digital potentiometer power ratings

Thread Starter

jtnewgard

Joined Mar 16, 2020
4
Hi all,

I am planning to programmatically control the speed of a motor loading a seafloor anchor (just a model anchor in the lab for now). The idea is to mimic cyclic wind and wave loads the anchor will see at sea.

To do this, I would like to replace:

the 5 k-Ohm, 0.5 W analog speedpot that came with my Dart 130HC100 motor controller

with a digital potentiometer:

something like the MCP4161 or AD5160.

I can successfully communicate with these devices through Arduino and see the resistance change according to the codes I send. But when I turn on my motor controller with a digital potentiometer connected, the resistance immediately goes very high (measured this with multimeter)! Correct me if I'm wrong, but I believe the current flowing through the controller is overloading the digital potentiometer which fails open-circuit.

My question is, does anyone know some programmable/digital potentiometers that have higher power ratings? I noticed on the AD5160 data sheet that the maximum current across the wiper terminals is only about 5 mA.

And using Power = I^2 * R, I think the 5 k-Ohm, 0.5 W analog speedpot can handle about 10 mA, so about 2x what I am trying to replace it with.

Thanks much for any insights!
Jeff

PS. I attached the data sheets for each of the digital potentiometers mentioned, and also the 130HC100 controller.
 

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dl324

Joined Mar 30, 2015
11,926
Welcome to AAC!
My question is, does anyone know some programmable/digital potentiometers that have higher power ratings? I noticed on the AD5160 data sheet that the maximum current across the wiper terminals is only about 5 mA.
Redesign the circuit so you don't need so much power.
 

paulktreg

Joined Jun 2, 2008
791
A quick look at the 130 controller datasheet suggests it can be controlled by a DC voltage.

"Speed control --------------------------------- via 5Kohms .5W potentiometer or 0-10VDC isolated signal"

This may be a better solution?
 

joeyd999

Joined Jun 6, 2011
4,434
Correct me if I'm wrong, but I believe the current flowing through the controller is overloading the digital potentiometer which fails open-circuit.
Also, none of the terminals of the digital pot may be outside the range of Vss to Vdd.

You definitely do not want a digital pot to dissipate power beyond a very few mW.
 

Ohmlandia

Joined Mar 2, 2020
32
But when I turn on my motor controller with a digital potentiometer connected, the resistance immediately goes very high (measured this with multimeter)!
Are you measuring the resistance with power applied? This will not give you a true reading! The multimeter will be confused by currents and voltages other than those applied by itself to the circuit.
 

KeithWalker

Joined Jul 10, 2017
1,307
The result you are getting is probably because you are using the potentiometer outside it's operating range (see post #4). If you look at the attached detail, you will see that if any of the potentiometer terminals are connected to potentials outside the range of GND to VDD, the switching operation of the value selecting MOSFETs will be compromised. You will have to measure the voltages on the terminals of the existing control to see if it is possible to use a digital potentiometer. If the voltages are within the range of 0 to 5V, you will need to match the reference (GND) of the pot supply to a suitable offset voltage to be compatible with the voltages on the controller.
Keith

Dpot.jpg
 

Thread Starter

jtnewgard

Joined Mar 16, 2020
4
Wow these are very helpful!

I see my first task is to measure voltage across the controller terminals to make sure the controller is not applying voltage outside Vss-Vdd to any two of the potentiometer terminals.

I might have goofed and tried to measure resistance while the power was on too, leading to the false conclusion the potentiometer was overloaded. I did have the controller on, but did not run the motor up and down (the motor doesn't actually run unless you move a separate switch on the controller)

I am most intrigued by using the 0-10VDC isolated signal to control the speed. I have a nice NI multifunction board with some analog outputs that could do this no problem! Maybe that is easiest!

I had considered redesigning the circuit to incorporate some potentiometers in parallel (believe the attached diagram would cut the current across the pots in half?)
 

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Thread Starter

jtnewgard

Joined Mar 16, 2020
4
I can't believe someone went through the controller datasheet and noticed something I hadn't! Thanks everyone for your inputs!

I'll try some things in the lab tomorrow and report the results.
 

Thread Starter

jtnewgard

Joined Mar 16, 2020
4
OK just wrapping up in the lab -- here's the results in case anyone's interested!

The motor controller uses the analog speedpot as a voltage divider. It divides down 10V across HI to LO terminals and uses the potential on the W-wiper to control the motor speed. So when I powered up my digital potentiometer with 5V from Arduino, I did in fact exceed this voltage once I connected it to the controller.

And the digital potentiometer is reading OL (I think means "overload") on my multimeter, with the power off this time. So it seems I definitely broke the digital pot. Thank goodness these parts are cheap. Would exceeding Vss and Vdd break the wiper movement on the digital pot?

I'd like to make the digital pot solution work just out of curiosity (obstacles seem to be maintaining voltage between Vss and Vdd, not trying to dissipate too much current) ....

but after speaking with tech support from Dart, I believe using the 0-10VDC signal will be easier. I'm lucky to have access to a nice multifunction DAQ board. Now just have to calibrate the motor speed and program it in LabVIEW.
 
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