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I need to short-circuit a power source using a microcontroller to measure the short-circuit current capacity (less than 10A). So the impedance of the short-circuit circuit should be the lower as low as possible.

I was using a 5V relay commanded by a BJT transistor and it works very well. But for space reasons I would like to test other options, like using a MOSFET transistor working as a switch. I tested with the logic level NMOS IRL540N as showedshown in diagram and it worked fine. Putting 5V at the gate closes the short-circuit circuit with about 36mΩ resistance.

But the problem is when the power supply is connected wrongly with reverse polarity the NMOS always conducesconducts, even with a low microcontroller output. I don't understand why it conducesconducts as VGS remains null even with reverse polarity connected.

How couldcan I protect the MOSFET to be always off when reverse polarity is connected to drain-source?

The power supply could reach up to 25V. Is the MOSFET always secure? The absolute maximum rating for Gate-to-Source Voltage is 20V, but in this application VGS is always lower than 5V. Thanks.

schematic

simulate this circuit – Schematic created using CircuitLab

I need to short-circuit a power source using a microcontroller to measure the short-circuit current capacity (less than 10A). So impedance of short-circuit should be the lower as possible.

I was using a 5V relay commanded by BJT transistor and it works very well. But for space reasons I would like to test other options, like using a MOSFET transistor working as a switch. I tested with the logic level NMOS IRL540N as showed in diagram and it worked fine. Putting 5V at gate closes the short-circuit with about 36mΩ resistance.

But the problem is when the power supply is connected wrongly with reverse polarity the NMOS always conduces, even with low microcontroller output. I don't understand why it conduces as VGS remains null even with reverse polarity connected.

How could I protect the MOSFET to be always off when reverse polarity is connected to drain-source?

The power supply could reach up to 25V. Is the MOSFET always secure? The absolute maximum rating for Gate-to-Source Voltage is 20V, but in this application VGS is always lower than 5V. Thanks.

schematic

simulate this circuit – Schematic created using CircuitLab

I need to short-circuit a power source using a microcontroller to measure the short-circuit current capacity (less than 10A). So the impedance of the short circuit should be as low as possible.

I was using a 5V relay commanded by a BJT transistor and it works very well. But for space reasons I would like to test other options, like using a MOSFET transistor working as a switch. I tested with the logic level NMOS IRL540N as shown in diagram and it worked fine. Putting 5V at the gate closes the short circuit with about 36mΩ resistance.

But the problem is when the power supply is connected wrongly with reverse polarity the NMOS always conducts, even with a low microcontroller output. I don't understand why it conducts as VGS remains null even with reverse polarity connected.

How can I protect the MOSFET to be always off when reverse polarity is connected to drain-source?

The power supply could reach up to 25V. Is the MOSFET always secure? The absolute maximum rating for Gate-to-Source Voltage is 20V, but in this application VGS is always lower than 5V.

schematic

simulate this circuit – Schematic created using CircuitLab

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MOSFET like relay for short-circuit

I need to short-circuit a power source using a microcontroller to measure the short-circuit current capacity (less than 10A). So impedance of short-circuit should be the lower as possible.

I was using a 5V relay commanded by BJT transistor and it works very well. But for space reasons I would like to test other options, like using a MOSFET transistor working as a switch. I tested with the logic level NMOS IRL540N as showed in diagram and it worked fine. Putting 5V at gate closes the short-circuit with about 36mΩ resistance.

But the problem is when the power supply is connected wrongly with reverse polarity the NMOS always conduces, even with low microcontroller output. I don't understand why it conduces as VGS remains null even with reverse polarity connected.

How could I protect the MOSFET to be always off when reverse polarity is connected to drain-source?

The power supply could reach up to 25V. Is the MOSFET always secure? The absolute maximum rating for Gate-to-Source Voltage is 20V, but in this application VGS is always lower than 5V. Thanks.

schematic

simulate this circuit – Schematic created using CircuitLab