Trim with the Garmin G3x

[Eric Page]

5) I am not an engineer, but believe that there is a simple and inexpensive way to get the required current to the KF trim, and that is through the use of a MOSFET. MOSFETS are voltage devices and will conduct when voltage is applied to the gate. Depending on the MOSFET chosen, you can supply very high currents with a small voltage (from the VPX) applied.

6) So far so good, since MOSFETS are only a few dollars, way cheaper than the booster. You would need one MOSFET for trim up, and one for trim down.

Unfortunately, it's not quite that easy. Reversing a DC motor requires that you have the ability to invert the polarity of power to the motor. You can't do that with a single MOSFET in each direction.

There are two wires to the motor. Each can be positive or negative, depending on desired direction of rotation. This means that to rotate clockwise, wire "A" should be positive (connected to 12V) and wire "B" should be negative (connected to ground). To rotate counterclockwise, "B" is positive and "A" is negative.

Doing this requires four MOSFETs in an H-bridge configuration. The two transistors connecting to 12V must be P-channel, and the two connecting to ground must be N-channel. Controlling them takes some finesse, as accidentally turning on both FETs on the same side of the bridge results in a direct short from 12V to ground (called shoot-through).

There are dedicated ICs for this control task, or a carefully coded microcontroller could be used. There are also dedicated half-bridge ICs that perform -- as the name suggests -- half of the H-bridge function. They just require a signal telling them which of their internal FETs to turn on; helpfully, their internal logic enforces dead-time to prevent shoot-through.

7) I spoke with a tech guy at VPX today who told me that the way the VPX regulates the speed of the trim motor is by varying the voltage sent to the trim motor. That confounds things, because if you do use a MOSFET, it will turn on hard with voltage applied to the gate and completely conduct above some minimum input voltage. That effectively does away with the speed dependent function.

If what the VP-X tech told you is true, it's worrying. Controlling the speed of a DC motor with varying voltage is a poor technique for a couple of reasons. First, low voltage does make a motor turn slowly, but it also proportionally reduces torque. Second, as you pointed out, using variable voltage makes it very difficult to buffer the output to accommodate motors with higher current draw.

Both of these problems are overcome if you control motor speed via pulse width modulation (PWM). Essentially, you turn the motor on and off at high frequency, applying full voltage in variable-length short bursts. This controls motor speed while maintaining torque and permits easy buffering by interposing an H-bridge between the control electronics and the motor.

If you look at the specifications tab for the Safety Trim Booster on TCW's website, you'll see this: "PWM follows autopilot signals." So, TCW's product is designed to pass PWM control to the motor. Perhaps the VP-X guy was "dumbing it down," but I would be concerned if he wasn't.

B) For those of you who are using the booster, are you sure that the speed of the trim motor is really dependent on airspeed? If so, how have you determined that?

A booster has nothing to do with controlling direction or speed of the trim motor. All it does is apply power to the motor in the direction indicated by the HAT switch, autopilot or EFIS, for as long as it's told to do so. Essentially, the TCW Safety Trim Booster is a smarty-pants H-bridge-in-a-box. If the pilot sends a constant-on nose-up trim command from the HAT switch, that's what the booster sends to the motor. If the EFIS or autopilot send a half-speed nose-down command via PWM, the booster switches the motor on/off at the frequency and pulse width of the drive signal. All subject to run-away trim time limits, of course.

[Shadowrider]

"A booster has nothing to do with controlling direction or speed of the trim motor. All it does is apply power to the motor in the direction indicated by the HAT switch, autopilot or EFIS, for as long as it's told to do so. Essentially, the TCW Safety Trim Booster is a smarty-pants H-bridge-in-a-box. If the pilot sends a constant-on nose-up trim command from the HAT switch, that's what the booster sends to the motor. If the EFIS or autopilot send a half-speed nose-down command via PWM, the booster switches the motor on/off at the frequency and pulse width of the drive signal. All subject to run-away trim time limits, of course."

Eddie I think this is your answer. I will look at my settings but I think the G3X is turning on and off the trim motor so that it is resetting the 3 second timer. You just need to change the speed of the motor slightly so that the 3 second time is reset if that makes sense?