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I want to control a low voltage (hobby class) motor with a 8Mhz/3.3v Arduino Pro Micro. I'd like to power both from 2 alkaline cells. The motor draws approximately 1 A @ 3V with new batteries. I would be doing PWM of a couple of channels at the normal Arduino rates.

My thought is to connect the batteries to the Vcc pin (not Vraw) and run the Arduino unregulated at about 3V, but somewhat less when the motor is running. I'm willing to reprogram the fuses for a lower brownout voltage (or none) if needed.

I'm feeling my way on the rest of this - I'll lay out my (perhaps naive) approach and get some feedback about whether I'm understanding it, and how to proceed.

With such a low battery voltage, a TIP120 darlington seems to drop too much voltage and substantially reduce the motor speed/power. So I'm thinking that I need to use a power N channel MOSFET, and it appears that I need to be fairly selective to get one which can sufficiently turn on with a <3V gate to source voltage.

I'm considering something like the

If I read the datasheet correctly, Vgs of 3V and a load Ids of 1A would result in less than 100mV drop Vds. But this transistor may be overkill, and the TO220 package is larger than I'd prefer.

If I'm willing to deal with SOT-23-3 surface mount packaging, there are some options like:

If I'm reading it right, with 2x1.5v (nominal) batteries directly powering an Arduino Pro Micro, I can sufficiently drive Vgs to handle 1A Ids at a small Vds and a low enough power dissipation, with any of these three.

I would put a reverse diode and a cap across the motor. I've seen devices with a small electrolytic across the motor terminals, and I've seen a smaller value ceramic cap - what's the best practices?

Is this a feasible and reasonable approach? What are the gotcha's that I need to take into consideration? Any advice about choosing among these (or other) transistors?

My concerns are: not understanding MOSFETs enough, and the unregulated power supply being too noisy for the ATMega328p when it's also powering the motor. Can I handle the latter by adding another cap or two across the Arduino Pro Micro's Vcc/Ground?

Backup option: run 3x1.5v alkalines and power the Arduino Pro Micro through its regulated Vraw input.

  • The FET seems to work, tho using PWM creates a lot of audio noise unless I use the highest PWM frequency. My current concern is motor noise on the power supply. As it turns out, I also want to operate a radio transceiver like the nRF24L01+, which is somewhat finicky about its power supply. This issue of a noisy battery power supply and how to mitigate it is part of the original question (other than adding the nRF) - should I pursue that aspect here or start another Q? – Zeph Nov 29 '14 at 18:44
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I don't think the mosfets will be an issue. Looking at the last one (BSS806NE); in figure 5 in the datasheet you see that for 1A and 1.4v (Vgs) you'd get a Vds of only .1v.

I think a bigger problem might be the voltage of the batteries dropping under load. You'd have to measure that with a multimeter while the motor is running. A simple cap might help with a little noise, but not with the voltage dropping while the motor is running. Not sure how much the AVR would mind having its supply voltage dropping all of a sudden.

Of the top of my head the minimal voltage of the Atmega328 @8mhz is 1.8v, while default fuse settings use 2.7v, so I'd probably change to fuses to make BOD at 1.8v. Other fuses don't need to change.

Backup option sound good, though you might have to go with 4 x 1.5v depending on the voltage drop under load. Another alternative might be to have two sets of 1.5v batteries in parallel, halving the current load on each battery.

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