LM317. How hot is hot? - Page 1
Forum section: electronics / projects

author: pope
author_rank: Frequent Contributor
timestamp: April 18, 2025, 06:49:16 pm
content: So, one of the LM317 bodies measures around 73°C under maximum load, with a large heatsink and 253VAC mains (the upper limit for EU mains voltage). At 240VAC the temperature drops at around 66°C. Room temperature is 22°C.Is this 73ish°C safe, or is it destined to fail?By 'fail,' I don’t necessarily mean the regulator itself but even if it shuts down, it will potentially damage downstream components.Thank youW.

author: TimFox
author_rank: Super Contributor
timestamp: April 18, 2025, 06:57:27 pm
content: The TI data sheet specifies max “virtual junction temperature” (125oC) and thermal resistance (in K/W) from that point to several locations (see section 7.4) and different packages.To evaluate this, you need to start with the actual power dissipation in the device under your operating conditions to determine the temperature rise inside the package from a location where you can measure the actual temperature.

author: pope
author_rank: Frequent Contributor
timestamp: April 18, 2025, 07:08:50 pm
content: Quote from: TimFox on April 18, 2025, 06:57:27 pmThanks. I have seen this table but quite frankly I can't understand it. The max load is around 300mA and I measure the 317 body (to-220) temperature with a thermal camera.

author: TimFox
author_rank: Super Contributor
timestamp: April 18, 2025, 07:14:52 pm
content: The power dissipated in the LM317 is essentially the product of the voltage from input to output multiplied by the output current.Multiply that by the thermal resistance to one of the places on the body called out in the data sheet that you can see with your camera.The product of power x thermal resistance tells you how much hotter the internal “virtual junction” is, above your temperature measurement.Above 125oC at that point, you exceed TIs spec.

author: bdunham7
author_rank: Super Contributor
timestamp: April 18, 2025, 07:19:28 pm
content: Quote from: pope on April 18, 2025, 06:49:16 pmIt's very common for LM317s to be run at 70C or so case temp (and last for decades) when you have a small or nonexistent heatsink. A typical LM317 might have 5C/W between junction and case, so it will be within specs up to about 10W of power dissipation. I would probably want it to run a bit cooler for reliability purposes so if it is dissipating 5W at max power I think you're OK, 10W and you're pushing it.You would need to state the exact make, model and package and the calculated power dissipation for us to be of any further help. Note that even the TO-220 case styles have some variations, including the wretched "single gauge" thin-tabbed models. Also, what is the temperature of the heatsink itself under these conditions?Quote from: pope on April 18, 2025, 07:08:50 pmCan you also measure the voltage drop from input to output under the worst-case conditions?

author: pope
author_rank: Frequent Contributor
timestamp: April 18, 2025, 07:31:14 pm
content: Please bear with meVi-Vo=10.5V10.5V x 300mA = 3.15WI don't use a TI 317 but the data sheet I use states "Thermal Resistance, Junction−to−Case JC 5.0 °C/W"3.15W x 5W = 15.75If I measure 75°C on the body, does this mean that the junction temperature is 75+15.75 ≈ 91°C and therefore well within the 125 max temp? Would a 317 with max 150°C junction temp would be a better option?I use TO-220 and when the temp on the body is 73ish the heatsink measures around 60°CQuoteI hope this is the worst case condition as I run the power supply at 253VAC (please read the first post about the mains volatge)

author: TimFox
author_rank: Super Contributor
timestamp: April 18, 2025, 07:40:53 pm
content: I tend to be conservative in thermal matters.TheTjspecs for TI and ON are defined slightly differently.I believe TI specs 125ofor preferred operating conditions and 150ofor damage may occur above that.

author: bdunham7
author_rank: Super Contributor
timestamp: April 18, 2025, 07:43:17 pm
content: Quote from: pope on April 18, 2025, 07:31:14 pmYes, that looks right and I think everything is fine if you're at 91C worst case and 10C cooler normally.I wouldn't opt for a higher temp rated device, but just out of curiousity which specific device (by part number) are you using?

author: pope
author_rank: Frequent Contributor
timestamp: April 18, 2025, 07:50:46 pm
content: Sorry, I put the wrong data sheet before. I don't use ON 317 butLM317T-DGSTMicroelectronicsI'm definitely open to better recommendations though! I've been using this part on many projects for many years and I never had a problem. However, all are running on lower temperatures.

author: bdunham7
author_rank: Super Contributor
timestamp: April 18, 2025, 07:53:22 pm
content: Quote from: TimFox on April 18, 2025, 07:40:53 pmThe 150C in the beginning of the OnSemi datasheet only apply tothe 'NCV' automotive-qualified part numbers, the rest are 125C. But thermal shutdown is at 180C, so while I agree with you about being conservative, in practice where someone has been a bit more reckless they typically roast everything else, including resistors and the PCB, before the LM regulators themselves fail.

author: bdunham7
author_rank: Super Contributor
timestamp: April 18, 2025, 07:55:44 pm
content: Quote from: pope on April 18, 2025, 07:50:46 pmThat one is optimal, probably as good or better than anything else you're going to find.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 18, 2025, 08:51:41 pm
content: May be a small consideration, but did you apply emissivity compensation while doing the measurements?The plastic body will have a very different emissivity from the tab, and that from the heatsink.

author: pope
author_rank: Frequent Contributor
timestamp: April 18, 2025, 09:07:17 pm
content: Quote from: UnijunctionTransistor on April 18, 2025, 08:51:41 pmNo actually I haven't. All the measurements are done with 0.98 emissivity.How should I find out about the correct values for plastic, aluminum, etc?EDIT: foundthis table