Attached is a schematic for a MOSFET switch. The unregulated 12vdc input is regulated to power an op-amp circuit (not shown) and when the ouput from the op-amp goes high, the MOSFET provides a return path for unregulated 12vdc to the output.

What problems does anyone see with the circuit?

Thanks.

 

The 7812 will not work with a 12 volt supply. It needs a few volts of head room to work with.
You need a 1nf capacitor on the out-put of the 7812. Mount it as close to the regulator as you can.
I don't know what's driving the gate but a 10 ohm resistor may be too small.

 

The 7812 will not work with a 12 volt supply. It needs a few volts of head room to work with.
You need a 1nf capacitor on the out-put of the 7812. Mount it as close to the regulator as you can.
I don't know what's driving the gate but a 10 ohm resistor may be too small.

Thanks.

Actually the unregulated input is 14+ volts. I'll add a small ceramic cap to the output of the 7812. The gate is being driven by the output from an opamp running on 12v.

 

Actually, it should be 0.1uF (100nF) on the output, and a 0.33uF (330nF) cap on the input.

You can add more if you'd like, but you need the small value caps in parallel with them to take care of high frequency transients.

 

Actually, it should be 0.1uF (100nF) on the output, and a 0.33uF (330nF) cap on the input.

You can add more if you'd like, but you need the small value caps in parallel with them to take care of high frequency transients.

Just five minutes ago, I thought to myself, "I believe SgtWookie already told me about the need for small caps on the input and output of the regulator. I better go look for that post."

So, my memory isn't gone; it's just slow.