Reverse polarity protection circuit for negative voltage sources

[gopintj]

Dear members,

Please recommend me a simple MOSFET circuit or useful links to protect my load from reverse polarity inputs voltages.

My Supply voltage : -12V/-5V.
Supply current : 2.5A.

Load to my circuit is an Op-amp (consumes around 100mA of current, but I need a circuit of around 2.5A for future designs).

Note: Previously, I designed a reverse polarity protection circuit using a P-MOSFET to protect my load from positive voltage sources and it works fine. Here is the schematic.

Thanks in advance.
- N Gopi.

 

[crutschow]

So what's wrong with using the P-MOSFET?

 

[gopintj]

So what's wrong with using the P-MOSFET?

The input supply voltage of the above circuit is +5V. When I interchange the ground and Vcc lines in P16, the P-MOSFET will not be biased and the circuit becomes open.

My current requirement is, to do the same setup (The circuit should be open) by using -5V as my voltage source. Will this circuit work when I use a -5V instead of +5V in P16 ? I hope it will not. Please provide me any suggestions.

Regards,
-N Gopi

 

[crutschow]

Okay. Sorry I missed the fact that it was a negative voltage.
In that case just substitute an N-MOSFET of at least a 10A rating for the P-MOSFET (again with the MOSFET source terminal connected to the input negative voltage source).
For a -5V source, the N-MOSFET must be a logic-level type (fully turns on with a Vgs of 5V).

 

[schmitt trigger]

Also...................reverse the polarity of all the diodes in the circuit.

 

[crutschow]

What is the purpose of the diodes in the circuit?
I don't see that they are needed for a maximum supply of 12V.

 

[gopintj]

Okay. Sorry I missed the fact that it was a negative voltage.
In that case just substitute an N-MOSFET of at least a 10A rating for the P-MOSFET (again with the MOSFET source terminal connected to the input negative voltage source).
For a -5V source, the N-MOSFET must be a logic-level type (fully turns on with a Vgs of 5V).

Dear Mr Crutschow,

I tried as per your suggestion and got -12V on the DRAIN side by providing the SOURCE with -12V and connecting the GATE to ground through 56K resistor. As my ultimate aim is to open the circuit when the power is applied in reverse direction, I conducted it and got the below result.

As I connect the SOURCE to ground and GATE to -12V, I am able to get only of around 0.57V on the DRAIN. But when I measure the voltage between -12V (of SMPS) with respect to DRAIN (currently it is 0.57V), I can able to measure upto -12 volts. This represents the N-MOSFET is conducting the current when I reverse the boards supply voltage. So, I believe the circuit requires some additional changes to acquire my requirement.

Note:
1. I am currently using -12V as my input source, as I didn't receive my -5V SMPS yet.
2. For testing purpose, I am currently using a voltage source, a N-MOSFET, a 56K pull down resistor to GATE and a 256 ohms / 9W resistor as load between DRAIN and ground.