Getting started with Pico Pi controlling 5V PTC heating element 100C

Question

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.

Answer 1

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-V_gs 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:

• 2N6040−2N6045 Series,
• TIP120−TIP122 Series,
• TIP125−TIP127 Series

With a (typical) DC Current Gain (h_FE) 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.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

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.

Answer 3

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.