View Full Version : What size variable resistor for VFD?

02-20-2009, 07:00 PM
I have a VFD that needs the typical 0-10V DC to control speed. I want to use a potentiometer to adjust the speed.

I'll be supplying 12V DC from a small power supply. I just need to vary this so that the MAX voltage is 10V, and the minimum is close to 0V.

I don't know the current allowed. Does someone have some experience with this that can tell me what size pot to use?

Thanks in advance!

Wayne C

02-20-2009, 07:09 PM
The minimum usually is 2.5k. You can probably go up to 10k.
Lets assume you have a 2500ohm pot, you want 10v maximum across the pot.
This results in a current of I=E/R = 10/2500 = .004 or 4ma.
Now you have 2 volts to lose somewhere, so put a resistor in series at the 12v end.
The resistor will have to drop 2v at 4ma, R=E/I 2/.004 = 500ohms.
For any other pot value, use the same formula, the input impedance seen by the pot slider can usually be ignored for this calculation.

02-20-2009, 11:38 PM
Al, thanks for the quick reply. I have both a 5K and a 10K pot, and I'm just having a heck of a time understanding this.

When I measure the resistance with my Simpson I get a varying resistance as I turn the knob. But when I measure voltage across it, it doesn't change no matter the setting. Its wire the exact same way- one leg to the center terminal, the other to one of the side terms.

I even tried it with just a flashlight battery to get as basic as I could. Again, no variance with the 5K pot. And no change when I put resistors in series.

Am I doing something wrong, or should I expect voltage to be the same?

Thanks, BTW, for the Ohms law refresher. I wasn't 100% sure that I was ending up with the right milliamps.

I'm wanting to simplify my rig abit and take my CNC4PC speed control out and try just a Pot in a Box for speed control. In that box I'll have the resistor and a simple switch that will break the relay voltage and shut the relay off.

Thanks alot for your help.

Wayne C

02-21-2009, 12:02 AM
Try connecting the pot outer ends across the 12v, on the slider or centre terminal measuring from one of the ends you should get a varying voltage.
You should get this result before connecting to the VFD.
On the VFD, you need the bottom end or zero voltage end connected to the VFD common and the slider connected to the analogue input, the 12v end is not connected to a VFD terminal.
BTW, most VFD's supply a reference voltage for the Pot, are you sure you do not have a 10v source on it?

02-21-2009, 10:11 AM
So THATS what that voltage was for! Excellent. I was planning to tap into the same power supply that I use for the CNC4PC speed control gizzie.

I fried a parallel port buffer board when I was dinking around inside the control box trying to troubleshoot an intermittent connection, so my computer speed control is out of commission until I get the replacement board. The pot will be a backup, but I'll be using it for now.

Now I can probably get the thing working.

At the risk of piling on with questions, do you have any idea of why my Mach 3/Gecko system lurches when you click RESET in Mach? Its only when you remove drive control, not when you first power up with RESET. Sometimes its a half inch of jerk.

It happens only when the spindle VFD is ON but the spindle doesn't have to be turning. Am I dealing with EMI noise here?

Thanks for letting me pile on with the questions.


02-21-2009, 10:26 AM
I am not a Mach or Gecko user, so I may not be the one to offer advice here, It could be a grounding/bonding issue, if your VFD is powered but not outputting there should be no switch noise at that point.
I would guess there must be a spurious command signal getting to the drives?
Regardless, it would pay to go over your ground connections and make sure every metallic part of the machine is bonded together.
Did you see this post? http://www.cnczone.com/forums/showthread.php?t=71923

02-21-2009, 07:21 PM
Thank you AL! I now UNDERSTAND how to wire a POT, and I can wire up speed control.

I have my CNC and computer etc grounded pretty well, all to one central ground, except for my VFD. If there is a ground issue, I suspect its to the VFD. Mine is 220V single phase, and its grounded only via the power negative leg. Can I attach a ground lead to the ground terminal and run this over to my star central ground location?

Your knowledge is a great help to lots of people, ya know. I'm grateful.


02-21-2009, 07:40 PM
Normally you do not run a neutral to a 240v VFD. By all means the service ground should go the the VFD ground terminal and carry on through to the motor frame ground.
This ground should originate from the central machine ground plate that also has the service ground connected to it.