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fet on when switch is disconnected

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mrd

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Hi,
I'm trying to steal another switch position by using the middle of a ON-OFF-ON switch.. so I'm looking to flick a relay when the switch is disconnected in the middle position.

Not sure the best way to do this.. I've been thinking something with a 2N7002 maybe would work.. My FET knowledge is a bit rusty ... I've attached a circuit I drew up which I think would work but unsure of any potential pitfalls. Would I need some protection on the gate or is the 16V (regulated DC supply) small enough, it'll be ok?

Thanks for any help
 

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I would make R2 & R3 smaller to turn on the fet a bit faster. Other than that it seems like it should work.
 
A negative voltage is not necessary to shut off the transistor. Just connect ground to the gate via the switch.

Boncuk
 
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Hi,
I'm trying to steal another switch position by using the middle of a ON-OFF-ON switch.. so I'm looking to flick a relay when the switch is disconnected in the middle position.

Not sure the best way to do this.. I've been thinking something with a 2N7002 maybe would work.. My FET knowledge is a bit rusty ... I've attached a circuit I drew up which I think would work but unsure of any potential pitfalls. Would I need some protection on the gate or is the 16V (regulated DC supply) small enough, it'll be ok?

Thanks for any help
where is the relay in your diagram? If it is the "1000 load" then 2 points.
1/ you have drawn the symbol for a resistor not a relay coil.
2/ more important, the 1N4148 diode will not be suitable for this circuit, you need a diode which can handle much more current.
I would suggest a 1N4002 or even a schottky diode with at least a 1 amp rating.
Norman
 
I'm with Boncuk. Eliminate R2 and connect the gate directly to the switch and the other end of the switch to ground.

Norman suggested using a larger diode. That's only needed if your relay coil draws more than 200mA.
 
I'm thinking the -16 is already there since he said he was stealing from an existing switch.
 
I'm with Boncuk. Eliminate R2 and connect the gate directly to the switch and the other end of the switch to ground.

Norman suggested using a larger diode. That's only needed if your relay coil draws more than 200mA.
I see what you are saying so maybe I have misunderstood this. I thought that the back emf could be quite high and so no matter what the current through the relay coil, when it is interrupted, the voltage could be quite high and so the initial current could possibly be much higher than the steady state current. In which case, if I am correct, it could easily exceed the rated current for a 1N4148.
 
In which case, if I am correct, it could easily exceed the rated current for a 1N4148.

If you're afraid of a back EMF too high use a 1N4004. It's good for 1A of back EMF. (pulsed higher)
 
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I see what you are saying so maybe I have misunderstood this. I thought that the back emf could be quite high and so no matter what the current through the relay coil, when it is interrupted, the voltage could be quite high and so the initial current could possibly be much higher than the steady state current. In which case, if I am correct, it could easily exceed the rated current for a 1N4148.
The current through the diode will never be greater than the coil current at the time of the switching.

The back emf voltage can be very high if there is no path for the inductive current, but there is no amplification of the current.
 
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