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Electronics => Beginners => Topic started by: cellularmitosis on June 18, 2018, 10:11:15 pm

Title: TO-220 MOSFET, is the tab live?
Post by: cellularmitosis on June 18, 2018, 10:11:15 pm
Heyo, bit of a noob question:

How can I tell if the tab of a TO-220 MOSFET is electrically connected to the drain, gate, source, or is disconnected?

I've seen some datasheets explicitly call this out, but I couldn't find it in the IRF510 or IRF610 datasheets, for example.

https://www.vishay.com/docs/91015/sihf510.pdf (https://www.vishay.com/docs/91015/sihf510.pdf)

http://www.vishay.com/docs/91023/91023.pdf (http://www.vishay.com/docs/91023/91023.pdf)

I suppose I can just break out a multimeter when I get home from work, but I thought I'd ask in case there is something obvious I'm overlooking in the datasheets.

Thanks!  ;)
Title: Re: TO-220 MOSFET, is the tab live?
Post by: RandallMcRee on June 18, 2018, 10:22:22 pm

Hmmm, most datasheets tell you explicitly. TO-220 tabs are always connected to either pin 1,2 or 3 in my experience.

So if the datasheet doesn't tell you try googling?! for irf510 pinout or similar...quickly found D is also tab.

Just tested a real one, yep, it is.


Title: Re: TO-220 MOSFET, is the tab live?
Post by: Zero999 on June 18, 2018, 10:27:03 pm
The tab is normally connected to the middle pin, which is usually the drain on a MOSFET.

Devices with isolated tabs do exist, but the tab is normally non-conductive and it will explicitly state it on the data sheet. If there's no mention of an isolated tab, on the data sheet, then assume the tab is connected to the drain.
Title: Re: TO-220 MOSFET, is the tab live?
Post by: james_s on June 18, 2018, 10:30:53 pm
I don't recall ever seeing a non-isolated TO220 package where the tab wasn't connected to the middle pin.
Title: Re: TO-220 MOSFET, is the tab live?
Post by: wraper on June 18, 2018, 10:36:38 pm
Only for triacs isolated tab is a common thing. As of isolated MOSFETS, they come in variant of TO-220 package in which tab is fully covered in plastic (TO-220F).
Quote from: Hero999 on June 18, 2018, 10:27:03 pm
The tab is normally connected to the middle pin, which is usually the drain on a MOSFET.
Yep, middle pin connected to the tab is a standard thing, and not only for MOSFETS but basically any device in TO-220 package.

(https://d2t1xqejof9utc.cloudfront.net/screenshots/pics/ae80d5283ef6658004bddfee183a4a18/large.JPG)
Title: Re: TO-220 MOSFET, is the tab live?
Post by: lordvader88 on June 18, 2018, 10:47:29 pm
I guess it's good and bad. Sometimes it makes it easy to check a voltage, but myself-included, we have no idea how dangerous and easy it is to touch something by accident.

Sure hope no one finds out.
Title: Re: TO-220 MOSFET, is the tab live?
Post by: cellularmitosis on June 18, 2018, 11:19:03 pm
Thanks so much guys!
Title: Re: TO-220 MOSFET, is the tab live?
Post by: Belrmar on June 18, 2018, 11:27:04 pm
you can always test it with a multimeter if the datasheet does not give you enough info
Title: Re: TO-220 MOSFET, is the tab live?
Post by: aju11 on June 19, 2018, 12:13:02 pm
Yes. and usually it is connected to the drain pin.
You have to be careful while mounting it on a heat-sink.