Ferrite rings on servo leads

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Tony Newman
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Joined: January 20th, 2010, 3:59 pm
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Ferrite rings on servo leads

Postby Tony Newman » July 6th, 2011, 7:24 pm

On some large models where the servo leads can be longer than 1 metre, I am aware that ferrite rings are used on the servos leads to increase the lead inductance and provide a more reliable data link. I have observed jittering servos myself with long leads although it is not always repeatable.

In an ideal world the servo leads would be treated as a classical transmission line and be terminated at both ends with 50 or 75 ohms. Lead length is then non critical. Power considerations do not allow us to adopt this approach in our models although it is used in the fullsize world e.g the 1553 data bus as on most military aircraft.

The use of ferrite rings on our models is a sub-optimal but practical approach but I have not seen any good practice guidelines for use on large model. e.g Where in the lead should the ferrite ring be placed - Rx end , middle or the servo end ? How many times should the lead be looped through the ring - this effects the inductance value ? What is the minimium lead length to use a ferrite ring ?

Has anyone any practical or theoretical advice on these issues and should such advice be in the LMA Handbook ?

Alan King
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Joined: May 8th, 2011, 3:38 pm
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Re: Ferrite rings on servo leads

Postby Alan King » August 26th, 2013, 6:01 pm

Hi

following this question i contacted a well known gentleman in South Africa who originated from these shores and who runs a shop called Radio Control repair centre in Pretoria his name should be well known to some of the older model enthusiasts , Dave Armitage, Dave has publised many articles and designed many eloctronic systems besides being a JR official repair specialist, below is his answer to the posted question.

I must admit to being a bit surprised at you saying thet the LMA handbookdoesn't contain any advice on long servo leads

Mind you the use of 2.4 GHz rather than 35Mhz has helped dramatically with the interference that some long leads can cause

I will use ferrite rings on any servo lead over a metre long. I only use a heavy twisted cable (.5mm minimum copper conductor) and I fit the rings as close to the Rx as I can get them.

With the rings I have I can get 5 or 6 turns through the ring and I then fit a piece of heat shrink tubing over the ring to keep it all in place.

I also try not to have any plug and socket joints in any servo cables either. On my own installations I actually solder the cable into the servo and the

only joint then is where it plugs into the receiver or power box.

My Boomerang Torus has elevator and rudder servo cables over 2 metres long without a joint .

There are ferrite rings on these cables and they terminate in a power box which helps dramatically with any noise coming into the back of he Rx. I've had no twitching servos with this setup

Most servo oscillation hassles can be traced to either too many joints in the cables increasing the resistence or poor quality cables themselves. I have seen very many Far Eastern extension leads with very thick insulation and very little copper inside. The servo oscillates because when it is commanded to move or even centre at switch on, the voltage drop down poor leads will make it basically lose control so it overshoots. When it coasts to a stop it is then commanded to reverse and the same thing happens. A powerful digital servo starting up can draw as much as 2.5 to 3 amps instantaniously and that 3 amps down a cable with a resistance of half an ohm in it can cause a 1.5 volt drop.

The new Futaba "S" bus system that is coming in uses a form of serial data bus which means the servo lead lengths are not so critical. The only thing about this system is making sure the furthest servos from the power supply are still getting the power they need to operate correctly.


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