RBT and Pro-160 / 360 P3 Pot Fault

The RBT hand controller has been in production for many years and has proved a popular way of controlling many of our products.

But the introduction of the Pro-160 / 360 controllers has brought to light one issue. These controllers have a more sophisticated pot testing circuit than our earlier products, one aspect of this is that for safety reasons they continuously check for breaks in the pot wiper circuit, if such a break is detected then fault code P3 is shown and the controller shuts down.

Apart from obvious wiring problems, there are two possible causes for breaks in the pot wiper circuit….

1] As mentioned elsewhere it is possible to force the RBT handset plug into the chassis lead socket with incorrect alignment, one consequence of this is that it is possible to apply a short circuit from B+ to ground via the pot wiper at it’s zero point. If this happens, then the track in the immediate vicinity of the zero point will be destroyed. Here is a picture of an example with the damaged area circled in red…

This damage can be detected by using the pot learn function in the PRO-160 / 360 to check the reading for the zero point. On a good pot this reading will be between 3 – 35, a bad pot will show much higher values, around 450 on the units we have tested here. If you are seeing values greater than 35 particularly if the lever is pushed gently sideways then it is likely that the pot is damaged and needs replacing.

2] The green 50′ plastic pots used in the RBT have a plastic shaft running in a plastic bearing and they only have a rated life of 20 000 operations. The sideways load of the return spring can cause the bearing to wear and the wiper track will then start to wear away [circled blue above]. As this wear progresses, the chances of a disconnection in the pot wiper circuit increase, and hence the likelihood of a P3 pot fault. If you are getting intermittent P3 faults which clear for a while after switching off and on again it is almost certainly due to the pot requiring replacement. Note: As of 4th May 2023 the design of the 50° version has been improved to make it longer lasting. See this article for more details. RBT-24Vs with serial numbers 23051 onwards have the new version fitted.

Software update.

In software version 2.4.1 we changed the way that a P3 pot wiper fault is handled, previously this fault required a power off / on to restart. With 2.4.1 the controller will restart if the fault is cleared, this makes the Pro-160 / 360 more resilient to worn pot tracks.

Potentiometers do wear out, how quickly this happens depends on a number of things. This example was taken out of one of our test boxes, it had been in daily use for about 2 years. If you are getting P2 or P3 pot faults, then the first thing to do is to change the pot.