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I have just started with the Pico Pi and have no idea where to get info on how to power 5V 3A PTC heating element using the Pico and a MOSFET. Would there be anyone who could give me a pointer as I have been going in circles.

Colin Thomson
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    https://raspberrypi.stackexchange.com/questions/70931/how-do-i-do-a-basic-mosfet-switch-with-pi may be of help. It applies to a RPi, rather than a pico, but the advice still stands. TLDR: most MOSFETs do not work well with 3.3V, so find a different solution... – David Arno Feb 01 '23 at 08:50
  • What device you are using to control the unspecified element is irrelevant. The circuitry would be the same regardless of controller. – Milliways Feb 01 '23 at 09:00
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    I appreciate the pointers, thank you – Colin Thomson Feb 01 '23 at 09:02
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    This might help: https://www.circuitlab.com/circuit/b45c4a7snvv9/irl540n_2023feb0103/ – tlfong01 Feb 01 '23 at 12:58

3 Answers3

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Don't use a MOSFET with Pico (or Pi) GPIO; use a bipolar transistor.

Why? RPi and Pico GPIOs switch between Gnd & 3.3V; most MOSFETs need a higher gate-source voltage than that to reduce the drain-source resistance. The MOSFETs that do operate as a proper switch at 3.3V gate-source generally come with tradeoffs that are undesirable in prototypes & hobbyist construction. For example:

  • They often come in tiny packages designed for surface-mount construction techniques.

  • In general, MOS devices are more vulnerable to ESD-induced failures due to handling and unplanned events.

If one is building a performance-critical, production prototype device, there may be strong incentives to use low-Vgs MOSFETs in a Raspberry Pi project. But there are tradeoffs associated with most all decisions; e.g. ease-of-use & component ruggedness vs better performance. As your question is currently worded, you're better-suited to make those tradeoffs than we are.

OTOH, there are numerous bipolar Darlingtons that will handle your 5V, 3A load:

With a (typical) DC Current Gain (hFE) of 2000 or higher, one of these Darlingtons can efficiently switch 3A with a tiny 1.5 mA source current from the GPIO - well within their range. They are available in "leaded" packages (e.g. TO-220, etc) for easy breadboard construction, and fairly cheap (<$1 USD last I checked).

A heating element can be driven with a "low side" switch arrangement as shown below.

You can find data sheets online for the Darlington devices listed above. Let us know if you have questions.

schematic

simulate this circuit – Schematic created using CircuitLab

Seamus
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I would recommend using a MOSFET, a darlington will lose about 1.4V when saturated leaving you 5-1.4V = 3.6V for the heater. A MOSFET will have a much lower drop. Look at the Si8424DB, Si8424DB, RFP30N06LE as a starting point. There are many others that will do more current with more becoming available every week.

Gil
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I have been running a rp for at least 7 years 24/7/365 as a radius server. It is on stable power and running headless. Other than clearing the log files once in a while and blowing off the dust, that's all I need to do. I do complete backups of the data every night.

George F
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