Digital Voltage Divider

Hello to all, new to the community and only just starting out with Arduino. Really enjoying my starter kit projects.

I have an aircraft double moving coil instrument I want to control with Arduino (well actually lots of them!).

I’m looking for some concept guidance. In the aircraft a 28V supply is split between two coils in the instrument. Each ‘leg’ then goes to a potentiometer acting as a movable voltage divider. So at the zero position each ‘leg’ of the circuit has 14V, when the potentiometer is moved, the voltages become unbalanced and the needle of the instrument deflects up or down accordingly.

I have attached a simple schematic diagram of a test circuit I have made that makes the instrument respond.

So the question is, how do I replace the physical potentiometer in the test circuit with a digital circuit driven by the Arduino?

I thought about using an L293D H-bridge which will handle the 28V and I could use this to vary the voltage using PWM from the Arduino, I just need some ideas how I might physically connect it all up or approaches that might be taken to make a digital voltage divider to replace the potentiometer in the test circuit.

Hope that all makes sense. Afraid I’m more use to electromechanical things than microcontrollers.

Potentiometer Test Circuit 3 terminal.doc (26.5 KB)

You can buy digital potentiometers. They even come in dual chips so you can control 2 things with one chip.

Thanks MorganS, I did not know that so will research them. Will they be able to directly handle a 28V supply? I need to also interface with a flight simulator, so the data will be coming from the simulator to drive the instrument, should have said this earlier.

Yes, 28V is unusual for small chips. I've done a search on Mouser and they do have listings for some that go up to 36V but they're "non stocked" which means they are very rare chips that nobody uses. Other retailers may keep stock of these 36V units.

You could possibly operate the instruments on lower voltages.

Perhaps your original idea of a PWM motor controller is best, for the voltage and high (for Arduino) current of these coils.

I may need to stick with the PWM route with the motor controller as looking briefly most of the digital potentiometers did seem to be for much lower voltage and current requirements as you say.

One of the observation I made with the instruments is though I can operate them at lower voltages, for some reason when power is switched off/on they will only reliably return to the zero position when subjected to higher voltages. So this seems to be a constraint in the project that the instruments require the native 28V for reliable zero readings.

Lightning_Simulation:
I may need to stick with the PWM route with the motor controller as looking briefly most of the digital potentiometers did seem to be for much lower voltage and current requirements as you say.

One of the observation I made with the instruments is though I can operate them at lower voltages, for some reason when power is switched off/on they will only reliably return to the zero position when subjected to higher voltages. So this seems to be a constraint in the project that the instruments require the native 28V for reliable zero readings.

How about connecting each coil to just a single Mosfet to Ground?
Then apply PWM to Gate #1a.
Then use 1/6 of Hex Inverter IC to drive Gate #1b.

Or instead of Mosfets …

  1. Hex Open Collector 50 volt Non-Inverting Driver to all “a” coils: 1a, 2a, 3a, etc
  2. Hex Open Collector 50 volt Inverting Driver to all “b” coils: 1b, 2b, 3c, etc
    PWM #1 to both Input Channels 1a & 1b
    PWM #2 to both Input Channels 1a + 1b

    PWM #6 to both Input Channels 6a + 6b

16 Channel PWM …
16 CH PWM

Of course, reverse bias diode across coils.

mrsummitville, this might be a very promising solution. When I was initially thinking of the H-bridge I was not taking into account the instrument is a sealed unit and the fact I do not have easy access to each 'leg' of the internal circuit which contains the coils (there in fact being only one pin for 28V in) - one of the aims of the project is to not dismantle instruments when it can be avoided. Using mosfets on the ground side might be just the way to go. Thanks also for the link to the 16 channel PWM too.

If you need some fraction of 28 v., and you can only get 5 v. digital potentiometers, go look up the non inverting amplifier based on an opamp. Opamps are very easy to use (for slow analog circuits), and well worth looking into. They solve lots of analog problems with minimal circuitry.

This sounds like it calls for an MCP41HVXX high voltage digital potentiometer (comes in the MCP41HV31 and 51, with 7 and 8 bits of resolution, respectively). Can handle 36 volts.