MOSFET as switch

Packhorse · Dec 7, 2009

I have been using this MOSFET as a switch controlled by a reed switch in past projects.
Its always worked wello but in the latest project with only a 3.7v Li Ion battery pack it does not seem to turn the SST-90 fully on.

Can anyone help?
Is the voltage too low for the FET?
Is there a recommended FET to use?

I need to switch up to 9 amp.

Thanks

georges80 · Dec 7, 2009

Packhorse said:
I have been using this MOSFET as a switch controlled by a reed switch in past projects.
Its always worked wello but in the latest project with only a 3.7v Li Ion battery pack it does not seem to turn the SST-90 fully on.

Thanks

You need to find a different FET that is fully enhanced with a lower gate voltage.

The datasheet for the part you are using shows a Vgs of up to 4V. Vgs(threshold) needs to be lower when running a single li-ion battery. Look for something with a Vgs max of 3V or lower.

cheers,
george.

Packhorse · Dec 7, 2009

Thanks.
I am struggling to find one.
Many data sheets I look at quote Vgs as 20v so I guess thats not the rating I should be looking at right? But Vgs (threshold) or Vgs (off)??

This FET is available locally. Its quite expensive compared to others but if it works its worth it I guess. It seems to be designed for an audio amplifier. Will it work for my needs?

Nereus · Dec 7, 2009

Packhorse said:
Thanks.
I am struggling to find one.
Many data sheets I look at quote Vgs as 20v so I guess thats not the rating I should be looking at right? But Vgs (threshold) or Vgs (off)??

This FET is available locally. Its quite expensive compared to others but if it works its worth it I guess. It seems to be designed for an audio amplifier. Will it work for my needs?

I am afraid it won't. According to the datasheet's "drain-to-source voltage vs. gate-to-source voltage" graph this mosfet can not handle the 9 amp load.

A logic level, very low on-resistance mosfet might suite your needs. Try googling using those words as keywords.

Should this thread be in the Flashlight Electronics sub-forum?

-N

Aircraft800 · Dec 7, 2009

Packhorse,

ledaholic just built a SST-90 Mag with a MOSFET as a switch, I wonder if he had the same problem?

See this thread

It may not be an issue for him because he's running 4.8V (4 NiMh C cells), but that's a lot of Amps to handle. I'd check with him.

ledaholic · Dec 7, 2009

I used a IRLZ34 in mine just because it was what I had lying around. I don't remember the rated specs off hand, I believe about 5V but I can turn it on with 3V easily. I have an extra if you would like to try it.

Der Wichtel · Dec 7, 2009

irf8113 or irf8736.
Don't look only for big mosfets. Smaller mosfets with high current rating means that the mosfet has lower resistance and therefore doesn't need a big package for heat-transfer.

Packhorse · Dec 7, 2009

ledaholic said:
I used a IRLZ34 in mine just because it was what I had lying around. I don't remember the rated specs off hand, I believe about 5V but I can turn it on with 3V easily. I have an extra if you would like to try it.

That would be great. PM sent.

Packhorse · Dec 9, 2009

ledaholic said:
I used a IRLZ34 in mine just because it was what I had lying around. I don't remember the rated specs off hand, I believe about 5V but I can turn it on with 3V easily. I have an extra if you would like to try it.

As per PM I found some local(ish).

Anyway Its in and working great. Thanks

willieschmidt · Apr 10, 2012

ledaholic said:
I used a IRLZ34 in mine just because it was what I had lying around. I don't remember the rated specs off hand, I believe about 5V but I can turn it on with 3V easily. I have an extra if you would like to try it.

jason 77 · Apr 11, 2012

Try using a FQP50N06L fairchild it might work... I use them all the time for my constant current circuits. I think the Vgs is like 1-2 volts minimum...

MOSFET as switch

Packhorse

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Nereus

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