that's not the expected behavior: it should put out 1.25V. I wouldn't trust that part anymore and would consider it as good as dead. Have you got any LM317s left to test?QuoteAnd you're sure this step down regulator of yours is working properly under load? Seems to me adding more variables might lead you further astray from a solution. I think you should get the basic LM317 circuit working first, test it with a light load, then try to power something else.Also, the minimum voltage headroom for proper regulation, according to the datasheet, is 3V. Assuming a functional chip, the 2V headroom you gave it could cause it to lose regulation.QuoteIf anything, it was under more stress when the output voltage dropped out (assuming it wasn't your step down converter sagging), since the power dissipated on the LM317 is (Vin-Vout)*current. If it were a functional LM317, I'd suspect this was due to current limiting kicking in, but again, that would assume the LM317 to be functional. Lots of assumptions here. The chip has already proved itself faulty by not putting out the expected 1.25V.Alternatively, here's a thought: you can get ~5.7V from a 7805 if you connect a diode between its GND pin and ground. ~6.4 if you connect two in series. Either voltage should be close enough. Heck, your camera flash might just work straight from your computer PSU's 5V line.

author: augustol
author_rank: Contributor
timestamp: July 04, 2021, 01:13:23 am
content: Quote from: george.b on July 03, 2021, 10:00:25 pmI get your point. The more I add to the circuit, the more problems can be created. I do have two more regulators around to work with. I was actually curious if it were really dead or if it was just some overheating at the moment. I didn't disassemble the circuit this time so that I could take a look at it afterwards and discover if it has any problem.--So, you are reading one line after the other, but I stopped and decided to swap the IC for a new one. Supplying it directly from 12V. Same behaviour: when I turn on the flash, it drops to ~3.2V. I waited a few seconds for the output to get back up, but when the voltage was around 4.5V (few seconds, actually) the heatsink was already getting warm. I turned it off. Also tried the adjust-pin-to-ground thing with the new one and it measure 1.3V. Close enough, right?I suspect by now that the current limit is kicking in. I'll take a look at the emitter-follower configuration and see if it can help me. i don't have any power transistor here, but i'll consider it.QuoteNow that you say that, it seem pretty obvious. But I never though of this possibility haha And the PSU is rated for 20A at 5V, so... If this isn't enough, nothing will be.I can't damage it with undervoltage, right?I might not get it fully working, at most.

author: amyk
author_rank: Super Contributor
timestamp: July 04, 2021, 01:23:25 am
content: It may take longer to charge up at 5V compared to 6V, but shouldn't cause any damage... but if the original battery was 6V lead-acid, which actually has a nominal voltage of 6.3V, then it will take significantly longer. Lead-acid batteries have a very low internal resistance so voltage drop under load is low.

author: rsjsouza
author_rank: Super Contributor
timestamp: July 04, 2021, 02:16:13 am
content: Quote from: augustol on July 03, 2021, 03:24:24 pmSome technicians and developers likeProesi. I have never purchased from them, but I purchased equipment (not components) from bothParesteckandBaú da Eletrônica- both provided good service.I have no affiliation with anyone of these guys.

author: MarkF
author_rank: Super Contributor
timestamp: July 04, 2021, 03:49:15 am
content: Use a large heat sink or piece of metal to dissipate the heat.I use these heatsinks:Then, I would test them with small power resistors.Start with 50 Ω and adjust the output voltage to 6V.That should be 120mA. With 12V input, the LM317 will dissipate 3/4 watt.Do they get hot?Then, reduce the load resistor to 20 Ω. Same output voltage.That would be 300mA and the LM317 will dissipate 1.8 watts.Do they get hot?Keep reducing the load resistor watching how hot the LM317 gets.I would NOT go below a 5 Ω resistor (1.2A load).IT WILL GET F___ING HOT! 7.2 watts.

author: augustol
author_rank: Contributor
timestamp: July 04, 2021, 03:11:56 pm
content: Quote from: rsjsouza on July 04, 2021, 02:16:13 amI have bought some components from Baú da Eletrônica and all of them seem to work perfectly well. 555's, logic gates, low power FETs and transistors and a bunch of passives. Actually, I made my starter kit with their components, so most of my components come from them. I'll definetly check the other two suppliers you mentioned. Thanks!Quote from: MarkF on July 04, 2021, 03:49:15 amMy heatsink is not that big and I don't have any piece of metal laying around. But I'll definetly do this test of current/heat you mentioned.With an LED and 330R resistor it does not get hot, so, that's a starting point. I'll start from there.

author: george.b
author_rank: Frequent Contributor
timestamp: July 04, 2021, 06:00:36 pm
content: Quote from: augustol on July 04, 2021, 01:13:23 amYup, that does sound like current limiting kicking in, or even thermal protection (Tcase is not the same as Tjunction anyways).QuoteRight.QuoteDo keep in mind that an emitter follower isn't going to make any miracles: the power it will dissipate will still be (Vin-Vout)*current. You just won't be limited to the 1.5A max from the LM317. Your power dissipation requirements will still be the same (or greater, if you're pulling more current). If the LM317 is heating up now, expect the transistor to heat up at least as much.QuoteI think a 1-volt difference shouldn't be much of a problem, no. I'd try it.It could be interesting if you cracked the packaging of the dead LM317 open to check the die size. Should be doable with pliers and some brute force.

author: MarkF
author_rank: Super Contributor
timestamp: July 05, 2021, 04:23:38 am
content: Quote from: augustol on July 04, 2021, 03:11:56 pmYou'll want to trying attaching something bigger and/or use a fan to get rid of the heat if you want to try loading the LM317. Even with a large heatsink and a fan, I doubt you can load it down to 10 Ω. Any old piece of metal will do in a pinch until you can get a proper heatsink. Just make sure it doesn't touch anything!If you can't hold you finger on the LM317, I would stop loading it further.With the 330 Ω resistor, that's only 18mA (assuming 6V applied to it). A trivial load.Maybe an electronic lesson would be helpful here (Sorry, I don't know your background):

author: E Kafeman
author_rank: Regular Contributor
timestamp: July 05, 2021, 07:24:07 am
content: LM317 is very bulletproof and selfprotecting from thermal view.However, placing a big capacitor as output load may destroy LM317 as it not can handle reverse voltage.Check datasheet from Texas where they specify a reverse diode to avoid killing LM317.Same goes for similar analog regulators such as LM7805.There exist a version of 7805 with improved handling of reverse current but it have not been produced for 30 years now.Always check datasheet before designing anything.