I just finished assembling the MK138 Thermostat. The LED and the Relay both work when I adjust the trimmer, but when I test it it does not appear that the change in temperature triggers any change in the relay. I put it in a cool place, adjust the trimmer so the light just goes out and advance the temperature - does not turn on. I adjust the trimmer until the light goes on and move to a cool location and light does not turn off.
I have examined the board for solder “spill over”, checked the orientation of the diodes and the ic. The input voltage from my 12 vdc adapter is a little high (16 volts) - could that be the issue?
Anyway - is there a checklist of voltages or tests to fault isolate why the thermostat is not responding to temperature change?
Thanks,
I got some excellent help from support, so I thought I would post the help in return, just in case some other newbie has similar problems.
Number one, the voltage appeared to be too high, at least for long term operation - a 12 volt regulated supply was recommended. I found a dc adapter with the larger “brick” (more than just a transformer) that usually means there is some circuitry involved. The output was a consistant 12.3 volts, so I switched to that for a power supply.
As far as checking the thermometer out, support recommended the following:
“The best and easiest way to test the unit
Set RV1 so the LED has just turned off
Fill a glass with ice water and let the tip of the sensor touch the glass.
After the sensor cools to the point below the set range the relay should click and the LED should light up.”
Well, that was exactly the opposite of how I thought it was suppose to work…so once I had that information, and the unit did checkout OK, it was fairly simple to set it up using a halogen light to warm the sensor and a IR thermometer to measure the on/off points, using the surface temperature of the sensor. It now turns a fan on at 80.7 degrees and off at 73.4 degrees - almost exactly what I wanted.
Hope this will be helpful to any others working with this kit!