Elevator Trim Braking Circuit

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Coen Recommendation

Larry Coen offers this solution, and reported benefits:

Dynamic breaking circuit 001.jpg
Brett, The circuit you are showing is different from the one I posted. Yours works best for those that want to add braking to their existing pitch trim. The circuit I used has some advantages for those starting from scratch.

1) The relays switch the motor current instead of the switch in the stick. Improved durability.

2) Only 3 wires to the switch instead of 4.

3) Current for the relay is much smaller than the motor allowing the use of smaller gage wire.

4) No 12v hot wire to the stick.

Larry Coen

Ginsberg Recommendation

The tip below is from Bob Ginsberg, courtesy of Velocity Views Volume 6

From Bob Ginsberg,
Fuquay Varina, NC:
Pitch Trim Motor Dynamic Brake


The Warner motor supplied with the kit has considerable "coastdown" when you release the switch, making it sometimes difficult to get the trim fine-tuned. I solved this problem, with the help of Bill Betts of Sparkchasers avionics shop, by the use of a dynamic braking circuit. What this circuit does is short the motor windings when you release the trim switch, thereby making the motor its own load. This stops the motor almost instantaneously, but does not change the motor's speed when it is operating.

The circuit is show below. It taps into the two leads going to the motor from the switch. Each of these leads can be hot or ground, depending on which way you have the switch. The relay is picked up by power from the motor leads, but is grounded separately, making sure it releases when you release the switch.

I mounted the relay and the two diodes on the fuselage wall, just behind the pilot side of the instrument panel, with a 4 pin connector and a separate connector for the relay ground. This way, if the relay fails, I can open the connectors and pull the relay and the diodes all at once, facilitating a relay or diode replacement.