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Old Oct 19, 2014, 10:23 PM
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smewr's Avatar
United States, PA, Easton
Joined Jan 2009
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Induced signal into servo

Hi, Sort of new to 4-servo glider wing set-up.... with flap and ail. servos tied into the same power and ground, I was testing my aileron servo with my trusty HK servo tester, and noticed that my flap servo was affected to the point of moving to a far stop position.... my flap servo was unplugged from the tester, but the flap servo still has power thanks to the single power/ground feed going out to the servos... took me hours of checking splices/you name it, and finally pulled the wiring out of the wing between the center of the wing where they all pop out, to the flap servo...and figured out that if the unpowered servo signal wire it laid next to the signal wire of the other servo that is being tested, for about 1 1/4", strange stuff happens.... the only thing I can figure, is that the servo that you are actually testing is INDUCING a signal into the other servo, (which is actually powered) even though it is unplugged (see above). One "fix" seems to be; don't allow the two signal wires to lay next to each other, which isn't really realistic in my case since I have to stuff 4 servo wires through a small channel.... the other option is; don't unplug any servos.... if you leave all servos plugged in, this problem doesn't happen.... my main concern about this issue is if you unplug a servo, and move another servo (either with a tester or by radio), and lets say the flap servo gets moved to the far stop via induced signal, you can harm your servo/linkage/ etc, for instance you only want your flap to move down, and now it gets jambed up, which is tough on the hardware..... anybody out there seen this issue?
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Old Oct 20, 2014, 06:39 AM
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Canada, ON, Rockland
Joined Aug 2008
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What servo ???? (HK servo?)
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Old Oct 20, 2014, 09:22 AM
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United States, PA, Easton
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HS-85MG.....new
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Old Oct 20, 2014, 12:24 PM
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ivanc's Avatar
United States, TX, Round Rock
Joined Dec 2004
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If the signal input of the servo is high impedance you will get induced signal from an adjacent signal wire IF the wire in question is not connected to a receiver output and long enough. Once the wire is connected to the low impedance receiver output the induced small current will be dissipated by the receiver output stage.

Think about it like this - a loose wire connected only to a high impedance input acts like an antenna.
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Old Oct 20, 2014, 01:48 PM
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Greeley, Colorado, USA
Joined Feb 2000
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When the power leads of the wing servos are ganged together and you connect one to a servo tester or receiver, it's pretty common for the other's to see some effect.
Usually they don't move drastically, but often twitch a little.

Not a big deal though, as in normal use the servos will all be connected to the receiver.


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Old Oct 20, 2014, 02:28 PM
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Quincy IL
Joined Jul 2006
528 Posts
Just change to twisted servo extensions if more than one is parallel to each other as in a servo lead channel in a foam wing or in a tube internal to the wing.

Cross talk is not your friend.
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Old Oct 20, 2014, 11:50 PM
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United States, PA, Easton
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Roger everyone......thanks for the info.... the issue had me baffled for hours, but for some reason, and this is rare, common sense kicked in and I didn't tear everything apart looking for the "short"....
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