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I'm trying to fix this water dispensing pump, and got stuck in between MOSFET vs transistor working principles.

Mosfet SI2302

For some reason the MOSFET stopped working and the pump as well.

Now I am trying to find a similar MOSFET, but unfortunately I could not find the same part.

While looking for an equivalent MOSFET I did find some, but they don't work at all.

The actual MOSFET on the device was a 2302. No letters, just 2302. I googled it and found it is an N-channel MOSFET

I tried to replace it with an IRF730 but it doesn't work. I tried an IRF2807, which didn't work either.

I thought of using a transistor instead of a MOSFET, so I grabbed an NPN pipolar transistor P2N2222A. That also did not help me pass the negative source to the motor in order to start the pump.

The power supply source is a 3 V, 1500 mAh battery. The motor is drawing 500 mAh, the gate of the MOSFET supply is 3 V.

During this troubleshooting, I learned that the MOSFET required a minimum gate threshold voltage. I checked the datasheets of the IRF730 and the IRF2807 - they both need a minimum 2.0 maximum 4.0 V to operate, despite satisfying the requirement those MOSFETs don't work. Obviously I'm missing something.

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    \$\begingroup\$ Minimum gate threshold voltage isn't everything you think it is. You want the Vgs required to achieve the MOSFET's RdsOn. \$\endgroup\$
    – DKNguyen
    Commented Aug 10, 2022 at 17:48

1 Answer 1

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The gate threshold voltage means you need at least 2 - 4 V to open the MOSFET. Where in this range it exactly is changes depending on production tolerances. So if you are unlucky and the particular specimen of the MOSFET you use has a threshold voltage of 4V, you cannot open it with 3V. Also, it depends on the drain current, so may be even higher (see Ids vs. Vgs graph).

A bipolar transistor may not open as it needs a certain input current, not just voltage like a MOSFET.

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    \$\begingroup\$ Pneumatic terminology such as "open" (and close) mean exactly the opposite in electronic terminology. \$\endgroup\$
    – Andy aka
    Commented Aug 10, 2022 at 11:37
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    \$\begingroup\$ The gate threshold voltage is the voltage where the mosfet is just barely starts to conduct, not where it's fully conducting (which you seem to mean). \$\endgroup\$
    – marcelm
    Commented Aug 10, 2022 at 11:40
  • \$\begingroup\$ I agree, what I meant was a MOSFET starts conduction at the gate threshold voltage (certainly not fully), hence the reference to the Ids vs. Vgs graph. \$\endgroup\$
    – Hyp
    Commented Aug 10, 2022 at 11:59
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    \$\begingroup\$ The only people who will fully understand the nuance of what you said are the ones who already know it and don't need to be told to begin with. So you should add the information someone unaware actually needs to know: if using the MOSFET as a switch then the threshold Vgs is not useful and that what they actually want is the Vgs listed beside the rated RDson in order to achieve it. \$\endgroup\$
    – DKNguyen
    Commented Aug 10, 2022 at 17:42

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