6 replies to this topic

[Fred]

I've got an issue with a Netduino Mini project (although I suspect it would be the same on the Plus if I tried). I think I know what things to try but thought I ask for opinions. Mainly because I'm under some time pressure and haven't got some of the components I suspect I'll need. I've got a project that uses servos - 2 x Hitec HS55. They're powered from the Mini's 5V out and operated with PWM as you'd expect. Powered via a wall wart they work fine. Powering the Mini using a battery and they go crazy. (Tried a 9V and also 6V although I know it's a bit low.) I would have thought the battery would be OK driving the servo motors, so suspect they're mucking up the 5V supply when battery driven. The options I'm considering: - Use a separate 5V regulator to drive the servos. (Not got one to hand right now.) - Stick a kickback diode and a capacitor across the 5V line in case the motor's generating back EMF. Do these sound sensible? Any other ideas? I'd have thought both of these would be an issue running of a wall wart too. Fred

[Chris Seto]

"Use a separate 5V regulator to drive the servos. (Not got one to hand right now.)" That's the best option. Servos can pull more current than the Mini's Vregs are suppose to handle. Also, 9V is _much_ too high to be feeding into servo Vin. You want 4.8V to 6V maximum, unless otherwise specified.

[Fred]

Thanks, Chris. I had a feeling you might be the man for anything servo related. I wasn't thinking of supplying 9v direct to the servo - just through the Mini's regulator. Odd that 9v (or even down to 6v) from the wall wart works OK. I might try 6v direct to both the servos and the Mini's Vin. I know 6v is a bit under-spec for the Mini, but it seems to work using the wall wart at least. It's OK according to the Servo's spec sheet. I might still get issues, but worth a try whilst waiting for the regulator order.

[ItsDan]

The 6v may work fine even though it's under spec depending on the application. If you are only turning on a pin or two vs using all 20. As long as you're aware you're underpowering it and may need to adjust that down the road you should be fine.

[Fred]

A quick try this morning - 6v to servos and mini still misbehaved. Then adding a 100uF capacitor to smooth out the supply a bit seemed to do the trick. I should probably stick a flyback diode across there too - something else I'm out of at the moment. It does seem it's an issue with noise on the power line rather than power draw. A serapate regulator would definitely seem the best option. Tough little things these Netduinos - coping well with all sorts of unreasonable treatment!

[Mark H]

What sort of battery are you using? Can it supply enough current?

[Fred]

I was using a 9v PP3 battery, and solved the problem the quick and dirty way by just wiring 3 9v batteries in parallel. They're just too puny on their own.

I think that in the end the problem was that when the servos drew current it pulled the Vin for the Netduino too low. Supplying the Netuino from a separate 5v regulator (but same battery) didn't work.

The proper solution would have been to use a better quality battery, but the project only had to work for an hour so I just got it over and done with. This was the finished project, by the way.