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I'm doing some antenna deployment work on a CubeSat testbed prototype for a school club. Here's a wiring diagram of what I'm doing:

Arduino powered by two 18650 batteries wired to a nichrome cutter and a MOSFET.

I'm using two 18650 batteries to power an Arduino which programmatically sends a brief digital signal to saturate an IRFZ30 MOSFET. I want to essentially short the battery across the nichrome wire to heat up and cut through some fishing line. In my own tests without the MOSFET, the wire shorted across the battery terminals draws 1.5 A and the line is cut.

The IRFZ30 datasheet shows a \$V_{GS}\$ of 2-4 V, and I'm measuring the Arduino's digital output in that range. Unfortunately, I'm getting less than 1 mA of current through the nichrome. I'm certain that I'm missing some very obvious things here. What do I need to do to get this to work?

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  • \$\begingroup\$ Have you considered the topology shown in neilUK answer? electronics.stackexchange.com/questions/264142/… (you probably could do with a single transistor...) \$\endgroup\$ Apr 13, 2023 at 1:55
  • \$\begingroup\$ When you have found a matching FET, I would like to mention another aspect: The activation of the cutter introduces an imbalance in the cell charge. Consider using the full voltage of both cells and a low frequency 50% PWM signal for the FET. This will discharge both cells equally and recharge is simple. \$\endgroup\$
    – Jens
    Apr 13, 2023 at 18:53

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\$V_{GS,th}\$ is not the voltage at which the MOSFET turns on. It's the voltage at which the MOSFET barely starts to turn on; typically, it's defined as the gate voltage that produces some arbitrary small current (frequently between 10 μA and 1 mA) on the drain. In the case of the IRFZ30, that current is 250 μA according to the datasheet you linked in the question.

You typically want to drive a MOSFET several volts higher than its threshold voltage.

From the linked datasheet: enter image description here
Notice how \$R_{DS,on}\$ is specified with 10 V on the gate. 10 V is a typical drive voltage for these common 2-to-4 V threshold FETs. They definitely won't work on the 3.3 V or 5 V you get from your typical microcontroller output pin.

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  • \$\begingroup\$ Should I use something like a relay for this then? Or are there other MOSFETs that would be able to handle this current with the voltages that I have available? I can make the VDS 8 volts by putting the cutter circuit across both batteries. Would that let me get more current? \$\endgroup\$
    – Jake Nixon
    Apr 13, 2023 at 1:52
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    \$\begingroup\$ @JakeNixon No need to get a relay, just get a logic-level MOSFET. I'm fond of the Si2318, personally. Or an IRLZ44 would work just fine, if you want a through-hole part. \$\endgroup\$
    – Hearth
    Apr 13, 2023 at 2:07
  • \$\begingroup\$ I'll see if my school stockroom has any logic-level MOSFETs tomorrow; if not I'll order some IRLZ44s on Mouser tomorrow. Thanks! Should it be a simple 1:1 swap? \$\endgroup\$
    – Jake Nixon
    Apr 13, 2023 at 2:16
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    \$\begingroup\$ @JakeNixon Yes, logic-level FETs are just regular old FETs with lower threshold voltage and higher transconductance to allow you to turn it fully on with a low gate voltage. There's nothing special you need to do differently, other than that some of them might not like 10 V on the gate--but you're only going to be putting 5 V on the gate. \$\endgroup\$
    – Hearth
    Apr 13, 2023 at 2:18

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