# how to make 3K linear pot into 2K linear pot

• posted

Hi - I have a slightly odd problem. I have a 3K pot that I'd like to have be more a 2K or 2.5K pot (not positive about the range just yet - but somewhere around 2-2.5K).

This reason for doing it is this: I recently acquired some hobby servos. Unlike the servos I've used before, they were limited to about 120 degrees of rotation (others I've used have been able to go about 190 degrees). I have confirmed that all the mechanics of the servo allow about 190 degrees of rotation. It's just that the control chip will only allow it to go about

0-120 degrees. The control chip gets its position sensing from a pot (that can turn about 190 degrees) that is attached directly to the output shaft. Thus - it would be possible to trick the servo into going the full 180 degrees if I could multiply the resistance by about 2/3. (2/3 = 120/180).

Any ideas on how to do this? I should mention that it appears one of the outside leads of the pot, as would be expected, is entirely disconnected.

Thanks!

-Michael J. Noone

• posted

If it is being used as a potentiometer and not a variable resistor then the absolute value of the resistance isn't that important -- what's important is the voltage at the wiper with respect to the supply voltage. That's going to be the same whenever the pot resistance is significantly less than the input resistance of the chip.

I suspect that you'll need to pad each end of the pot for that, otherwise you'll have all of the response crammed up on one end of the servo travel.

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Tim Wescott
• posted

Are there 3 connections to the 3 k pot, or two? If there are 3, you can probably just put a fixed resistor in series with one end and get more rotation out of the servo.

• posted

all you need to do is put maybe a 680 ohm R in series to the high side of the pot. this will force the controller to move the shaft more since the voltage at the top of the pot will not be less. depending on how the controller works, you may need to put the R on the low side of the pot to now allow it to see the low value as soon. or you could put like 330 ohm in series on both sides.

• posted

Fred Bloggs wrote in news: snipped-for-privacy@nospam.com:

Eh no secrets... The board looks fairly complicated so I thought that the easiest solution would be to just work with the pot. I uploaded a bunch of pictures here:

- I included resized versions for those with more limited bandwidth. If somebody needs me to trace out the circuit I can do that.

Also - I wonder if I could just replace the pot? Is that a standard shape/size? I expect 2K pots are probabaly made...

• posted

Are you perhaps not approaching this the wrong way? Is the circuit not something like a *fixed* resistor in series with a pot, where the wiper is used to indicate position? If that is the case, can you not leave the pot at 3k, and change the fixed resistor?

About the only way I can think of chaning the pot's effective value is to put a resistor in parallel. But then it will no longer be linear.

Another possible option, is to put a second linear pot in parallel. If you have a 10 k and a 3 k pots in parallel, and join the wipers, then you will get an overall linear pot of 30/13 k (which is somewhere between 2 and 2.5). (The degrees of rotation must be the same of course.

• posted

The formula for resistors in parallel is 1/R = 1/R1 + 1/R2 + ....

6000 ohms in parallel with 3000 ohms makes 2000 ohms.

If you connect 6k across the two terminals of your variable resistor, you'll get a zero to 2k range, but it won't be linear.

You could look at the rest of the circuit, see if there's another resistors in series with it and try adjusting that.

• posted

What's your problem that you want to keep the "control chip" and its particular configuration circuit a secret? Will it burst your fantasy bubble about a magic pot changing solution? Your damned "control chip" doesn't give a damn about your pot and its resistance- it only knows voltages and currents- that's why *wires* are attached to the pot.

• posted

"Michael Noone" schreef in bericht news:Xns967D91B1D9BE7mnooneuiucedu127001@216.196.97.136...

Michael,

Apparently the pot is used for a variable resistor. The servo electronics use a 0-2k resistor value for a 180 degrees deflection. As the servo system does not "know" the deflection but from the variable resistor it's at its limit when this resistor reaches 2k. So if you replace the 2k pot for a 3k pot, it uses only 2k or 2/3 of it and so wil happen to the deflection.

The most simple way to make 2k from the 3k is mounting a 6k parallel to the latter. But you can bet the linearity will be gone. Another aproach is disconnecting the pot and putting a piece of electronics between it and the servopart, to emulate the 2k pot. To be honest I don't see a lineair electronic 2k pot be build easily. Guess it will become more expensive the a new servo.

You can try to find out what voltages apear on te pins of the pot. Maybe you can drive the servo by pushing the required voltage on the pins of the pot. A resistance to voltage convertor should not be that difficult. Of course you may have to replace the pot by a fixed resistor to use the pot as an input for your electronics.

If you want to find more detailed solutions, you'd have to find out more about the servo electronics. Once you know more details of the input circuit, you may find an easy solution. (And we can help searching for it.)

petrus bitbyter

• posted

"Michael Noone" schreef in bericht news:Xns967DC6B6EDA64mnooneuiucedu127001@216.196.97.136...

Having seen the pictures I'm sure you can replace the current pot by a 2k one. Lineair 2k pots are widely available but you may look for some type that is intended for use with servo's. For a good lineairity and performance you'd better avoid the cheapest ones as they often have a poor quality.

When a pot is used a variable resistor, I advise to connect the not-used terminal to the one in the middle. It's especially usefull when the resistor get some serious current but it never harms. But looking to the picture I'm sure the pot is used as a pot so beware.

petrus bitbyter

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