Hello everyone,

I have several DC drives for DC motor speed control (recycled from old machines). I want to reuse them since they are operational, with a little "upgrade" to automate its operation.

I was hoping I can replace the manual potentiometer (which is used to control the motor's speed) with a digipot, but I coundn't find a suitable one (I live in Brazil and some electronic parts are impossible to find).

The potentiometer supplied with the drives are 10K. I connected one drive to a small motor and measured the voltage between P3 and P1 of the potentiometer, and got aprox. 9V. That's my main problem, all 10K digipots I found here are rated at max. 5V (bought a few to make some tests).

The drive manual (attached) also indicates another way to control the speed, using a voltage following connection with a 0-9V analog signal between P1 and P2, but it must be isolated because there is a risk of damaging the drive and the control equipment.

Then I found this article:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/843


Which explains how to use a digipot and an op amp to replace a potentiometer. Looks interesting but I'm no expert and need some advice.

Here are my doubts:

At the op amp, VDD must be connected to the P3, wiper-out to the P2 and VSS to P1 terminals of the drive, where the potentiomenter was connected?

And since the drive uses a 10K potentiometer, I'm assuming I will have to use R1 = R2 = 5K to get 10K total between P1 and P3?

If I remove the push buttons/debouncing components to use SPI and an external power source for the digipot, should I worry about the signal isolation?

And finally, is there a better approach, like using the voltage following connection with a DAC?

Thank you!

 

On these drives, they do not have galvanic isolation between the circuit common and the AC supply, so care has to be taken if connecting any external circuit other than the pot.
If using a transformer fed L.V. DC supply for any digital to analogue convertor then it can be done without isolation, just that all circuitry has to be enclosed and not accessible to the operator.
Max.

 

Thank you, using the DAC seems to be easy and simple, I'll try that.

 

If I remove the push buttons/debouncing components to use SPI and an external power source for the digipot, should I worry about the signal isolation?

Probably.

It's easy to isolate SPI.

In the real word, sometimes an isolated current source across a resistor is used. The resistor is placed at the input of your "motor controller"

e.g. http://www.analog.com/media/en/technical-documentation/data-sheets/1B22.pdf

So, your DAC+an isolated voltage source should do the trick.

 

On these drives, they do not have galvanic isolation between the circuit common and the AC supply, so care has to be taken if connecting any external circuit other than the pot.
If using a transformer fed L.V. DC supply for any digital to analogue convertor then it can be done without isolation, just that all circuitry has to be enclosed and not accessible to the operator.
Max.

When looking for a DAC I remembered that the DC drive manual indicates a 10K or less impedance at the 9V source - the DAC output.
Couldn't find the output impedance in some DAC's datasheets, how do I find that? Or it's something I can ignore safely?
I'm attaching a DAC datasheet as an example, where the output impedance is mentioned only when the DAC is off but not when the DAC is in operation.

Thanks again!

 

Here is the typical analogue input for the KB drives, also The isolated ±10vdc KB board will give you an idea how they interface to the pot input.

 

Here is the typical analogue input for the KB drives, also The isolated ±10vdc KB board will give you an idea how they interface to the pot input.

Thank you!

 

Couldn't find the output impedance in some DAC's datasheets, how do I find that? Or it's something I can ignore safely?

Short circuit current is enough information. With 10 V out and 15 mA minimum, you can drive a 666 ohm load.

 

Short circuit current is enough information. With 10 V out and 15 mA minimum, you can drive a 666 ohm load.

Thanks!

 

I´m make some tests with the DAC and post the results, just in case someone else is on a similar situation

 

DAC worked fine, used a Raspberry Pi and an MCP4921, and it worked ok (the tests were only for a few minutes).

Thanks for your help!