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I am working on a project where I need to control this water pump using a Raspberry Pi 3B+.

Currently, I've chosen the IRLD014PBF N-channel MOSFET to drive this water pump. For added protection to my RPi GPIO, I've decided to incorporate the TLP620-2X opto-coupler into the circuit.

RPi MOSFET water pump circuit

I have attached the schematic for review and would appreciate feedback on the component selection. Specifically, I am unsure about the chosen MOSFET and opto-coupler, and the necessity of resistors R1 and R2 (would like to know if they are required, as well as recommendations for their values).

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  • \$\begingroup\$ Add a resistor between the gate and ground (1k Ohm). \$\endgroup\$
    – Antonio51
    Nov 11, 2023 at 10:37

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You should consider using this modified circuit: -

enter image description here

Regarding the MOSFET (rated at just over 1 amps continuous), it's unclear what current the pump takes despite reading the data sheet. I suspect that the MOSFET may overheat if you don't choose something with a little more capability.

OK, I've re-read the data sheet and, it seems to imply the pump takes between 65 mA and 500 mA. However, if the pump got blocked (quite a common thing) then the current may exceed the rating for the MOSFET.

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  • \$\begingroup\$ Thank you for the modified circuit! Regarding the water pump, I may be implementing an additional circuit for current sensing, to prevent the current from exceeding the MOSFET's ratings. As for the need for a heatsink on the MOSFET, I've completed the following calculations: with a current consumption of 750mA and an Rds of 0.2 ohms, the power dissipation is 0.1125 W. Given the MOSFET's thermal resistance of 120°C/W, the temperature is expected to rise by approximately 13.5°C. Do you think it would be adequate to operate without a heatsink? \$\endgroup\$ Nov 11, 2023 at 10:48
  • \$\begingroup\$ @Engineering007 I personally think you are pushing it too hard and that a MOSFET with lower on resistance would be a more reliable choice (maybe 0.1 ohms with a Vgs of 5 volts). It's just a little too close to the "unreliable" area to make me convinced. \$\endgroup\$
    – Andy aka
    Nov 11, 2023 at 10:57
  • \$\begingroup\$ Regarding the heatsink, because the localized temperature around the MOSFET rises (due to power dissipation) you ought to factor that in. I mean, I don't know what external ambient temperature you are expecting or, how the MOSFET is enclosed to make a judgement call on this. \$\endgroup\$
    – Andy aka
    Nov 11, 2023 at 10:59
  • \$\begingroup\$ Agreed, the initial MOSFET choice was somewhat optimistic. Based on your advice, I've opted for the IRLD024PBF, featuring an Rds of 0.1 ohms @ 5V and a higher continuous rating of 2.5A (up from 1.7A). This MOSFET dissipates about 56mW @ 750mA, resulting in a ΔT of 6.75 °C. Regarding the electronic enclosure, intended for an RC submarine, it's completely sealed without airflow, yet the surrounding water may provide a cooling effect. \$\endgroup\$ Nov 11, 2023 at 11:51
  • \$\begingroup\$ I feel more comfortable about that choice but, for a sub maybe you should have an internal temperature monitor that shuts down stuff should it rise too high. I guess if it's a metal body it'll be alright. \$\endgroup\$
    – Andy aka
    Nov 11, 2023 at 11:54

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