Trim potentiometer problem

[airguy]

I'm trying to diagnose a problem I'm having with my elevator trim position indicator, I'm using a Ray Allen servo and feeding +5 volts and ground to the potentiometer on the servo, and returning the wiper signal back to the EMS. I've pulled the servo apart and confirmed that it's working properly, but I'm still getting a widely variable voltage coming back from the EMS on that pin. What is the impedance level on the EMS sensor pins? When I have the trim unit disconnected and the Dynon powered up, I am measuring approx. 4 volts on the pin that is connected to the Dynon EMS for the potentiometer wiper - but I don't know if that is just a "floating" non-connected voltage, or a sign of bigger problem.

 

[dynonsupport]

Our instructions in our EMS-D10, EMS-D120, and FlightDEK-D180 Installation Manuals on this sensor could be improved, and we hope to do so.

The White/Orange wire of the sensor should be connected to +5V (Pin 18, White/Red wire of a Dynon Avionics EMS harness).

The White/Blue wire of the sensor should be connected to Ground (preferably one of the black wires on the EMS harness).

The White/Green wire is the OUTPUT of the sensor - as the servo is moved, you should see the voltage change between "somewhere near ground" and "somewhere near 5V" as the resistance changes during movement.

The White/Green wire is what's connected the General Purpose Input that you select - either Pins 4 (Violet/Blue), 22 (Violet/Yellow), or Pin 23 (Violet/Green).

I'm trying to diagnose a problem I'm having with my elevator trim position indicator, I'm using a Ray Allen servo and feeding +5 volts and ground to the potentiometer on the servo, and returning the wiper signal back to the EMS. I've pulled the servo apart and confirmed that it's working properly, but I'm still getting a widely variable voltage coming back from the EMS on that pin. What is the impedance level on the EMS sensor pins? When I have the trim unit disconnected and the Dynon powered up, I am measuring approx. 4 volts on the pin that is connected to the Dynon EMS for the potentiometer wiper - but I don't know if that is just a "floating" non-connected voltage, or a sign of bigger problem.

 

[airguy]

Your instruction in the manual were actually just fine, and I have it installed as it needs to be. The problem I'm seeing is that I am getting erratic readings on the Skyview trim widget from the potentiometer wiper. They wiper voltage does indeed vary as it should, but it has about 11k-ohm of resistance within the wiper connection, before it makes contact with the 5k-ohm potentiometer winding. I'm not sure if that is a non-standard condition with the potentiometer, or if the current resulting from the 11-16 k-ohm total resistance is not enough to correctly register on the EMS pin - which is why I asked what the impedance on the EMS connection is - which you did not answer.

I'm also registering a floating voltage of around 4 volts for the EMS pin without anything connected to it when measured with a high-impedance Fluke meter, is this normal?

 

[dynonsupport]

Suggest you re-do the trim calibration - the calibration should "adapt" to whatever the voltage output is at the stated positions.

Else, it sounds like a issue with the potentiometer - have you checked with the trim servo manufacturer that your observations are normal?

Your instruction in the manual were actually just fine, and I have it installed as it needs to be. The problem I'm seeing is that I am getting erratic readings on the Skyview trim widget from the potentiometer wiper. They wiper voltage does indeed vary as it should, but it has about 11k-ohm of resistance within the wiper connection, before it makes contact with the 5k-ohm potentiometer winding. I'm not sure if that is a non-standard condition with the potentiometer, or if the current resulting from the 11-16 k-ohm total resistance is not enough to correctly register on the EMS pin - which is why I asked what the impedance on the EMS connection is - which you did not answer.

I'm also registering a floating voltage of around 4 volts for the EMS pin without anything connected to it when measured with a high-impedance Fluke meter, is this normal?

 

[airguy]

(sigh)...

I guess the impedance of your EMS voltage inputs is a state secret or something, since I've clearly stated my question twice and you've still not answered it.

 

[dynonsupport]

The impedance technically matters which pin you are hooked to on the EMS and how you have them configured. There are variable pull up and down resistors inside, defined by the sensor config file.

When configured as a "position" input, EMS pins 4,6,7,8,9,10,11,12,20,21,22,23,31 (on the D37) have a 10k pull UP to 5V.

You should see right around 5.00V on any of the pins configured as a position input when measured with a high impedance meter. 4V is not normal unless it has a 40K load to ground on it.

 

[airguy]

To close the circle on this for Dynon Support - I was able to diagnose a double failure, both a potentiometer problem in the trim servo and a bad wire crimp at the EMS for the 5V power to the potentiometer. I needed to know what the pullup resistance was to make sense of the readings I was getting, thanks for finally supplying that.

Servo has been sent in for repair and the wiring repair is in process.