An H bridge is basically performing the same switching function as
your pair of DPDT relays, the difference is the switches are power
transistors instead of electromagnets and springs pulling little
contacts together and apart. It has the advantage of no moving parts
or contact points. They usually offer a speed control and brake as
well. The brake just short accross the motor contacts so that the
motor stops as soon as you release the switch and also so the aero
forces won't be able to spin the motor and move the flight surface.
The friction of the mechanical system might stop this on its own. The
issue for you is going to be to find a nice packaged module with lug
terminals so you can just wire it up and go. Do a search on google
for "H bridge module" or something like that.

National makes a very nice 3amp componant level solution with a 6amp
peak capability http://www.national.com/pf/LM/LMD18200.html
This is just the raw componant and is probably too difficult for you
to use directly.

Or you could build your own H bridge from solid state relays like
Sharp's S108T01.

Find out if your EM actuator has a built in snubber circuit or not.
The snubber circuit is what disipates the energy stored in the
magnetic field of your motor windings. You want that energy disipated
in a safe way so it doesn't fry any of your high dollar avionics.

And on the other end of the sophistication spectrum is just using a
momentary contact DPDT on-off-on switch instead of the switch
currently built into your stick grip. Maybe panel mount?



"Jeff" my last name @fidnet.com wrote in message ...
Jay,

I am not totally set on using a relay. As you can tell from my post, I am
not an electrical engineer by any stretch of the imagination. I can wire
things up and am very comfortable with stuff like that, I just need a little
nudge in the right direction. Please tell me more about what an H-Bridge
is.

Thanks!!

Jeff

"Jay" wrote in message
om...
Unless you're in love with relays, you could use a solid state
solution such as an H-Bridge. One thing you're going to want to think
about in your system is when you turn off the juice to that big motor,
it's inductance is going to generate a large voltage spike. So you'll
want some way of absorbing that other than channeling into into your
radio equipment and pitting your relay contacts.

Regards

"James R. Freeman" wrote in message
...
Jeff:
To do it right , I would use 2 of this type of relay unless You are
sure
the SPDT switch can handle 5 amps D.C. in the long term. You may contact
me
off list for wiring details either wayfor wiring details.

remove the no-spam
Jim
"Jeff" my last name @fidnet.com wrote in message
...
I have a 12v DC linear actuator, it just has 2 wires coming from the
motor.
If I connect the red wire to the positive terminal of the battery and
the
black to the negative, the motor extends. When I reverse the wires
the
motor retracts. The motor draws about 5 amps. Pretty straight
forward
even
for an electrical dummy like me :)

I have a grip for my control stick that has a 3 position switch to
control
this motor. It is an ON-OFF-ON SPST switch that is capable of about 2
amps.
So I went down to Radio Shack and bought a DPDT 12v relay part number
275-218. Here is the Radio Shack web site for this relay:
http://www.radioshack.com/searchsku.asp?find=275-218

I need some help wiring the thing up so I can use my motor. Which
wires
go
where? Did I get the right kind of relay?

Thanks,
Jeff