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Use MOSFET to perform circuit ON/OFF

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winson

New Member
Hi friends,

I have try to use a n-channel MOSFET as a switch to ON or OFF my circuit(give power supply or cut the power). The MOSFET that i'm using now is STP36NF06L, i have facing problem with this MOSFET that is the circuit look like cant receive any power supply from my 9V battery(regulate by a 7805 IC) when i apply 5V to the gate of the MOSFET.

The connection is simple, the battery's positive terminal is connect to the input of my circuit(PIC microcontroller, LCD and other) which is the input pin of the 7805 IC and the common ground of the circuit is connect to the drain of the MOSFET. The MOSFET's source is connect to the negative terminal of the battery. Suppose when 5V is apply to the gate of the MOSFET then my circuit should be power-on and running but now what i observe is that the circuit does not running at all and the LCD also does not show anything.

Anybody can help?
Any suggestions will be appreciate.

Thanks.
 

Sceadwian

Banned
That's a pretty beefy Mosfet to be using but it should work, are you sure the Mosfet isn't bad? They're pretty static sensitive, improper handling can easily kill one.
 
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ericgibbs

Well-Known Member
Most Helpful Member
hi,
Your circuit description on how the components are interconnected sounds incorrect.

Can you post a sketch showing how it is connected.?
 

winson

New Member
Hello,

Thanks for reply.

Every time when i handle it, i will just hold the upper iron portion of the MOSFET, so is this an "improper handling"?

Regarding the connection of circuit i was not having knowledge on the software for circuit drawing so just post a sketch which is drawing by paint. Is it connected correctly?

Here the diagram,
 

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Sceadwian

Banned
That schematic is too primitive and electrically doesn't make sense, we're going to need to see a REAL schematic of how you have it laid out. Pictures might help but only if they show all components as they're connected.

As you have the mosfet currently set up with the basic block diagram it seems reasonable it should work, unless the mosfet is faulty or there's something else going on in the circuti that that simplified drawing doesn't show. And simply not touching the leads doesn't garuntee you're not going to hurt it, it's static discharges that are the problem, even one's you can't see. Keep in mind the upper metal portion of the mosfet is usually connected to the drain (could be wrong there) but they're not usually issolated (for best heat transfer)
 

winson

New Member
I have forget to show that the 5V that apply to the gate was directly take from the output of 7805 IC, so just to ask when the mosfet is OFF then is it possible that the 7805 still able to supply 5V to the gate? If it cant, then i think this may be the problem, because no supply to the gate then will just make the whole circuit OFF.

Here the new diagram,
 

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Sceadwian

Banned
If the 7805's ground is connected to the same common ground as the Mosfet drain, then no voltage difference exists in the circuit. Don't tap the voltage for the gate of the mosfet from the regulator, tap it directly from the battery. You can't bootstrap a circuit from 0 volts =)
 
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winson

New Member
Hi everybody,

Just try it out in my circuit and it WORK nicely.

Yes Roff, just now i was facing the problem that the MOSFET will not turn OFF by itself when the gate voltage is remove and have to connect gate to ground then MOSFET will OFF. Actually, i plan to use a decade counter like 4017 to control the ON/OFF of the MOSFET, when a button connected to the clock pin of the 4017 is pressed(give a clock pulse) then the IC will count 1 and pin that represent "1" is connect to the gate of the MOSFET and thus turn ON the MOSFET, when the button is press again the counter will jump to "2" and pin represent "2" will connect to reset pin of counter IC then will reset the counter to "0".

But just dont know this will able to turn the MOSFET OFF or not when the counter's output is not "1".

Anybody have any opinion?

Thanks a lot
 
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