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Old 22nd April 2013, 01:59 AM   #1
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Default question about transistor package types

For the various medium to high power transistor package styles that have some sort of metal heatsink mounting tab (to-220, to-126, to-246, ect), is that tab electrically isolated from the actual transistor?

For instance, I know sometimes people stick to-92 types together with thermal grease to make them track thermally. Can one do a similar thing with the above mentioned types, and similar ones? Not that I specifically want or need to do this. I just want to know if it works that way.

Something I think that I would want to do is mount my heat sinks to the chassis. Can one do this, and then bolt the tabs of these transistors straight to the heat sink? Or do you need some sort of insulator between the tab and the sink.
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Old 22nd April 2013, 02:12 AM   #2
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Usually the transistor's metal case is connected to the collector; often this fact is mentioned on the transistor's datasheet. It's also possible to use an ohmmeter to find out what the case is connected to (if anything).

There are also some all-plastic versions of the TO126 and TO220 packages, which have no metal case. Check the datasheets of the BD139 and BUJ302 for examples.
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Old 22nd April 2013, 02:17 AM   #3
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There are 3 types:
1/ Completely plastic package.
2/ Metal tab but with plastic bush where the bolt goes through.
3/ Metal tab and metal bush.

2+3 need some sort of isolation.
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Old 22nd April 2013, 02:28 AM   #4
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ok, how is this isolation accomplished? I assume any sort of plastic or other insulator between the tab and the sink would be highly detrimental to proper heat transfer. Besides, in the types with no metal bushing, keeping the mounting bolt isolated seems problematic.

Does this pretty much mean that the heatsink itself needs to be isolated from the rest of the circuit, ie, mounting direct to the chassis is a no-go?

I presume that when I see photos of lines of power transistors bolted to the same sink, they are all in parallel, and the collectors are fine sharing this connection.
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Old 22nd April 2013, 02:33 AM   #5
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There are insulating washers for the transistor body and plastic bushes for the bolts.

Once the transistors are insulated from the heatsink you are free to bolt the heatsink to an earthed chassis.

I built up an amp last week and fell foul of the classic problems.
1/ I forgot to deburr the holes and they broke through the insulating tab when I bolted them up.
2/ I got some swarf behind am insulating tab and that broke through the insulating tab.

A case of more haste less speed.

I always test now with a DMM that there is no short between transistor tab and the heatsink before I power up for the first time.

Here is an example of a few insulating washers.

http://uk.rs-online.com/web/p/thermal-gap-pads/1692234/
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Old 22nd April 2013, 02:36 AM   #6
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so what is going on when I see transistors attached to the sink with thermal grease? I assume these aren't isolated with washers. Or are they?

Are these washers and bushings a standard thing that I could order from mouser?
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Old 22nd April 2013, 02:39 AM   #7
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You still need insulating washers and bushes.

The thermal grease just fills in the pits and holes and makes a better thermal contact.
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Old 22nd April 2013, 02:45 AM   #8
jcx is offline jcx  United States
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it is possible for hard anodize on a Al heatsink to be the insulating layer - but explicit insulation is safer for hobbyist

for driver Q using PCB mount individual heatsink - you can just let the heatsink float with the Q collector/drain V - although it can be a shock hazard on probing/while debugging

Last edited by jcx; 22nd April 2013 at 02:48 AM.
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Old 22nd April 2013, 02:46 AM   #9
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so one would grease both sides of this washer? What is the washer made of, and why does it not thermally insolate the transistor from the sink? Or does it indeed insolate it a little, and is just an undesired but necessary side effect.
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Old 22nd April 2013, 02:47 AM   #10
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You still have the bolt to contend with.

I wouldn't rely on the anodisation.
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