Heatsink for an LM-317 - Soldering to PCB - Page 1
Forum section: electronics / projects

author: amaningd
author_rank: Contributor
timestamp: July 11, 2022, 04:40:28 pm
content: I'm using a Texas Instruments LM-317 (in the "typical application" shown on thedatasheet, fig. 9) in a TO-220 package to power a 7.5V/1A load. Since the regulator will be dissipating P = ΔV*I = (12V - 7.5V)(1A) = 4.5W, I plan to use both a fan andthisOhmite RA-T2X-25E vertical heatsink coupled to the regulator with thermal paste. I see that the heatsink has solderable pins. My question is: since the TO-220 tab is connected to the output pin, should I connect the heatsink mounting holes on the PCB to the same net/traces as the output pin, or leave the holes disconnected from the rest of the PCB and use them purely for mounting? I was wondering whether this might affect heat dissipation, e.g. whether the heatsink material would add significant resistance.(Also, this is a complete aside, but I've been curious about this for a while: on the LM-317 datasheet, R1 is 240Ω in most of the example circuits. Is there a particular reason/requirement for R1 to have this value or be this order of magnitude?)Thanks!

author: edavid
author_rank: Super Contributor
timestamp: July 11, 2022, 04:50:16 pm
content: 1. the links in your post are broken2. if you solder the heatsink pins to the output copper pour, you will get a tiny bit of extra heatsinking that way3. I don't see why the resistance of the heatsink material (thermal? electrical?) matters (you should still solder the TO-220 center pin)4. I'm surprised you need a fan, but I haven't looked at the heatsink datasheet5. the 240Ω resistor provides the required minimum load current - it's inconsistent with the datasheet 10mA spec, but it obviously works fine - if your circuit provides a minimum load, you can use a higher value

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2022, 04:57:21 pm
content: Quote from: amaningd on July 11, 2022, 04:40:28 pmThat's a huge heatsink if you have a fan with any airflow at all, so I wouldn't worry about the PCB thermal issues either way. I'd just leave the mounting holes isolated. I'm not sure those solder-mount tabs will conduct that much heat. I just made a PS using a similar but smaller heatsink with an LM317 @ ~2W and using an IR camera I don't see any significant heating of the PCB even with the heatsink at 125C and without any fan.The 240 ohm is to provide a minimum load current of 5mA through the divider. You can use higher values if you have an assured minimum load or you don't care about regulation at very low loads.

author: amaningd
author_rank: Contributor
timestamp: July 11, 2022, 08:45:05 pm
content: Quote from: bdunham7 on July 11, 2022, 04:57:21 pmThanks for the responses, guys -- they're super helpful. The fan is there for another off-the-shelf PCB in the same enclosure that requires forced-air cooling. Now that you guys mention it, that heatsink I linked is definitely big.@bdunham7, do you happen to have a part number for the smaller heatsink?

author: MarkF
author_rank: Super Contributor
timestamp: July 11, 2022, 09:38:54 pm
content: I used the Ohmite E-Series on mine:https://www.mouser.com/ProductDetail/Ohmite/EA-T220-38E?qs=6weo7U0lPxCX9kcQhGcuZQ%3D%3DAlso, I mounted them onto isolated pads. Avoiding the need for mica insulator to increase heat transfer.

author: magic
author_rank: Super Contributor
timestamp: July 12, 2022, 09:08:23 am
content: Isn't 4.5W a level where one could get away with heatsinking by the PCB? (Using D2PAK or that sort of package).