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Topic: I'm getting a lot of static when driving servos. (Read 772 times) previous topic - next topic


I just remembered something. 

My boards don't have an onboard voltage regulator.  I use an external regulator or USB power to supply the 5V I need.  Or I power it directly from 3 AA batteries.  This means I couldn't add a linear regulator for my audio, because to supply 5V to the audio circuit I'd need to supply the regulator with more than 5V.

I suppose I could create a servo control module of some kind which uses an external power source, but the whole point of this board is to be as small as possible and be a light, sound, and motion controller.  A separate servo module would likely not fit in a small handheld prop with servos.

Any other ideas for how I could prevent noise from getting into my audio circuit?  Inductors where the digital power and ground planes connect to the analog ones perhaps?  I've seen inductors in some papers I've looked at.  But my budget and space constraints didn't really allow for them at the time and I thought I could get away without them.  Is it possible to get really tiny inductors? 

Oh, and the reason I used an external regulator was because late in my design process I discovered the linear regulator I was going to use wasn't going to be able to supply more than 2A without melting my board down and I didn't have space for a heat sink, or the money, space, or time to add a switching regulator, so I ended up purchasing a bunch of fixed 5V switching regulators to get the job done.


Jan 09, 2013, 08:15 pm Last Edit: Jan 09, 2013, 08:18 pm by scswift Reason: 1
Well, I just did a couple tests with some interesting results.

First, I discovered that my cheap chinese switching regulators which are supposed to supply up to 3A seem to fail when I use them as my power source and try to drive a servo.  Mostly I've been running off USB power when doing my tests, and I know that can only supply up to 500mA.  So something weird is going on.  I know the switching regulators can actually supply up to 5A peak because I tested them with my multimeter. 

So perhaps the issue is the switching regulator is unable to supply power quickly.  There's some 220 ohm caps on the input and output of the regulator though, so I'm not sure what more could be done there.  The regulator on these boards is the LM2596S: http://www.ti.com/product/lm2596

Or, maybe the problem is the power source itself, which is six C cells of unknown age.  I don't have any LiPoly batteries to test with though.  I suppose I could try fresh C cells.

Anyway, the other test I did was to run the main board on USB power, power the servo with the voltage regulator, and connect the grounds.  This worked like a charm.  The huge amounts of static I was getting disappeared.  So I know the static isn't coming from the servo's signal line, or from emf getting into my audio lines.  It must be from the power.  Perhaps an inductor or some caps would help there, but until I know the voltage regulator can even power the servo at the same time as the board with an adequate power source, I'm not gonna mess around with that.

At least I know now I can drive servos without noise if I use a separate power source.  That at least is a workable solution.   

Oh, and I did finally test the vibration motor going with a quiet sound effect going and I do notice that I can hear some quiet pops with that.  So even the vibration motor isn't immune from causing noise.  I guess I should have used an inductor on that, but it's not really a major concern since the only time you really use the vibration normally is with loud sound effects.


Just a quick update to mention that I found one of those clip-on ferrite things on a spare cable I had and stuck it on the servo wire to see if that would have any affect on the noise, but it didn't make any difference.   

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