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servo filter capacitor

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user9253

Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan

Posted: Wed Aug 04, 2010 6:09 am Post subject: servo filter capacitor

The plans call for a 2200µfd capacitor to be connected to the positive power wire at the autopilot servo.  The servo is not electrically connected to the aircraft frame at the servo.  Instead it has a black negative power wire going back to the instrument panel.  Should the negative capacitor lead be connected to aircraft ground at the servo or should it be connected to the black wire?  Or does not it matter?

Thanks, Joe

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Aug 04, 2010 6:41 pm Post subject: servo filter capacitor

At 09:09 AM 8/4/2010, you wrote:

 	  Quote: 	
		  



The plans call for a 2200µfd capacitor to be 

connected to the positive power wire at the 

autopilot servo.  The servo is not electrically 

connected to the aircraft frame at the 

servo.  Instead it has a black negative power 

wire going back to the instrument panel.  Should 

the negative capacitor lead be connected to 

aircraft ground at the servo or should it be 

connected to the black wire?  Or does not it matter?


	


    It may not matter. I'm skeptical of

    any such device hung on the outside of

    a product. I had to do it once . . .

    a cabin vent blower speed controller

    that generated so much noise that

    we had to bolt a fat electrolytic

    capacitor to the outside of the housing

    (UGH!). That's a goof that follows me

    around and chews on my buns every time

    I work in forward avionics bay of the

    King Air.



    Does anyone put down a reason in writing

    why this capacitor is necessary? Noise

    from the actuator? System noises getting

    into the actuator that upset its operation?



    Understanding the antagonist/victim/coupling

    combination that is presumably broken by

    adding this capacitor is key to wiring

    it for greatest effectiveness.



    Who's autopilot are you talking about?

   Bob . . .

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user9253

Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan

Posted: Wed Aug 04, 2010 8:05 pm Post subject: Re: servo filter capacitor

 	  Quote: 	
		  Does anyone put down a reason in writing why this capacitor is necessary? Noise from the actuator? System noises getting into the actuator that upset its operation? 	




No, there is no explanation.  The capacitor came with the kit and is called a "filter" and is to be installed at the servo.



 	  Quote: 	
		  Who's autopilot are you talking about? 	




It is a Dynon servo connected to the FlightDEK-D180 and AP74.



Thanks for the reply,

Joe

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Aug 04, 2010 8:48 pm Post subject: servo filter capacitor

At 11:05 PM 8/4/2010, you wrote:

 	  Quote: 	
		  

 > Does anyone put down a reason in writing why this capacitor is 

 necessary? Noise from the actuator? System noises getting into the 

 actuator that upset its operation?

No, there is no explanation.  The capacitor came with the kit and is 

called a "filter" and is to be installed at the servo.


	


   Hmmmm . . . I would hope that their instructions

   are specific as to where it grounds. Are these Dynon

   FlightDEK or third party instructions? I think I

   know a guy at Dynon I can ask about it.

   Bob . . .

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user9253

Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan

Posted: Thu Aug 05, 2010 6:36 am Post subject: Re: servo filter capacitor

 	  Quote: 	
		  Hmmmm . . . I would hope that their instructions are specific as to where it grounds. Are these Dynon FlightDEK or third party instructions? I think I know a guy at Dynon I can ask about it. 	




Third party.  The 2200µfd capacitor came with the RV-12 autopilot kit.  And yes, the RV-12 directions say to ground the negative lead to the airframe.  I questioned that because any noise generated by the servo motor will have a longer path through the airframe.  I assume, but do not know, that the servo motor is the antagonist.  Actually there are two servo motors (pitch and roll) connected in parallel, but there is only one capacitor.  It is connected to the pitch servo which is the one closest to the instrument panel.



It is possible that the intent of the 2,200µfd capacitor is to help smooth the voltage from the single phase Rotax dynamo.  But there is already a 22,000µfd capacitor connected to the input power to the instrument panel.

Joe

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Aug 05, 2010 9:00 am Post subject: servo filter capacitor

At 09:36 AM 8/5/2010, you wrote:

 	  Quote: 	
		  

 > Hmmmm . . . I would hope that their 

 instructions are specific as to where it 

 grounds. Are these Dynon FlightDEK or third 

 party instructions? I think I know a guy at Dynon I can ask about it.

Third party.  The 2200µfd capacitor came with 

the RV-12 autopilot kit.  And yes, the RV-12 

directions say to ground the negative lead to 

the airframe.  I questioned that because any 

noise generated by the servo motor will have a 

longer path through the airframe.  I assume, but 

do not know, that the servo motor is the 

antagonist.  Actually there are two servo motors 

(pitch and roll) connected in parallel, but 

there is only one capacitor.  It is connected to 

the pitch servo which is the one closest to the instrument panel.



It is possible that the intent of the 2,200µfd 

capacitor is to help smooth the voltage from the 

single phase Rotax dynamo.  But there is already 

a 22,000µfd capacitor connected to the input power to the instrument panel.


	


    Yeah, hard to tell. If the servo grounds locally

    to airframe, I'd put the capacitor ground there

    also. The capacitor across the single phase

    rectifier regulators has minimal benefit. See:



http://www.aeroelectric.com/Pictures/Curves/SD-8_Noise_Data.pdf



    While there ARE effects for adding the capacitor,

    they don't reduce the ripple voltage markedly.

    Nor the the battery have any real effect on

    ripple voltage. This is what Mil-STD-704 tells

    us to EXPECT on a DC power bus . . . the righteous

    designer learns to live with it. Adding capacitors

    after the fact is probably whistling in the dark.

   Bob . . .

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