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MOSFET terminology

twiglet123

Dec 30, 2012
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Dec 30, 2012
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Hello all, I'm after buying a bunch of MOSFETs to run a few relays that i'm about to buy from rapid online.

I'm controlling a fairly powerful motor from an mbed micro-controller that can output 3.3v and the solenoids I'm planning to use take 5v to operate.

These ones here..
http://www.rapidonline.com/Electron...C-16A-SPDT-Low-Profile-PC-Power-Relay-60-1474

Any recommendations for good cheap MOSFETs that will allow the desired voltage 5v to pass when I stuff 3.3v in at the gate?

I'm looking at these ones at the moment..

http://www.rapidonline.com/Electronic-Components/P-Channel-Low-Power-MOSFET-Transistors-518576

However I haven't a clue about any of the MOSFET terminology and haven't had much luck googling for results (VDS, RDS, ID cont and IDM mean nothing to me!).

If anyone could provide a link to a good human friendly glossary of terms and make sure i'm buying the right thing i'd appreciate the help

All the best
twig
 

BobK

Jan 5, 2010
7,682
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I would not use a MOSFET in that application, and certainly not an P channel one. I would use an NPN bipolar transistor.

The reason I would not use a MOSFET is because even the so called logic level ones typically require more than 3.3V on the gate.

The reason I would not use a p-channel MOSFET is because when the gate is driven by a 3.3V micro and the source is at 5V this gives you either -1.7 or -5V on the gate, relative to the source. If the -5V would turn it on fully the -1.7 would likely turn it on partially.

Also, why does it require a bunch of MOSFETs to run a few relays? I would have though that a few would do :)

Bob
 
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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you thinking something like a 2N3904 or 2N2222A transistor?

As long as the relay current is well within their Ic ratings, yes.

Remember to place a diode across the relay to absorb transients as the relay switches off (the diode has the cathode at the +ve end of the relay coil so it doesn't conduct while the relay is on. A 1N4001 is typically fine for this)
 
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