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A single message from topic:  Building Yeovil Pen Mill in EM   in forum:  Trackbuilding topics
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Single message by Andrew Barrowman       Topic: Building Yeovil Pen Mill in EM
Posted: 20 Oct 2017 20:14
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Andrew Barrowman
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Martin Wynne wrote:  But I would simply add a pull-up resistor at the servo end. What is the sink/source capacity of the driver?

regards,

Martin.
 
Hi Martin,

You could certainly give that a shot, but if I understand correctly what's going on here, that might actually make the problem worse :(

I'm referring to the cheapest analog servos (SG90 type). When the input goes positive it fires a monostable and that starts a servo positioning cycle. The width of the input pulse provides the desired position information.

There's not too much data available on the chip inside the servo but from what I've seen the input goes straight into the base of a NPN transistor. If the input signal rises to around 0.7 volts, the transistor turns on and triggers the monostable cycle. Rather than increasing the noise margin a pull-up resistor can actually reduce it.

One solution that does seem to help is to elevate the servo's 0 volt rail above the 0 volt rail of the controller. That can be done with a diode, a resistor and a capacitor. The voltage drop through the diode raises the the servo's 0 volt rail by around 0.7 volts and that does add quite a bit of noise margin. Unfortunately it also violates the servo's spec (such as it is), but in practice it doesn't seem to cause a problem.

Oh dear! Maybe we should start a new thread on servos :D

Regards,
Andrew B


A single message from topic:  Building Yeovil Pen Mill in EM   in forum:  Trackbuilding topics
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