# Help identifying Potentiometer



## Abody711 (Jul 11, 2020)

*Potentiometer on new lathe appears to be bad. Turn knob half range before lathe starts. Then it jumps to full speed. Turn it all the way down, still full speed. Turn off and back on and is sitting still. If I missed something let me know. 
Attached a picture. What I made out is wx14-12 k7 +-5%. Is it rated at 7k? Suggested replacements?*


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## Abody711 (Jul 11, 2020)

Picture didn’t show


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## Ulma Doctor (Jul 11, 2020)

if i'm reading it right it's a 7k potentiometer.
a 5 or 10 k would work in it's place
i'd go for the 5k- you'll lose a little adjustability but it will serve well


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## Abody711 (Jul 11, 2020)

Is that 7k ohms?


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## CluelessNewB (Jul 11, 2020)

Can you make out the number before the K?  I suspect it might not be 7K but rather X.7K where X is whatever that number is.


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## Abody711 (Jul 11, 2020)

*Found this on eBay **WX14-12 10K ohm 4mm Dia Shaft Single Turn Wire Wound Potentiometer 3W. I am guessing it is 3. Should I go with a different part*


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## CluelessNewB (Jul 11, 2020)

I believe WX14-12 is the style but it comes in many resistance values.   

Do you have an ohmmeter?  If yes check the resistance between the two end terminals.  (This won't tell you anything if it is completely fried but worth a try if it isn't.)   

FYI 3.7K is a very weird value.  4.7K is common but it doesn't look like a 4 to me either.


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## homebrewed (Jul 11, 2020)

There's part of a digit preceding the "k".  It probably is a 4.7K pot, based on this product page from LMS.  It is used on quite a few mini lathe controllers.  If the pot has more than 3 pins, it has a built-in switch.

The value probably isn't critical if it is used as a voltage divider.  If it's used as a variable resistor, it probably is necessary to go with a pot that more closely matches the OEM part.


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## Abody711 (Jul 11, 2020)

Tested. Shows 9k


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## matthewsx (Jul 11, 2020)

Hi and welcome

Is the machine new? 
Or just new to you?

If you just bought it I would definitely check with the seller about a warranty replacement, I'm assuming the pot is part of a larger control circuit and it might not be the real problem. There's a guy who does board repairs on these things, https://olduhfguy.wordpress.com/about/ that others have used if you want to go that route.

Since you haven't stated the troubleshooting steps that determined the pot is bad how do you know it's actually the problem? I just hate to see you buy something off eBay, wait a week to get it, and then find out it's not actually what you need.

I also see Ulma Doctor has already given some advice on your other thread.



Ulma Doctor said:


> they have DC motors in them.
> the variable speed drives on these lathes are unfortunately kinda poor.
> they do the job most of the time, but since they are made to a low price point- expect some problems down the road.
> all is not lost, you can always replace the faulty stuff with another form of PWM ( pulse width modulation) controller for not a lot of money when calamity ensues



If I were you I would contact the seller and tell them how disappointed you are that their brand new machine doesn't work right. Don't tell them that you have taken it apart, but do let them know you want a replacement. They will take a while to get back to you and you will go back and forth but they will not want you to ship it back to them so you can ask for a partial refund.

Then do your research and replace the whole control system with something better, you really don't want to be chasing this problem since accurate speed control is critical to using the machine. You may come out with a better machine and a little cash back in your pocket.


john


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## Abody711 (Jul 11, 2020)

Machine is new. Checked resistance between the middle leg against the outer remain constant across the range of switch. With occasional drop outs. Does go to zero in the off position. Which could mean board has issue but I think pot is a issue for sure. I don’t want to wait two weeks on a $10 part to find out.


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## matthewsx (Jul 11, 2020)

The greater Knoxville area should have someone who carries electronic parts. I found this place 






						Home
					






					www.shieldselectronics.com
				




but didn't see components offered. There are lots of PWM controllers on Amazon but I think your first move should be contacting the seller and reporting the issue with eBay.

Good luck,

John


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## CluelessNewB (Jul 11, 2020)

My gut is telling me 4.7K ohm mostly because all the ones I find online with a "7" after the K are 4.7K ohm but the 9K measurement  end to end tells me more like 10K ohm.    Probably anything from 4.7K - 10K would work.


