The temperature in the control is jumping erratically. How do I fix it?

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For these older kilns

The temperature in the control is jumping erratically. How do I fix it?

Electrical Noise

External

  1. Sometimes there is electrical "noise" in your incoming line. This is hard to see (except with an oscilloscope). However, this can wreak havoc with the control and cause it to operate unpredictably.
  2. The best way to deal with this problem is to put your control circuit on a 120 volt line and power it with a filtered computer power supply. All the control transformers that we use can have 208-240 or 120 volts incoming. Normally we power the control circuit with the line voltage. (Note that 480 or other higher voltages use different transformers). However, it is possible to change the wiring and power it with a 120 volt home circuit and then plug that into an external computer power surge protector. Get the best that you can with plenty of filtering.
  3. Another way to deal with this problem up-front (or potentially as a retrofit) is to get our noise filter. This is not available on all models.

Internal

  1. Sometimes the cause of the noise is internal. This would be the power contactors causing voltage spikes as the turn on and off. We usually use MOVs across the coil of the contactor to absorb these spikes. The first thing is to check these and replace them if you suspect this is the cause of your trouble. 
  2. Another problem can be a ground that is not perfect. We suggest running a ground wire from the control directly to the earth ground in the control box. (In many of our control boxes we daisy-chain the ground to the relays and then finally to the earth ground - if yours is done like this run a separate ground direct from the control center tap to the earth ground in the box leaving the other ground wires in place.)
  3. See these parts.

Thermocouples

  1. Another source of noise that causes erratic operation of the control is noise that is induced in the thermocouple circuit. This can be caused by power wired running parallel to the thermocouple wires or can even be caused just by the voltage that bleeds through the firebrick and in the case itself.
  2. Thermocouple circuits are very low voltage and a small change in the voltage produces a big change in temperature reading. If the voltage in the wire fluctuates by any external influence then the control will think the temperatures are changing rapidly and erratically and no end of trouble will ensue.
  3. We control this to a large extent by grounding the negative lead of the thermocouple wires. The first thing to do is to check all of these connections.
  4. Also check for loose connections.
  5. Sometimes it is helpful to replace all of the thermocouple extension wire and terminal boards if all else fails.
  6. Another idea that was passed onto us by Bill Campbell is to wrap a piece of high temperature element wire around the outside of the thermocouple ceramic as it goes through the brick into the kiln and then ground that wire to the metal case (which is grounded) with a screw.
  7. Another idea is to put a ferrite bead around the thermocouple extension wire near the control.

Earth Ground Problems

  1. The problem could be a faulty earth ground. Grounding is important for safety of course but a bad earth ground can also prevent the electrical noise that you are so carefully "capturing" from going anywhere. If you suspect this try to make a direct gound from the kiln to a cold water pipe and see if this helps. If it does help then you know you have a grounding problem.