LM340 max heatsink temp - Page 1
Forum section: electronics / repair

author: cncjerry
author_rank: Unknown
timestamp: February 10, 2025, 10:25:54 pm
content: I have two "detanglers" that switch AESEBU digital signals between 8 devices. I bought one used and the power supply was shot. The soldering from the factory didn't take into account the heavier transformer wire so one was partially open and the solder melted. The seller sent me another that had an open ground on the LM340 regulator. The output went to the input DC of 9.8V. Luckily there was a 6V zener on the output which I caught just before it went into flames. I replaced all the burned components and repaired both supplies.The digital circuitry draws around 1A. The maximum free air temperature on the LM340 regulator is 125C. The heatsink on both repaired units is running around 74C which seems a little warm to me. What do you think? I can put a small fan on it but given this is a high-end audio device, I'm trying to keep the noise level low. There's no ventilation in the box.ThanksJerry

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2025, 02:20:37 pm
content: Where on the heat sink was that temperature measured?What's the actual temperature of the LM340's tab?I would say it's probably fine. The actual die temperature is unlikely to be more than 20 or so degrees higher.

author: magic
author_rank: Super Contributor
timestamp: February 11, 2025, 06:29:14 pm
content: Based on the 9.8V output without ground I suppose the input is about 12V. The output is perhaps 5V?Then at 1A load you have 7W power dissipation. Multiply this by the "junction to case thermal resistance" from the datasheet and you have the difference between LM340's metal tab and die temperature. Hopefully you can figure out the rest

author: cncjerry
author_rank: Unknown
timestamp: February 12, 2025, 05:09:01 am
content: I took the temperature right at the tab. Both units are between 70 and 75C. It just seems really hot. It's probably ok but I wouldn't have designed it that way. I've been kicking around putting a 2amp switching wall wart in them as I have a half dozen of them. Output is 5v by the way. It's all digital so the supply doesn't need to be super clean.Thanks

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2025, 09:29:56 am
content: It's fine. The LM340 has a maximum die temperature rating of 150°C, 125°C recommended. The die will be hotter than the tab, but is thermally protected. If it gets too hot, it will shut down, not smoke.Replacing it with a switched mode power supply is a good idea.

author: floobydust
author_rank: Super Contributor
timestamp: February 12, 2025, 07:21:52 pm
content: About the extra hot, mains voltages are higher nowadays and the Vreg's thermal grease might be dried out. Is there airflow inside? I doubt it (fan) because coolant residue is a terrible mess.LM320 is from a long time ago, it only was out for a few years for some reason, when the 8080 was all the rage. So these regs running hot, hasn't been an issue in prior decades?What size are the filter caps?You could change over to an SMPS IF the hold time is there. You don't want nuisance reboots whenever there is a mains dip due to a big motor load starting etc.I would get a decent quality SMPS, overrated for the load and just swap that in.

author: wraper
author_rank: Unknown
timestamp: February 12, 2025, 07:27:44 pm
content: If it's with a small heatsink with a few W power dissipation, then it's somewhat hot but totally fine. It would be an issue on a large or actively cooled heatsink because it would mean high power dissipation where the die to heatsink thermal resistance plays a large role, thus the die is quite a bit hotter than the heatsink.Another question is how do you measure the temperature? Asking that because very often people measure temperature incorrectly, and get much lower reading than actual temperature.

author: TimFox
author_rank: Super Contributor
timestamp: February 12, 2025, 07:53:46 pm
content: Yes, 70 to 75oC is hot to the touch (well above normal restriction on domestic hot water temperature). To remove heat from a small sink to the ambient requires a high temperature at the sink. As discussed here, you need to calculate (since it is hard to measure directly) the temperature difference between the junctions and sink to determine if the junction temperature is within spec.