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I made a pcb which uses an N-Channel Mosfet for enabling/disabling the NTC thermistor as in the schematic below. The problem is I get 2V at the Drain pin instead of 3.3V. I measure 3.3V at NTC_TRIG and power pins with a voltmeter so there isn't any issure there.

Currently using this N-Channel mosfet whose gate threshold voltage(1.6V typical) seems suitable for my application. MCU is STM32F030C8 powered with 3.3V

Schematic:Schematic

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2 Answers 2

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You can either swap drain and source and use a P-channel MOSFET, or drive the gate with several volts higher than the 3.3V supply and stay with the 2N7002 (likely not convenient for you).

You have made a source follower so for output current 250uA it will drop Vgs(th) from the gate voltage (source voltage effectively reduces Vgs).

In the case of using the P-channel MOSFET, input low will turn the MOSFET “on” and high will turn it “off”.

Suitable P-channel MOSFETs might include the BSS84 and the AO3401A for lower Rds(on).

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  • \$\begingroup\$ Wouldn't it waste some power through a pull-up from gate to the source? I forgot to mention that i need a low power solution since this is a battery powered device. MCU will not be able to drive high anymore when it goes to the sleep mode. Can a transistor pair be used to drive the gate? There is 8V available before the regulator. \$\endgroup\$
    – Berkays
    Dec 6, 2021 at 23:40
  • \$\begingroup\$ Ah, move the 10K resistor to the high side to minimize power consumption in the off state. In sleep the pin should stay high if you set it high, as long as power is kept on the MCU there will only be leakage currents. \$\endgroup\$ Dec 6, 2021 at 23:45
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With NMOS connected at high-side (Drain to Vcc) you cannot supply NTC with more then 3v3-Vth.

Use NMOS at low-side (Source connected to GND) if you can.

Or PMOS according schematic below - remember the logic will be swapped, so Pin=High will disconnect the NTC.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Useful answer though i need to mark the other one because of recency bias. Hope you dont mind :). \$\endgroup\$
    – Berkays
    Dec 7, 2021 at 0:47

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