Can you heatsink a transistor without a exposed collector? - diyAudio
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Old 26th January 2013, 09:51 PM   #1
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Default Can you heatsink a transistor without a exposed collector?

Some TO126's have exposed collectors and some dont. Do heatsinks not work without an exposed collector? Pic attached for reference.
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Old 26th January 2013, 09:54 PM   #2
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Good question. I hope someone can give us an answer.
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Old 26th January 2013, 09:56 PM   #3
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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.
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Old 26th January 2013, 09:58 PM   #4
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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.
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Old 26th January 2013, 10:05 PM   #5
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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.
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Old 26th January 2013, 10:09 PM   #6
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just remember the nasty greasy stuff
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Old 26th January 2013, 10:11 PM   #7
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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!
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Old 26th January 2013, 10:23 PM   #8
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Quote:
Originally Posted by Andrew Eckhardt View Post
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 ?
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Old 26th January 2013, 10:37 PM   #9
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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.
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Old 26th January 2013, 10:44 PM   #10
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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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