Running LM317 at high temperatures - Long term issues? - Page 1
Forum section: electronics / projects

author: cobblers
author_rank: Newbie
timestamp: June 01, 2012, 04:23:05 pm
content: Hey guys.I bought some cheap and nasty 1W led downlights from eBay, to fit in my old VW van. The usual LED lights people use in camper vans are about £18 ea, and they're probably only about half a watt at best. I figured I could make something better myself.They came with the usual chinese 85-260v drivers:I was being a bit optimistic in hoping to be able to reverse engineer the circuit to work from 12v, but the markings on the IC don't bring up anything relevant on google, so I scrapped that idea.I've got a load of LM317s kicking around, so I whipped up a quick and very dirty constant current driver:It works fine. I used a 4R7 5w resistor (I had some from a previous project) and the current works out at 266mA which gives me just under 1w on the LEDs.As expected, the LM317s get really hot (I'm running two LEDs in series) because I'm dissipating about 1.5w of heat in the tiny enclosure. I've tested them and there's been no issues from 12.5v, however when I increase the voltage to 15v (to simulate really hard charging of the battery with the engine running)I get around 2.1W dissipation, and after a few minutes the LM317 starts to wind the power down to about 190mA.Practically this has only a barely noticeable effect on light output, and the thing seems happy to sit like that for hours on end.I've had two of them running on the bench for a couple of days now, but I wonder how long they are going to last when run stinking hot like that, and what is the usual failure mode of the LM317? do they fail open circuit or closed?In practice the lights aren't going to be often used when the engine is running for extended periods of time.

author: Neilm
author_rank: Super Contributor
timestamp: June 01, 2012, 04:40:59 pm
content: Anytime a piece of electronics runs hot it will be more likely to fail. I seem to recall a rule of thumb that says the the rate of failure of electronics doubles with every 10 degrees rise in temperature.How hot are they getting?Neil

author: cobblers
author_rank: Newbie
timestamp: June 01, 2012, 04:54:16 pm
content: Thanks for the reply Neil.I've no way of measuring the temperature accurately but I can say with confidence they are quite comfortably over 100C (a drip of water on a finger on the heatsink tab fizzles satisfyingly). My best efforts to measure the temp with what I have show around 120C, which is right up at the limit according to the datasheet.I'd heard that rule of thumb too, but it's all fairly relative hence why I'm considering just risking it in this case, if they do fail I can fairly easily replace them.These lights will only be used maybe for 10-15 hours a month, on average.

author: T4P
author_rank: Super Contributor
timestamp: June 01, 2012, 04:55:34 pm
content: You might not have the temperature data now, but please heatsink it,but when they say 1W they aren't really joking, i bought one of their 3W models and sent it to my friend who measured dead on 300lumensit's just that their currency exchange gets lost in translation, the local sellers ALWAYS overcharge as a rule of thumb, so 3$ is as expected for a 1W led thing from china of course

author: SeanB
author_rank: Super Contributor
timestamp: June 01, 2012, 05:54:22 pm
content: Please put a heatsink on the poor 317. Running it at the point where it is in thermal limiting is going to make it fail in short order, and the failure mode is always to go dead short. This will destroy the led's and the wiring, and can even burn down your van, probably with you inside. A smallish heatsink, basically big enough to turn into the back of the box you are using, along with a mounting kit and some thermal compound, will make it run much cooler, and will increase the life of both the LED's and the regulator.

author: cobblers
author_rank: Newbie
timestamp: June 01, 2012, 07:14:12 pm
content: I have some heatsinks on order, but was itching to get the lights installed this weekend.Finding out these things fail short circuit, I'll definitely hold off installing them until I've got the heatsinks, I'd rather not end up ruining the LEDs if they do fail and dump 14.xxV straight across the LEDs.

author: Zero999
author_rank: Super Contributor
timestamp: June 02, 2012, 09:06:35 am
content: The LM317 is a very inefficient solution. It's not hard to build a simple unregulated buck converter using a 555 timer, MOSFET, inductor and freewheeling diode.You can also lash up a simple, poorly regulated, buck converter wih a few transistors.http://www.romanblack.com/smps/a05.htm

author: amyk
author_rank: Super Contributor
timestamp: June 02, 2012, 11:58:08 am
content: What does the LED driver IC say? Have you tried running it from 12V?Even a 12V -> 120V boost converter in series with the original driver board would probably have higher efficiency than the LM317.

author: Jeff1946
author_rank: Regular Contributor
timestamp: June 02, 2012, 04:34:28 pm
content: 1. I assume this lights need about 3.2V and 300 ma. So if you can wire three in series you could just use a resistor to drop the voltage from 12.6 to 9.6. Since voltages can spike in automobiles should pick a resistor for lower current on leds, something like 200 ma to give plenty of margin.2. Use a buck converter. Buy a usb plug in. They will put out 5V at about 0.5 A. They are probably cheaper than buying the individual components. Then put a series resistor with the led to get the appropriate current. Most of these are based on an MC34063 chip. Look up the data sheet on these and you will see it is straightforward to change the output voltage by changing one resistor so you could save a little power and drop the voltage to 4V. Or raise the voltage to run two leds in series.

author: Zero999
author_rank: Super Contributor
timestamp: June 02, 2012, 10:52:46 pm
content: You can even use the 555 timer.L = 100uHC = 1nFR1 = 470RR2 = 13k