# electrical noob needs a potentiometer help



## road (Mar 25, 2013)

My Craftex mini lathe uses a 4.7 k  pot with a switch for speed control. 
The splined post broke off the pot.
So I need to change it. 

Being as cheep as I am,, A local electronic shop has 5k pots w switch for $3.99 & no shipping costs. 

would there be any concernable difference in the 5k as to the 4.7k ? 

As long as a 5k keeps the rpm stable from low to mid range, even if the rpm ramps up quicker at the hi end of the 5k it should be within range.. I hope..  

I hope I stated that right.. 

lmao


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## Roger Smith (Mar 25, 2013)

see below, just learning the forum


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## Roger Smith (Mar 25, 2013)

Road you are only talking about a 6% increase in resistance. It may not be as sensitive as it was with the 4.7K,(instead of 1 rpm change with a tiny turn, maybe 1.5 rpm) but will function quite nicely.


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## road (Mar 25, 2013)

Thanks Roger ..


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## bedwards (Mar 25, 2013)

I think most electrical circuits and components have a design tolerance like +- 5 or 10%. I imagine you are well within the tolerance of a new part. I am no EE though 



bedwards


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## Hawkeye (Mar 25, 2013)

If you consider how a pot is normally connected to the circuit, I wouldn't expect any noticeable difference. In a 3-wire connection, one side is connected to positive power, possibly 5 VDC. The opposite end of the resistance is connected to the board ground, or 0 VDC. The middle terminal is the wiper. It slides around the resistance, but it isn't measuring resistance. It's sending a variable voltage into the circuit to control the speed.

The only difference from using a slightly higher overall resistance will be a slightly lower available current. If the circuit is set up to respond to voltage, not current, it should respond exactly as before. In a linear pot (not audio taper), turning the pot 50% will pass 50% of the available voltage into the circuit, whether it is a 4.7k or a 10k. Make sure you get linear.


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## Ulma Doctor (Mar 25, 2013)

the 5k would be an acceptable replacement


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## arlo (Mar 25, 2013)

The difference in maximum resistance should be insignificant.  Be aware that potentiometers commonly have two different characteristic responses.  An audio taper is logarithmic while a linear taper is, er, linear.  It's most likely the original is a linear taper.  Getting a replacement with a different taper will work but will change the "ramp rate" you referred to.  

The replacement should have at least the same power capability (measured in watts) as the original.


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## Rbeckett (Mar 26, 2013)

The 5 K will be just fine.  If you really wanted to be exact you could just add an additional resustor and drop it to 4.7, but whats the point.  As long as it is rated for the wattage or above you will be just fine with no discernable difference in how it works.  Good call to check befpre you butched just in case it wouldn't work for some reason or other.

Bob


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## road (Mar 26, 2013)

Got my new 5k pot today.
I got 3 for $7.00 (future project in mind)

I had it all rewired before dinner. 
It works even better than before, nice n smooth. 

While I had the control box off I made a plastic cover for the board to protect it from swarf n chips and added some cooling holes too.


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## road (Nov 24, 2013)

Well this is a disapointment. 

After all this time , my 5k replacement pot. it has finally given up .. 

I guess its my fault for buying a el cheepo part...


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## Hawkeye (Nov 24, 2013)

If you still have the original 4.7k pot around, check the wattage rating on it. Most control circuits don't burn off a lot of wattage across a pot, but, if your 5k replacement was rated at a lower wattage than the old one, that may account for the failure. Or the day of the week it was made ... :thinking:


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## Chucketn (Nov 24, 2013)

If the pot is for a 
SEIG type mini lathe, Little Machine Shop will have the correct rated replacement.
http://littlemachineshop.com/products/product_view.php?ProductID=1282&category=5

Chuck


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## road (Nov 24, 2013)

It was a mechanical failure and it may have shorted out if i had not noticed the damage. 

I ordered a few new ones .


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