What is a tab on a TO-220 transistor normally connected to?

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mike_canada

Joined Feb 21, 2020
239
I was trying to figure out what the tabs on TO-220 transistors (specifically TIP41 and TIP42) connect to. Which pin?

I ask because maybe I can condense my circuit more to the point where I could possibly sandwich a heatsink between two transistors and not have to worry about conduction but that's if the connection between the correct pins are made on the PCB. and the correct pins are the ones that also connect to the tabs internally.

I looked up TIP 41 datasheet and ON semiconductor does not specify what the tab connects to.
 

MrSalts

Joined Apr 2, 2020
2,377
On many TO-220 packages, it is connected to the center pin. But sometimes it is isolated and not connected to any pin. I have never seen it connected to pin 1 or pin. 3 but check the datasheet. You never know what manufacturers try to do.
Check with an ohm meter if you are not sure.
 

Audioguru again

Joined Oct 21, 2019
5,189
I look at the datasheets of the originator of a transistor, not a copy cat.
Texas Instruments designed the originals of all the TIPxxx transistors a long time ago and show the collector connected to the metal tab.
 

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MrSalts

Joined Apr 2, 2020
2,377
I look at the datasheets of the originator of a transistor, not a copy cat.
Texas Instruments designed the originals of all the TIPxxx transistors a long time ago and show the collector connected to the metal tab.
It's best to look at the datasheet of the transistor from the manufacturer of YOUR transistor.
 

MisterBill2

Joined Jan 23, 2018
13,145
The result of having the tab connected to the collector is that there is a lot more capacitance to the surroundings as a result. So if you attach a conductive heat sink the capacitance from collector to surroundings is higher. That can be quite inconvenient for RF and higher frequencies.
 

crutschow

Joined Mar 14, 2008
30,806
The chip substrate is soldered to the tab for good heat transfer, and the substrate is normally the collector.

Some transistors are in a TO-220 type package where the tab insulated from the transistor substrate.
The disadvantage is that the thermal resistance from the substrate to the tab is higher.
 

WBahn

Joined Mar 31, 2012
27,418
I was trying to figure out what the tabs on TO-220 transistors (specifically TIP41 and TIP42) connect to. Which pin?

I ask because maybe I can condense my circuit more to the point where I could possibly sandwich a heatsink between two transistors and not have to worry about conduction but that's if the connection between the correct pins are made on the PCB. and the correct pins are the ones that also connect to the tabs internally.

I looked up TIP 41 datasheet and ON semiconductor does not specify what the tab connects to.
If you are looking for this information on a datasheet specific to a particular package, as opposed to a particular device, you won't find it since the JEDEC specification for the package just sees the tab as "terminal 4". How it is connected is entirely up to the manufacturer using it. I used one part (decades ago -- can't remember anything about it other than this) that actually used the tab as a fourth pin unrelated to the other three. But, other than that and an occasional device in which the tab wasn't electrically connected to anything, just thermally, I don't recall ever seeing any TO-220 device in which the tab wasn't connected to the center pin. In fact, many (most?) package vendors manufacture TO-220 (and variant) packages in which the center pin and tab are the same piece of metal.

So, while you can expect the tab to be electrically connected to the center pin, don't count on this. Always either check the manufacture's data sheet for the particular device (preferred) or use a continuity tester, ideally one that can't forward-bias any junctions.
 
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