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Boeeerb MotorPiTX Update

Jan 30, 2014 - 3 minute read -

If you read my recent review of the Boeeerb MotorPiTX, you will be aware that I had trouble with powering the Raspberry Pi and the MotorPiTX board from the MotorPiTX power socket, and resorted to ignoring the smart power circuitry and using the usual Raspberry Pi micro-USB power input. I'm happy to report that this was not actually a problem with the MotorPiTX board, but rather with the way I had chosen to power it.

I had been doing some experimentation with battery-powered circuits, particularly using an Arduino Uno, and I had on my desk a 9V PP3 battery pack with an appropriate barrel jack, so it seemed an obvious choice to plug that in to the MotorPiTX power input. The 9V was seemingly enough to power up the MotorPiTX regulator circuit, but I now realise that the battery was not pushing out enough juice to reliably start the Raspberry Pi. This was particularly apparent because, althoiugh the lights were flickering, it never got as far as displaying anything on a monitor, or even attempting to connect to the network. When I replaced the battery pack with a 9V 500mA "wall wart" power supply the Raspberry Pi started with no problems. The Relay does still clatter a little when power is first applied, but after that, the little button works great.

It seems that the system, when powered from the MotorPiTX board, is pretty sensitive to power, though. Flushed with success after getting the Raspberry Pi started and stopped a few times, I tried to run the simple motor control example I used in the previous post. I'm guessing that this drew too much power from the little wall-wart, as it immediately removed power and crashed the Raspberry Pi. A bit discouraging, as the whole point of the smart power-supply circuitry is to help prevent such hard crashes. I'm sure that with a sufficiently powerful energy source this would not be a problem, but it is pretty drastic. Presumably this could happen at any time to a deployed project when a motor happens to ask a bit too much of the available power. In an ideal world the CPU would keep running and be able report an error or try some other fallback if a motor fails or draws too much power.

For a reliable system, I would either want some sort of "ring fenced" power available to the Raspberry Pi which can't be drained by motors, or (probably more likely) two separate power systems, one for motors and one for electronics. If we still want the comfort of a smart power supply, then a separate unit such as a Pi Supply seems a reasonable choice.

Posted by Frank Carver ATX battery boeeerb current hardware motor MotorPiTX power shutdown voltage

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Comments

  1. Simon Walters

    Hi Frank I think you've completely misunderstood how this board is intended to be powered :) Its a motor controller board and would normally be powered from a set of AA batteries (I'd recommend 6x NiMh 2300mA Energizer Extreme Rechargeables giving nominal voltage of 7.2V). Using that sort of source, the MotorPiTx functions very well and can make your RPi run all over he floor without a trailing wire back to a mains socket. Hopefully, you can get yourself a set of batteries and try it out and THEN write about it :) regards Simon

  2. Frank

    Sure. Perhaps naively, though, I guessed that one 9V battery would be broadly equivalent to six 1.5V batteries, at least for the duration of a short test. However, it was not enough to properly start up the Raspberry Pi when I did that. I'll try a pack of six AA batteries tomorrow and see if that works any better!

  3. Frank

    As promised, I have now tried the same test with 6 fresh AA cells and the Pi starts and runs the test happily. I'm still slightly puzzled as to why it would not start when powered by a single 9V battery, though.

  4. Jason

    It's all down to the mAh (Milli-ampre an hour) rating of batteries, and the amount of 'push' they give. A typical 9v battery gives around 600mAh, whereas a rechargeable AA can give up to (depending on chemistry - NiCad, NiMH etc) 2,500mAh each. So a Model B Pi alone will require 700mAh which before the regulator exceeds the 9v capabilities. An Arduino can live with 25mAh, so can happily live on a 9v battery for hours. So lets add some motors, the type I use are around 400mAh each (using 2 of them) and a stall current of 800mAh, which means if it hits a wall with the motor at full power, the motor will pull up to 800mAh in an effort to move. So 6xAA @ 1.2v 2,500mAh = 15,000mAh = 15Ah (Ampere hours) at 7.2 volts. This can provide the motors with power for about 18 hours alone. So add on the Pi (700mAh), and a Wifi dongle (about 500mAh), the MotorPiTX circuitry (50mAh). In my testing I get around 5 hours usage before power problems start using 6xAA @ 2,000 mAh. Using Li-Po batteries will extend life much further as some can give 7.2v rated at 40Ah. I hope this makes sense, but battery choice for a robot is definitely a big choice to make as they all vary so much.

  5. AndrewS

    Yup, except you're confusing current (measured in Amps) with battery capacity (measured in Amp-Hours)... A battery rated at 2,000 mAh is theoretically capable of supplying 2 amps of current for an hour, before being totally drained, or 1 amp for 2 hours, 500 mA for 4 hours, 4 amps for 30 minutes, etc. Except the internal resistance of a battery means that there's only a maximum amount of current that a battery is able to provide; and AA batteries (especially NiMH) are able to output much more current than 9V PP3 batteries. A quick google finds more discussion here.

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