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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Nov 2019, 5:14 pm   #1
McMurdo
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Default TO247 transistor collector-heatsink

I'm interested in hearing your opinion on using TO247 transistors. The heatsink plate is labelled as the collector, like many tab-type packages.

Is is acceptable to use the tab as the only collector connection? Is the tab-to-heatsink connection likely to be adequate? After all, it is designed to be in very good thermal contact with a flat surface.

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Old 13th Nov 2019, 5:17 pm   #2
HamishBoxer
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Default Re: TO247 transistor collector-heatsink

I would prefer to hard wire rather than a tag under the screw due to heat sink compound.
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Old 13th Nov 2019, 6:09 pm   #3
G6Tanuki
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Default Re: TO247 transistor collector-heatsink

I've used them [and things like some voltage-regulators with the tab electrically-active] in situations where the tab is bolted down to a tinned area on the 'component side' of a double-sided PCB, and it seemed to work OK.
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Old 13th Nov 2019, 6:36 pm   #4
paulsherwin
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Default Re: TO247 transistor collector-heatsink

Assuming you are using heatsink compound, I think you should connect the collector leg. I've never had to deal with this though.
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Old 13th Nov 2019, 7:16 pm   #5
merlinmaxwell
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Default Re: TO247 transistor collector-heatsink

If you can ensure contact or not be bothered if it goes O/C (open circuit) it will be fine, the lead frame (bit inside the transistor) collector section is contiguous. Easy enough to solder to as well.
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Old 13th Nov 2019, 7:51 pm   #6
kalee20
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Default Re: TO247 transistor collector-heatsink

Well, the chip is internally bonded to the tab, and then the middle lead is extended from the tab, so the tab itself (collector, drain or whatever) is the most direct connection to the device.

Whether you find it easiest to make a reliable connection of your own to the lead or to the tab, is, of course, under your own control!
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Old 13th Nov 2019, 9:58 pm   #7
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Default Re: TO247 transistor collector-heatsink

Thinking out of box .
If using one of these in a series regulator circuit ( or similar), I'd be tempted ( where the negative rail is chassis) to use a negative device and bolt the collector direct to chassis, but I'd still electrically connect to collector tab.
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Old 14th Nov 2019, 12:08 am   #8
McMurdo
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Default Re: TO247 transistor collector-heatsink

the reason I ask is to do with replacing an old TO218 package with a newer TO247.

See excerpt from a product change notice from ON Semi below:

The hybrid mounting used the collector pin onto a pcb for a voltage reference and the tab mounted to a live heatsink assembly for the power.
I want to avoid having to modify the product if it's not necessary by putting links between the pin and the heatsink.
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Old 14th Nov 2019, 1:17 am   #9
Maarten
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Default Re: TO247 transistor collector-heatsink

The very good thermal contact isn't really obstructed by a thin layer of aluminium oxide on the heat sink, but the electrical contact is. If you can't find TO-218 devices from old stock or a second source manufacturer, I'd rather make some modifications.
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