Hi all,
I am new to this community and i hope someone can help my with my project.
I want to control the speed of a DC motor working on 48v.
It is now controlled by a potentiometer that i want to replace.
I measure 0-15V on this potentiometer and the resistance is 2.5k.
I read that digipots can not operate with a voltage over 5v when used with arduino output.
Is there an other way to "replace" the potentiometer and control the 0-15v voltage on the machine?
Thanks in advance,
can you upload a schematic of the existing wiring?
Also clarify if the power source for the motor is isolated from the electricity mains.
Assuming that there is a 48v supply for the motor then you could derive a 0 - 5v signal from a pwm output with an RC smoothing circuit and use a simple op-amp with a couple of resistors to set a gain of 3. The op-amp would need to tolerate a 48v supply, or you could drop that to say 30v with a cheap buck converter off ebay. You would need to be very sure the motor controller was mains isolated as 6v6gt says.
At this moment it is a existing (old) machine of which i at this moment have no documentation.
It has wheels and the speed is controlled on a panel. This panel has a 2.5k pod and a lcd screen that shows you the travel speed in cm/min. (0-258).
I measured 15v on the potentiometer and the speed is 0cm/min at 0v (pot 0%)
and 258cm/min at 15v (pot 100%)
I assume this is then processed by a drive controller to regulate the voltage to the motor (0-52v)
This is all the information i have at this point.
If more info is required i will upload this (later)
Battery powered or mains powered? How big is the motor?
Mains powered, all DC voltages , relative small motor. Did not find specs/sticker on it unfortunately
ok i will test that.
That means a potentiometer current of 6mA (90mW). Can you measure the current flowing off the pot wiper?
DAC for that voltage range are available at price >$20. A cheaper DAC with an added amplifier may do the job as well.
Thanks for the replies,
at this moment i can not measure the current, but i will do this later.
So a DAC wil accept 0-5v pwm input and regulate the 0-15v when connected to the circuit where at this moment the potentiometer is located?
A DAC is controlled Digitally by SPI or I2C. It can be connected in place of the pot to +15V, wiper and GND.
Thank you
Well if the pot has 15V on its top and this has a reasonable current capacity, then use that to supply an op-amp. $DAC = $20, $op-amp = $1?
Good point, i was looking for a dac but most i2c seem to be 0-10v output.
I will look into op-amp
My best suggestion is a DAC (as previously mentioned), with optical isolation from the control.
I use MAX532 MDACs which can provide +/-12V output in one of my applications. It uses a SPI interface that is easy to isolate. Prices have gone up since the worldwide supply chain problems started, but they are still available and not unreasonably expensive. Expect to pay in the neighborhood of USD$30 each (used to be around $16 before).
If the +12V output isn't high enough, then you could use an amplifier. In fact, since you'll need an amplifier regardless, you could use a less expensive DAC to get to this level.
There are also simpler methods. e.g., an opto-isolated PWM which is filtered to produce a DC level. With a 15volt driver, you can get 0-15V easily. In fact, now that I think about it a bit, I really like that idea since the entire 0-15V output can be generated for less than $1 
Is this the "speed" potentiometer that you wish to replace (A05) ?
It looks like the speed, voltage and current potentiometers share the same power rails (0,15v).
Interestingly, the voltage potentiometer (A03) already appears to use a PWM/Optocoupler method for controlling a higher voltage circuit:
There appears to be no similar diagram for the speed potentiometer.
Thanks again for the reply's
I also like the idea that this 0-15v can be generated with less then $1.
At this point i see how to do that but i will try to understand and find out.
Is this the "speed" potentiometer that you wish to replace (A05) ?
YES
It looks like the speed, voltage and current potentiometers share the same power rails (0,15v).
this would be +15V i guess instead of 0,15V?
This voltage and current potentiometers are the process parameters for the tool that is placed on the "cart" of which i want to control the speed.
they are, (for me) not related to the travel speed voltage and current. but they share the same +15v input (and probably ground) for their potentiometers.
Oh yes, sorry. I see it could be misinterpreted. I meant 0v & +15v power rails as you have clearly understood.
Edit
There are threads in this forum which deal with a similar problem to yours, that is interfacing to a (potentially) un-isolated motor controller. Controlling isolated 0-5V
How long can your machine be out of service while you work on making the change? Will you ever need to go back to the original configuration?
