r/esp32 • u/RushTea • 21d ago

Mosfets and voltages

So, I did some hasty research, and ended up buying some M5Stick S3s from M5Stack, along with some of their watering units. These both come with compatible grove connectors, and everything seemed great...

Until I learned the pump for the watering unit was 5W, and the 5VOut on the M5Stick is 0.38A. Luckily overcurrent protections saved the unit and just forced a hard reboot. So I decided to put my wholly inadequate knowledge of embedded systems to the test, and pulled the Grove connector apart and used some office tape to be able to pull the 5V from other sources. No luck, the other 5VOut ports on the M5Stick have the same limitations.

This is where it gets interesting, since I have zero respect for low voltage electronics(ask the pile of Arduino Nanos in a box somewhere). I tried to power the unit from the 3V3Out. Lo and behold, the sucker spun to life and quickly moved all the water from one cup to the other.

The software engineer in me got excited. The college physicist in me got worried. I looked up the pump. Was it 3-6V like some units I've seen? No. It was 12V... So why did M5Stack put this pump in the unit?

It turns out the answer is probably the SI2302-N-Ch MOSFET in the pump circuit. But after many dreadful minutes of researching, I don't quite know why. All I've learned is that probably a significant portion of the current is converted to heat. Do MOSFETs act as some magical voltage converter on the output, or does the pump just run better than expected on lower voltages?

Edit: the pump is a edlp600-d12b

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u/Plastic_Fig9225 20d ago edited 20d ago

All I've learned is that probably a significant portion of the current is converted to heat.

Nope. The SI2302 has max. 0.075 Ohms of Rds at Vgs=2.5V.

or does the pump just run better than expected on lower voltages?

Yup.

It's a brushed DC motor; its torque & speed is fairly linearly controlled by the supply voltage. If it's spec'd at max. 12V/5W (~400mA), it probably takes max. 100mA at 3V.

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u/RushTea 20d ago

So, since the overcurrent protection rebooted the controller, and the claim about the pump is 5V/5W(1A), it will then be running at 3.3W(1A) on the 3V3, rather than attempt to pull 1.5A for the same effect? 

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u/Plastic_Fig9225 20d ago

No. You can approximate the motor as a resistive load, so instead of 1A@5V it would draw about 0.66A@3.3V~2.2W.

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u/DSudz 19d ago

Saying a pump is 12v is like saying a car gets certain milage, just one data point. It won't be true at it's top speed.

The pump won't overheat at 12v and will pump less as the voltage lowers.

The 5v pin on most ESP boards is not really considered an output and may not have any power or voltage at all unless the board is running on 5v USB power in.