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Fusion916 26th January 2013 08:51 PM

Can you heatsink a transistor without a exposed collector?
1 Attachment(s)
Some TO126's have exposed collectors and some dont. Do heatsinks not work without an exposed collector? Pic attached for reference.

Frank Berry 26th January 2013 08:54 PM

Good question. I hope someone can give us an answer.

JonSnell Electronic 26th January 2013 08:56 PM

The insulated collector type of transistor have similar characteristics for heat dissipation to exposed types. I prefer the insulated type as they are easier to fix to the metalwork and just need an aluminum oxide disc or plate to ensure maximum contact rather than the mica washer and heat sink compound that is required for the exposed type.

nigelwright7557 26th January 2013 08:58 PM

As the exposed transistors have aluminium pads and none exposed plastic pads then you will get better heat transfer with the aluminium pads.
This can be a nuisance as then you need insulating washers which restrict slightly heat transfer.

sofaspud 26th January 2013 09:05 PM

Yes, but the junction-to-air resistance is higher. The insulated types therefore require a better heatsink or lower circuit dissipation for equal thermal characteristics. As example, I'm working with an LM1876 currently; seems the exposed tab is 1C/W versus the isolated tab at 2C/W.

tinitus 26th January 2013 09:09 PM

just remember the nasty greasy stuff

Andrew Eckhardt 26th January 2013 09:11 PM

Wayell I think the fully encapsulated devices are generally designed to be mounted to a heatsink, because if you don't mount them to a sink, it wouldn't matter if it had an electrically conductive bottom or not, right? The whole idea is to save hardware and assembly cost.

What sofaspud said.

What I don't understand is why they have to make the epoxy so darned thick!

tinitus 26th January 2013 09:23 PM


Originally Posted by Andrew Eckhardt (
What I don't understand is why they have to make the epoxy so darned thick!

could be because the heat is transferred by the epoxy, and not through it, and the mass is better ?

Andrew Eckhardt 26th January 2013 09:37 PM

No doubt the larger, all black surface has lower junction to air resistance, but I don't think it would be enough of a difference for me. If it has to lose a lot of power I'll put a sink on it.

I've actually ground down full encapsulated BD139/140 when I didn't have any with exposed metal tabs. That stuff is about .020" thick on the back of that case type.

sofaspud 26th January 2013 09:44 PM

As an aside, I have some TO-92 parts that have small P-shaped heatsinks epoxied on, with the part number printed on the sink. Can't recall the part numbers.

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