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# help needed!! Using mosfet to act as a relay

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#### cubert_y

##### New Member
I am currently using two mosfet to act as a relay. As u can see from the diagram,
First I tight my gate to 0V.Then I apply I/p voltage of 15V to charge the capacitor. After fully charged, I apply a 5v to the gate for the two mosfet to turn on and my capacitor will be able to discharge to the o/p.

Problem:I can charge my circuit to 15V but I cannot discharge my capacitor to the o/p.

Anything wrong with the diagram?
pls highlight.thanks

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nobody can help?? :cry:

The problem with the second MOSFET is that it takes a minimum Vgs to turn it ON. Applying 15V to the gate will not raise the output (the voltage at the source pin) to 15V because the gate to source voltage whould have dropped to less than the minimum. You could use a high side driver or else replace the second MOSFET to a P-channel one.

motion said:
The problem with the second MOSFET is that it takes a minimum Vgs to turn it ON. Applying 15V to the gate will not raise the output (the voltage at the source pin) to 15V because the gate to source voltage whould have dropped to less than the minimum. You could use a high side driver or else replace the second MOSFET to a P-channel one.

The Vth is 5V .So i just replace the 2nd mosfet with p-channel.Using the same connection will do ?

A P channel in the place of Q2 will need 15v on the gate to keep it off, and zero volts to turn it on. Actually, you can turn it on with, say, 5 volts (it depends on what type you choose), but zero volts will work for sure, and it's easy to generate. Just be sure you pick a PMOS that can handle 15v gate-to-source.

try changing the 2nd mosfet with a p-channel( IRF9530) .But when i measure my capacitor it only about 7v and the o/p voltage also about 7v.

Then i put an invertor(7404) between the gate .i still get the same results.

any kind soul can help mi improve the current circuit .As soon as possible.

Thank. Urgent!!!!!!!!!!!!!!!!!! :?

mosfet as relay

Why not take the 2nd fet, tie its source to the -end of the cap, connect the drain to the "bottom end of the resistor. When the fet is fully enhanced it's voltage drop will be miniscule. This way both of the fets share the same g-s potential.

Otherwise, you could add a high side driver to the 2nd fet. It's ground would be referenced to the source of the fet.

Here is the schematic showing the P channel MOSFET and the driver for it.
You'll have to pick a suitable P channel transistor. It doesn't have to be very large. The IRF 830 is overkill for what you are doing.

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i really appreaciate.

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