Newb question: identifying leads on unmarked MOSFETS?

[Guest 65ShelbyClone]

This may be common knowledge to most, but I have to start somewhere.

I've salvaged quite a few transistors and mosfets(?) from various appliances, but many are not clearly marked. I found out how to identify the tansistor leads and polarity, but not the mosfets. I tried google and the manufacturer websites, but it looks like nearly all are obsolete. Some are NECs, a few Fairchilds, a few International Rectifier IRF 540s, and a bunch of unknowns.

Anyway, can I use a multimeter to ID the leads and polarity? Thanks.

[audioguru]

Hi Shelby,
Since the gate of a Mosfet doesn't draw DC current, you could connect a 9V battery in series with a 100k resistor and apply it to two pins to try to turn it on. Then since you don't know if it is N-channel or P-channel you could try another 9V battery in series with a LED and a current-limiting resistor as a load, swapping the connections for the two polarities.

You won't know the ratings of the Mosfets so maybe it isn't worthwhile.

[Guest 65ShelbyClone]

How would that wiring look? I'm not the best at visualizing circuits.

Maybe you're right, especially when I get them for ~$1 each and the ones I have could very well be static damaged(no ground strap for me!). I still have a bunch of mystery switching/power tansistors, though. Its a bear finding specs on obsolete semiconductors.

[audioguru]

I get them for ~$1 each and the ones I have could very well be static damaged.

Digikey sells brand new Mosfets with spec's and a guarantee. An IRF540 is only $.63US today in small quantities and costs less if you buy 25 or more.

[Guest 65ShelbyClone]

Cool, thanks  :)

[steven]

:)the leg positions of the irfp460 mosfet are the same

irfp460a.pdf

[steven]

:)lay the mosfet on the table in front of you now starting with the left leg is the gate the middle is the drain the right is the source leg

[Guest 65ShelbyClone]

Is the left-to-right G/D/S orientation standard? I've done alot more research since Febreuary and have seen that leg order to be the most common(at least on boards that were silk screened with the actual leg labels). Tansistors are another story.

[steven]

:)as far as i know the legs are the same for all n channels mosfets