4 Pin Potentiometer Identification

Question

I am trying to repair a very expensive manlift joystick. I had great luck soldering in a new potentiometer on a different machine and getting it back to work, but this other machine has a potentiometer that I have not seen before.

It has 4 pins total, three like a normal rotary potentiometer and then a fourth across from the other three pins.

It is a Clarostat U-10K-OHM Type EJ pot. It has another number stamped "99148B 9507"

Clarostat has been purchased by Honeywell, so I am having a hard time finding any catalogs or datasheets for this guy. Can anyone point me in the right direction or advise what the fourth pin may be for? Would replacing with a standard three pin pot be a possibility?

Thanks!

Answer 1

I second Spehro in what he thinks it is: an heavy-duty pot with additional center tap.

TT Electronics seems to sell a product similar to the part sported in your photo (datasheet).

Here is an excerpt from the datasheet with the technical drawing:

And here is the electrical circuit:

Even if it is not what you are looking for, it may be a starting point.

Answer 2

Not familiar with that series, but it appears to be an extended-life pot.

There are two possibilities for the 4th connection. The first is that it is a center-tap on the element. This might allow them to control the output at the neutral position of the joystick better. The second (and less likely one) is that it's a ground connection.

A quick test with an ohmmeter should settle the matter. Here is a picture of a center-tapped 10K pot. It would measure 5K from either end of the element to the opposite (center-tap) pin. The wiper would short to the center tap when in the center of rotation.

Answer 3

Four-terminal potentiometers were common at one stage on hi-fi volume controls to give automatic "loudness" control - see What's a potentiometer with four terminals in this schematic? - but these were logrithmic law and a joystick is more likely to be linear.

My guess is that it's a linear pot and that the centre-tap is used to provide a definite GND or reference to assist with a deadzone in the centre of the pot. This would be intended to prevent creep when the joystick is centred.

^{simulate this circuit – Schematic created using CircuitLab}

I found a Clarostat brochure on Stephen Engineering and they seem to be handling the product now. They don't have any four terminal pots listed.