Testing a Dehumidifier circuit

Humidity sensors are difficult to check, as the sensor element is often a capcitive device that is driven by an AC voltage which is generated on the controller board. Without an oscilloscope to check for the AC waveform and/or a service document that specifies the expected voltages, this is hard to test.

This board has no humidity sensor. that is external and a mechanical one. what this board does or is suppose to do when you turn the knob on humidistat this waits for a couple of mins and then relay clicks and the dehumidifier starts. this doesnt click at all. I moved the relay to another board and tested and that is good. any other reference material or site/forum to get additional help would be appreciated.

John wrote:

Then it is a time delay, designed to prevent quick restart of the compressor. You need a circuit diagram, service manual or several years of experience to repair it.

Yes, or if youre lucky posting the cct diag for us to see might get you somewhere. No circuit, forget it.

NT

Yes, or if youre lucky posting the cct diag for us to see might get you somewhere. No circuit, forget it.

Forgot to mention, if its a time delay you could likely run the system without it, if necessary.

NT

If you replace it with a manual switch and don't quickly restart the compressor. But even then there may be an anti freeze or overflow switch.

There is an IC on the board which I believe has the time delay. Should I try replacing that ? I did a manual test and gave a little voltage across emitter and base and the relay clicked, so the transistor is ok. I can post the PCB picture if you guys think will help, let me know pls.

Homer J Simps>

How much experience have you with appliances and AC powered equipment?

IC's rarely fail, usually the problem lies elsewhere.

no

checked that the IC pin1 and pin8 should be getting 5v DC. its getting about 0.3v so the timer is not kicking in which in result doesnt start the relay through transistor. i have checked Diodes they conduct one way, also checked all resistors and one 47uF 65v capacitor. There are two small capacitors 0.1uF I believe which doesnt show me any signal. maybe too small to be tested ? How do I check the transistor, Its C556B PNP I believe. It looks to be ok because when I gave a small voltage across two pins on the transistor the relay contacted.

What does the 5V come from? Something may be shorted across it holding it down, or the power supply may have failed.

I will try and create a circuit diagram by hand for this board, its a small board so someone here can give me idea what failed thats stopping from giving 5v across the timer IC.

James Sweet wrote: