Trim and Flap Relays....to DIODE or NOT ..?

[Chris Lunt]

I wonder if I can lay myself open to ridicule and ask my fellow builders a really numpty question about using diodes in relays.....?

I'm using Vans' RV7 flap motor, Ray Allen Company(RAC) trim switches and pitch trim motor, and wish to incorporate servos to activate my flaps and trim motor.
I'm using AircraftExtra's servos but specific manufacturer aside, my question is far more generic.

I am trying to instal these servos in my flap and pitch trim circuits.....and good old Bob Nuckolls has provided the very schematics I need...

Flap Circuit: http://www.aeroelectric.com/articles/flaps.pdf

Trim Circuit: http://www.aeroelectric.com/articles/trim2.pdf

RAC have also been most helpful in supplying advice on their schematics, my set up uses a RAC rocker (RS2) and sticktop buttons ... shown on page 2 here: http://www.rayallencompany.com/RACme...S2withmods.pdf

I've attached my simplistic shop notes

I realise that collapsing magnetic fields around relays induce reverse current, and hence, the requirement for diodes in the RAC trim schematic and across the trim relay terminals in Bob Nuckolls' Trim wiring diagram above.

Sooooooooo, getting to my point...

Bob Nuckolls does not show diodes in his flap servo diagram above...
To my uneducated eye the flap motor uses the same servo, inherently the same problem must exist...?

I'm guessing I'm missing something somewhere....but should I instal diodes in both my FLAP and TRIM relays...?

Doffing my cap to Bob, RAC and you wonderful folks out there....

[RV6_flyer]

Here is an article about why a diode is used with a relay.


To make a long story short, the current flowing through a coil creates a magnetic field that activates the relay. When switched off, the magnetic field is still there and collapses. The magnetic field creates a reverse current and voltage in the opposite direction. The reverse current and voltage can damage electronic components that are getting power from the same source.

Read the article for a better explination.

[Chris Lunt]

Thanks Gary,

I'd garnered the gist of the reversal of flow,....

I'm confused as to why Bob Nuckolls includes diodes on his trim relay circuit, but not on his Flap relay schematic.

Many thanks for the article and your help sir.

Rgds, Chris

[fixnflyguy]

I have the Ray Allen grip that has additional momentary buttons I used for flaps(one controls down, the other up). I also have aileron and elevator trim, so mine is a little different. I used the standard Ray Allen module for the 4 trim functions, but my flap switches are simply run to the VANs electric flap set-up in lieu of the rocker switch. I have been running this set up for 10 years with no issues. Pat Hatch (on here) also markets some trim/flap control boards that I understand work well. I have attached a picture with the grip I have, which is a bit different than yours.

[Walt]

No diodes needed, the relay coils are generally not large enough to generate an appreciable amount of arcing.

[emsvitil]

Lookup

Flyback Diode

[Pat Hatch]

Think I'll chime in here since I'm mentioned by my good friend Bill E. Early in our relay board development we had a couple of cases where a particular brand of autopilot was being knocked off line whenever the trim button was activated. Come to find out that it was EMI from the relay coils causing this. Yes, I was very surprised, but we changed our printed circuit board to include spike suppressing diodes. Problem solved. Now, how much those diodes prevent arcing across the small switches in your grip is an open question. I suspect there is some, so the diodes will eliminate this probability. Perhaps not much, but the diodes are there and the cost is almost negligible.

[sblack]

Why change from simple Motors to servos? What is the advantage?

[Chris Lunt]

I have the Ray Allen grip that has additional momentary buttons I used for flaps

Many thanks Bill.......

I'm convinced I'm not the first to have figure this one out ��

Rgds

[Chris Lunt]

Now, how much those diodes prevent arcing across the small switches in your grip is an open question. I suspect there is some, so the diodes will eliminate this probability. Perhaps not much, but the diodes are there and the cost is almost negligible

Hi Pat,

Thank you for that input.

I'm no electrician for sure - but I've spent an embarrassing amount of time online reading up as best I can to make an educated guess as to how to wire up my trim relay board. As you've mentioned, the flyback voltage EMI can be significant it seems...

Sooooooo,.... it appears a couple of diodes 'across the relays' can't harm things, but may certainly help..!

Reason to persist then !

Kind Rgds