Servo Wiring question - RC Groups
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Feb 06, 2013, 04:36 PM
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gavoss's Avatar

Servo Wiring question

I'm setting up a 5M plane that will use a separate battery for the servos and the RX. While setting things up and trying them out, I have the following:

RX powered up with a 2400mah battery
Servos powered up with a 5000mah battery
I have a plug into channel 1 of an old RC Direct 72mHz rx that I can check each wire harness and individual servo. (I also tried a Hitec SS and it didn't change the results)

Now, when I attach the signal wire of any servo to plug I get nothing.

If I hook the servo power wires up to the plug the servo functions.

What gives?

All help is appreciated.
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Feb 06, 2013, 06:15 PM
Registered User
Can you post a diagram that makes it easier to visualise?

The servo will need positive and negative from the 5000mah battery (via a 5v BEC). It also needs its signal wire and negative connected to the Rx. It would make good practice to connect the negative from each battery together as well forming a common ground

Feb 06, 2013, 10:45 PM
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gavoss's Avatar
That's what I found out. I had to connect a wire from the 5000mah battery to the negative on the plug, thus creating a common ground. I'll draw a picture up and maybe someone else can be helped.
Feb 07, 2013, 03:57 AM
Registered User
That's exactly what you'd expect. You can't use a signal connection without anything to reference it to. One wire signal connections have never really worked that well .

Feb 07, 2013, 09:10 AM
Dave the Rave
dmccormick001's Avatar
If you look at the circuit board of any R/C receiver, you'll see that the (+) and (-) connections for every servo plug, including the one where the battery plugs in, are all common. Sometimes they are called "rails", and all they do is distribute power to all the plugs uniformly. The receiver is able to control each servo individually via the signal wire, and the signal wire's reference voltage in relation to ground (-). So at the very least, you need the signal and (-) wires from the receiver going to each servo, the (+) and (-) from the second battery going to the servos, and both (-) connected together.

But the big question I have is "Why?" The receiver draws very little current itself, most of your battery usage is because of the servos, it seems kinda "overkill" to employ a separate battery for Rx and servos. I'd recommend just using the bigger battery to power both, the extra current used by the Rx will almost certainly be so little that you'll never be able to tell any difference, and not carrying the extra weight of the 2400mAh battery should improve performance.
Feb 07, 2013, 09:33 AM
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But the big question I have is "Why?"
If 5m is the span, the plane is rather large (15ft) - the servos might see some pretty heavy loads and drag voltage down enough to cause brownout. If the rx has its own power supply that is independent of any servo activity, probability of brownout could be less. Whether separate rx/servo power supplies are really necessary, I can't prove one way or another, but I do understand the logic behind this. Ymmv.

Oh, btw, while it is true that all grounds need to be tied together, there shouldn't be any closed loops in ground wiring (just a remainder, I know you all knew that).
Feb 07, 2013, 10:56 AM
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gavoss's Avatar
It's a customers plane and he wants 5 cells for the servos and 4 for the rx. Also, the power wires are much larger than the signal wire, 20g vs 26.
Feb 07, 2013, 01:32 PM
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gavoss's Avatar
Photos attached.
Feb 07, 2013, 04:08 PM
Registered User
Your diagram with common ground is correct.

When I use separate power sources -- e.g. for retracts or swing-wings -- I achieve the same thing a different way: I splice in another black wire to the existing black wire on the servo that's going to get the external power supply, then one black wire and the white go to the receiver for the signal, and the other black wire and the red go to the separate power source. Makes it simple for me to follow, and a servo plug with two pins in it is less likely to fall out of the receiver socket than one with only one pin

If you don't wan't to mess with the servo wires themselves, you can make a separate harness that splits the wires the same way.

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