control a pot

What you suggest would work if the pot is a two-terminal variable resistance. If it is a three-terminal voltage divider you need to understand and describe the possible CNC outputs and pot circuit more thoroughly.

FETs have an ON resistance well below 1 Ohm without an offset at zero volts.

Its 0 - 24 volt, 2K ohm pot. Don't know how to measure impedance, but i assume high.

karl

this:

outputs

If it's two terminal, then it is a Rheostat, not a Potentiometer.

Ya, see your point. Its three treminal. To do this, I'd have to double the complexity and have resistors above and below the wiper. Pot turner looks better now.

karl

I've done this search and only find ICs for circuit board type stuff. Not something to just replace a pot in a machine. Maybe it exists but all the descriptions are written in EE not english.

Karl

There are several semiconductor companies who make digital pots and board level digital pots are available too. Google "digital potentiometer". Art

Disconnect the pot wiper wire and connect a milliamp meter in series, then record the current at both end settings.

jsw

With all the electrical noise a plasma cutter generates I think you are right.

jsw

Karl there are dc powered, motorized pots made by Alps and others for remote volume controls. but you could use a R/C servo motor to turn the pot, and a 555 timer IC to generate the PWM control signals. This was common on the old C-band feed horns to select the polarity, and trim the alignment as you changed channels.

Digital pots are not intended as drop in replacements. They are low voltage, low power devices that usually need microprocessor control. Any noise in the control wiring or supply rail corrupt the settings. They were intended to replace pots inside a piece of equipment, to allow ATE calibration of equipment.

Depending on the current required at that input, a DAC and OpAmp might be usable. If the DAC is parallel input you could store several preset levels, and select them by changing the status of the address lines of a EEPROM or EPROM. A lower tech method would be the use of diode arrays to control the DAC inputs.

BTW, we used DACs for volume controls on telemetry receivers. We fed the low level audio to the reference pin, then used the output to feed the audio amplifier.

There are motorized potentiometers like some stereo's use with remote volume control. There is also a digital pot that acts like a motorized pot. I've seen the equivalent circuits for digital pots, you could make an equivalent. For example, make a 2K ohm resistance using 10ea 200 ohm resistors in series, the voltage output depends on the junction you select with an analog switch. It may be a better solution to use some sort of digital pot, if you can't find a digital pot to work with 24V you could use an op-amp to change the voltage you can get from the pot (or DAC for that mater) to the 0-24V range.

Parallax has some digital pots on their website, complete with datasheets and source code for the Basic Stamp.

But for CNC control, I would probably just use a 0-10V analog output, like I do for my spindle speed, and amplify it to 0-24V. If you wanted you could control your pot output with your S command.

RogerN

A lot of the stuff I've used (dc motor controls, vfd's) that has a pot also has another way to control output, such as analog signal input terminals. I suppose you've exhausted the possibility of such for your plasma torch?

Pete Keillor

I'm terrible at circuit board work and useless at EE design, but I can have kits done. How about this one? My control can output 0-10 volt for analog control.

Karl

You've ALMOST got it right. Google R/2R or "Resistor Ladder" to get what you're looking for.

Awfully elaborate. Easier would be to run three wires from the box, (usually a potentiometer has three terminals) to an interface that contains multiple pots just like the original, and a bunch of relays to switch one or the other into the circuit. How many 'states' do you need to have the digital gizmo control? Fast-medium-slow-off? If one terminal is ground, wire those all together and put a double-pole relay on each channel. The digital gizmo just selects one of the preset pots.

As you will undoubtedly find, 'digital' potentiometers have low voltage and current limits, and are best designed in at the factory.

...

BINGO!

Just what I want to do.

Thanks

Karl

Errr.... maybe not. If the wiper of the pot is a variable voltage, and feeds a high impedance circuit, then fine. But, the wiper might be +24V and it proportions current in two legs, or the wiper might be a current source into a low-Z input, or the pot might be a nonlinear taper type... there's LOTS of potentiometer circuits, besides the one that has +V on one leg and GND on the other. DAC and amplifier is the same as R/2R ladder in most cases (and it's easier to wire up, with less parts).

It looks like it should work for you. That kit will handle up to four servos, too. You would want it well shielded when using around your welder or plasma cutter. If you do use it, ground the unused inputs, and put a 1K resistor from each unused output to ground.