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I'm designing a new PCB in which I'm powering a resistive wire via a MosFet, driven by a microcontroller kind of that way:

enter image description here

Vdd: +5V several Amps (depending on the heating wire)

Vin: 3.3V, an output of the microcontroller

I would like to be able to simply test if the resistive wire is present or not before Vdd is turned on. without Vdd on, I can't use a sense resistor in series with my load to sense the current so I don't know what to do.

There is probably a simple circuit or an IC capable to detect that and send the info to the microcontroller but I can't find it.

also, I only have one input pin left on my microcontroller which also complicate things.

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One way is to drive a small current through the heating element and measure the voltage at the drain of the MOSFET.

You could use your remaining GPIO. You haven't specified what voltage VDD is so you would probably require a couple of resistors to match the input of the microcontroller.

This requires that VDD be active in order to check the heating element. I suspect in your question you really meant "Sense that the resistive element is present before you activate it" not before you turn Vdd on.

The circuit to drive a small current through the element could just be a resistor that has a value that is much larger than the resistance of the element. Maybe many kilohm. It would be placed in parallel with the MOSFET. Since you will need a couple of resistors to shift the level down to the micro they will also provide the sensing current.

This resistor network would draw a small current all the time Vdd is present but it would cause negligible heating.

If you had another IO pin you could use it to enable the current used to sense the heating element presence to avoid the power drain.

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  • \$\begingroup\$ thank you for your answer, I actually changed the microcontroller for a bigger one and implemented your solution (the small current through the resistive wire) . I had to re-rout everything but that was the best option. \$\endgroup\$ Commented Mar 20, 2020 at 18:31

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