Question: I have an analog motor controller that has a B10K potentiometer on it. I'd like to be able to swap out the analog potentiometer for a digital one, but I'm not exactly sure what voltage is being sent to the original potentiometer as I don't have the spec sheets for it.

My current specifications, which work as a prototype, are:

```
- Power supply: 48V 4A transformer, connected to...
- Variable speed DC PWM Motor Controller which controls...
- A 130V 1A max DC motor. 

```

I'm fine with the power and speed that 48V gives me, but do I need to specifically look for a digital potentiometer that can handle high voltages, or would a standard 10K digital pot work?

Answer: 
You can increase the voltage drop and use a (very efficient) DC-DC converter to convert the higher voltage drop to a higher current. For example, if you need 3V at 20mA (60mW) you could drop 17V or so and if your DC-DC is efficient enough you will stay under the 4mA. Chances are you will have to design this, most chips and modules use too much quiescent current to make this work. 

Optionally, and this is common practice if you are not even close on the current, is to use an external supply and DC-DC isolation to provide the necessary current. There are many kinds of devices that cannot operate from the small amount of power in a loop-powered instrument. Of course it increases the wiring cost and may make intrinsic safety more complex. 

Another option (probably less popular these days) would be to use a different analog current loop standard such as 10-50mA.
![schematic](https://i.stack.imgur.com/osUds.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fosUds.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

*Figure 1. (a) 2-wire 4 - 20 mA sensor and (b) 3-wire sensor configurations.*

One way out of the problem - but perhaps not easily achievable with the XTR115 chip - is to add a third wire. You now have a greatly increased power budget.

[![enter image description here](https://i.stack.imgur.com/AVeBc.gif)](https://i.stack.imgur.com/AVeBc.gif)

*Figure 2. Different arrangements for 2-wire and 3-wire transmitters. Source: [e2e.ti.com](https://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/t/263177).*

2-wire 4-20mA Transmitter:

* Accepts loop-supply from 4-20mA receiver
* Sinks a return current between 4-20mA

3-wire 4-20mA Transmitter:

* Accepts power and GND from a local supply
* Sources an output current between 4-20mA

For the 3-wire transmitter you may be better using the [XTR111](http://www.ti.com/product/XTR111/technicaldocuments).

[![enter image description here](https://i.stack.imgur.com/EKiF7.png)](https://i.stack.imgur.com/EKiF7.png)

*Figure 3: XTR111 application note extract.*