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## silence dogood (Jul 11, 2020)

Talk about crappy labeling.  This pot looks like that it was made in Lower Slobovia.   Potentiometers of that style usually come in even number ratings such as 1k, 5k, 10k 25k.  Well, you get the idea.  Measure across the two outer terminals with a ohmmeter.  I bet it is near 10k, maybe 5k.  1/2 k off is not a big deal.   I believe that these pots in the control circuit is a linear pot.  You don't want a log style, those are used for audio volume control circuits.  If you can, get a Allen Bradley type J.  I think that they are still made in U.S. (someone correct me if I'm wrong)


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## markba633csi (Jul 11, 2020)

Not saying the pot isn't a bit flaky, but my hunch says the controller board is misbehaving.  Some of them do have an annoying "feature" which requires you to turn the knob all the way down before the motor will start,  but it sounds like yours is acting strangely even so
Definitely notify the vendor, see what they say. 
-Mark
ps:  at www.olduhfguy.com there is a troubleshooting section that might be helpful


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## RJSakowski (Jul 12, 2020)

An important parameter in selecting a potentiometer is the taper.  They typically come in one of three styles, linear, audio, and reverse audio.  A linear taper pot will be at 50% of its total resistance when the pot is rotated half way.  An audio taper pot will be at 50% of its resistance when the shaft is rotated to about  85% of its CW travel.  A reverse audio taper  pot will be a 50% if its resistance at about 15% of its travel.  For an application like a speed control, you want a linear pot so the operating range isn;t compressed

As to type of resistance element, a wirewound pot is the the least sisceptab;le to wear from the wiper.  The two issues with wire wound pots are they are limited to lower resistance values and the resistance necessarily increase in step as the wiper slides from one winding to the next.  This can be a problem if you want tight resolution.  Other types of resistance elements include carbon film, conductive plastic, and cermet. Of these, the carbon film is least reliable but cheapest.  It is best relegated to set it and forget it type application.  Cermet pots are reliable.  I haven't evaluated any conductive plastic pots so I can't comment on their reliability.

Another parameter is the power rating.  As with any resistor, when the power rating is exceeded, the pot will overheat and can fail prematurely.  This shouldn't be an issue with the OP though.

Finally, comes the resistance value.  If a pot is operating as a variable resistance or rheostat, this is important.  Too low a value will limit operation while too high a value will limit the useful range of the rheostat.  Operated as a potentiometer or voltage divider, the resistance is less critical as the voltage out it equal to the product of the voltage in and the fraction of rotation and independent of resistance.  Resistance can come into play regarding the associated electronic but usually isn't a problem as long as values are reasonably close to the design value.

edit:  I should add mechanical characteristics such as mounting type, shaft style, and single or multi turn.  Also, open design vs. enclosed vs sealed.


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## Abody711 (Jul 13, 2020)

Thanks for all the information. I bought a 10k potentiometer locally. Seemed lightweight but worked. Controlled the rpms. The lathe would only reach 830 rpms. Supposed to be 1440. Is the 10k potentiometer to high or to low. Old one measured 9k on multimeter.


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## RJSakowski (Jul 13, 2020)

There may be a series resistor to limit the upper end rpm.  Id you're lucky, it's adjustable in which case you just set it to bring your maximum rpm to 1440.  Ifr not you can increase the maximum rpm by decreasing the value of the series resistor.  The easiest way is to add another resistor in parallel with it.  When I do that, I just tuck the leads of the additional resistor  under the existing resistor leads and add a bit of solder.  Don't dolder the reswistor in until you have determined the proper value.  The new resistance value will be Rnew = (Rold x Radded)/(Rold +Radded).


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## Abody711 (Aug 3, 2020)

The best for the best answer goes to Matthewsx. Other issues have popped up. Olduhfguy confirm that I am having control board issues. Even offered to send me 4 or 5 other bad ones free. Stated a lot of Machines are being shipped the the control board I have and a large number are failing. I  ordered a US made board from him today. Thanks for all the answers. Learned a lot from them which I love. Old but still learning.


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