Heat regulation circuitry

Here’s the heating circuit I plan to use snaffled from here:

Mosfet switch

The Load is an SMD resistor about the same size as the MPU6050 chip, attached to it with thermally conductive sticky tape.  It needs to run near its power capacity (= melt down) when loaded with ≈ 5V.  SMD resistors of the right physical size have max. power settings ≈ 1W meaning 0.2A will trigger meltdown.  A resistance ≈ 5 / 0.2 = 25Ω will serve the purpose perfectly

The mosfet needs to have a low internal resistance itself, so the heat is generated primarily by the resistor.  The 2N7000 shown has a wide range of specifications depending on the manufacturer.  The best I’ve found is 2Ω, so the mosfet will drop 5V * 2 / (2+25) = 370mV and 0.37 * 0.2 ≈ 0.075W.  That’s about 12% of the chip’s maximum power rating of 625mW.

Alternatively, a BS170 has a 1.2Ω source gate resistance, dropping ≈ 240mV and ≈ 0.05W  – 6% of BS170 specified 830mW.

The GPIO is PWM driven so that resistor meltdown is avoided with feedback from the MPU-6050 internal temperature sensor.

There are a few niggles left:

• the phone charger battery bank I use to power the Raspberry Pi needs to be able supply up to another 200mA but in pulses – will it cope?.
• Will the Raspberry Pi polyfuse cope with this extra power drain?

I guess I’ll just suck it and see.

4 thoughts on “Heat regulation circuitry”

1. You might want to throw a capacitor in there – switching that much current is likely to cause spikes. You can also take the 5v from the source instead of the fuse.

• I’m considering both of those options, and I’m taking it step by step. The power comes from a battery (one of those phone charger lithium packs) which I suspect will have a capacitor in the built in switching regulator and so may may be able to cope with the power surges without causing spikes. But I really need the 5v to come from the GPIO pin so I’m hoping the polyfuse will hold.

Only testing will see. At the moment, I’m at the stage of using an LED instead of a resistor to check the PWM controls the perceived brightness as needed.

I’m taking this very carefully step by step to make sure I don’t blow anything, checking every step with my ‘scope for spikes etc.

• My electronics is very out of practice so I might be getting confused. (I think I mentioned that it’s been thirty years since I actually studied any electronics, and I certainly haven’t done any electronic design since then).
I assume you are worried about current supplied to the gate from the GPIO being too much for the polyfuse. I was unable to find an I value for the gate in the datasheet here, apart from a gate-body leakage current of 10nA under the Off Characteristics section, and that at 15V not 5V. In any case I am under the impression that a great thing about FET devices is that gate current is very low, so should not cause an excessive load on the GPIO.

• You’re absolutely right about the FEC gate – it’s just switching PWM with virtually 0 current draw.

My concern is that the 5v feed across the source / drain via the resistor could blow the polyfuse when combined with the current of the Raspberry Pi itself – at full power 5V over a 25ohm resistor is 0.2A.

My prototyping board is a Model B, so drains a fair current by itself, combined with the linear 3v regulator wasting energy, but once I move to the B+ board with a 2A polyfuse, I know all will be fine.

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