# Generating a 0-40 ohm signal

Hello,

Currently trying to supply a 0-40 ohm signal for a specific sensor I have. I looked into digital potentiometers and it looks as if they are all much high resistance (5k,10k...). I also was thinking maybe varying a PWM signal to achieve small resistances? Any ideas?

To my knowledge, there is no such thing as a 'x Ohm' signal. Volts, Amps, pulses, frequency, logic, but not Ohms.

Try to re-phrase what it is that you want.

Maybe my phrasing is incorrect? I need a 0-40 ohm input into a sensor. I currently use a stepper that varies a potentiometer between 0-40 ohms. I would like to improve upon this and have a cleaner signal to vary between 0-40 ohms.

Signals would involve current flow or voltage, which in turn can be specified by the circuit resistance. Please post a circuit diagram (hand drawn is much better than Fritzing).

The 0-40 Ohms is NOT the signal. It controls the signal by varying voltage (or current) seen as inout by the sensor.

That in mind, and with your second post, it sounds like you want smoother steps between changes. O.K. if you use stepper to control the pot you need smaller steps. You might be able to get 0-40 out of a 5K digi-pot, but you probably won't get any better resolution.

Are you trying to drive a known current ito a sensor? if so what current? ac or dc ?
if ac, what frequency?

How accurately do you need the 0 ohm end? if you can tolerate maybe 5 ohms or a bit less
something might be done.

Ifr you could provide details of the sensor we'd all be able to help you better...

regards

Allan

It seems we need to take him at face value.

Maybe he is trying to test an ohmmeter.

Rdrost:

You can use a MOSFET as a variable resistor. You need to measure the voltage across it and current through it. Adjust the gate voltage to set the current so that R=V[ across the MOSFET ]/ I [the current through the MOSFET].

Seems OP wants a digitally controllable load between 0 and 40ohm.
Maybe an 8-relay module with R-2R resistors.
Leo..

MOSFet's have an ohmic range, where careful control of the gate voltage provides nearly linear resistance from source to drain. That converts your problem to generating a variable voltage from an Arduino, which cabe done with PWM.

That requires measuring voltage and current.
“signal” could be DC, or not.
Leo…

Hi,
Welcome to the Forum

If we know its characteristics we can possibly suggest a suitable answer.

What is your electronics, programming, arduino, hardware experience?

Thanks .. Tom..

Think this does the trick…

got a gain of 2, but I dare say you can live with that. It will vary by 2.5ish% as the pot moves - if that’s unnacceptable add a simple follower in the pot wiper.

I don’t know what power you need, but power opamps are available.

regards

Allan

47drive.pdf (17.4 KB)

I append a modified circuit which

/ changes the gain to 1
2/ eliminates the gain changes as the pot moves

I’ve added a cap to guarantee stability - may or may not be necessary depending on opamps, layout etc etc

regards

Allan

47drive2.pdf (17.7 KB)

Sorry -forgot an R

regards

Allan

47drive3.pdf (18.2 KB)