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Originally Posted by shotgunefx
Any detailed online resources to learn more about how to calculate this?
For instance, on my car project, I'm using the equivalent of a 7812. It can deliver more than your vanilla one, but I don't have the part# on hand but I believe up to 1.5 amps with a heatsink.
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Most datasheets will give a value for thermal resistance of the device, and in the case of power devices a couple, junction to ambient (no sink) and junction to case (for sink calculations).
Junction to ambient is the easy one, if it's 65°C/W, like the 78xx, the maximum voltage you expect to drop x current = watts, junction temperature rise will be (65 x W)°C above ambient. Another number on the datasheet will give the maximum safe junction temperature, so you can work out the risks involved.
The thermal resistance figure given usually only applies if the device is soldered to a pcb (which helps remove some heat) so check the small print if you're working near the limits.
Using a sink, you need to take the junction-to-case figure and factor in the TIM (case-to-sink) and sink (sink-to-ambient) ratings, example calculation
here. There's a more accurate method in Micrel's LDO datasheets that includes in the load the extra current used by the device itself, but they only reckon about 1%.
So if you're using a low-drop-out 12V regulator on a car alternator, the maximum voltage drop will be about (14.5-12) = 2.5V; 2.5W at 1A load. Best figure I can find for a TO-220 case is 50°C/W so with no sink the junction would get to 125°C over ambient - only use in winter.
Good news is, you may not need much of a sink, doubling the tab area will help a lot. And check if the tab is connected to anything dangerous, IIRC 7812 tab connects to ground pin so can often be bolted straight to a chassis. Or the insulating TIMs for TO-220 come as a dry rubbery washer with a nylon bush, so it's easy to bolt the case to some other metal to keep it cool & electrically insulated.