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Basic Electronics » Motor Control/MCU power bus

May 27, 2011
by esoderberg
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I have a 12v power bus powering an H-bridge motor control circuit as well as going through a 5v voltage regulator (LM2931) to power my 328p MCU and I2C devices. The MCU and I2C link run very reliably without the motor control circuit in use. When the mc is in use(driving 12v/4amp max actuator), however, I get hangs up every 2-3 minutes in my mcu code and I2C link making me think that I'm getting some sort of momentary power surge back through to the 5v side. My 5v reg has a 100 uf cap at its output. Will a larger cap help here?

Eric

May 27, 2011
by bretm
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It's more likely a power sag than a power surge. A larger cap would definitely be something to try. Get a look at the 5V supply on a scope and you'll see if it's noisy.

May 27, 2011
by esoderberg
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Thanks Bretm,

Concur that the fluctuation is probably low vice high, unfortunately I'm without a scope to get a look as you suggest and may just have to experiment by throwing in a few caps and seeing if it works any better. Any recs on how big a cap is reasonable to try?

Eric

May 27, 2011
by bretm
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Try the biggest one you can find, within safety limits. If that doesn't help then you've eliminated that as solution. If it does help, then it's a matter of scaling down until you find the smallest one that works reliably.

If you just have multiple 100uFs, put at least 10 of them in there to start with. If it helps, then try it with half of them pulled out, etc.

May 30, 2011
by esoderberg
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Bretm,

Tried 900uF cap without much impact. But I also tried going the other way and put in a .1uF on the H-Bridge power side, and that seems to actually help. Also, I have been seeing a difference in reliability depending on PWM frequency going to the H-Bridge. When I get up to around 10KHz it tends to not work well at all. At .5 to 1KHZ (which is as slow as I want to update my actuator position) it works much better but still not good enough. I may have to just separate the mcu/sensor power supply from the H-Bridge/actuator supply, but it's convenient with just one supply so I'd rather avoid it if possible.

Eric

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