Temp controller fan

[yo1dog]

I have built a temperature controller using a cheap honeywell thermostat following this tutorial.

1st problem:
I removed the jumper between R and RC because I realized that the connection between RC and G was ALWAYS closed, even when the thermostat was off and had no power.

2nd Problem:
Now connected to R and G; When the Heat/Off/Cool switch is set to Heat or Off, my test fan that is plugged in turns off as expected. However, when I change it to Cool (and the fan mode to auto) no matter what temperature I set the thermostat to the fan still runs. The display does NOT show "cooling on" as it does when you set the thermostat below the current temperature, yet the fan still runs.

I know it is not the thermostat itself because this is the second one I have tried.

Thanks,
- Mike

[easternstargeek]

I'm confused- the link that you cite in your post has nothing to do with cheap Honeywell thermostats (CHT) of the kind you describe. Are you sure that is the right link?
It would also help to have a model number of your CHT, and a better description of what you are trying to control with it. Please provide the following:

Power Supply to the Thermostat
Type of Device (Load)you are trying to control (e.g., relay, motor, fan, compressor etc.)
Load characteristics: Operating voltage, power, current ratings
A brief description of what your system is for, and how you would like it to work.

The following diagram shows a simplified version of what goes on between a typical Heating and Cooling (HVAC) system and a Heat/Cool thermostat. (Note: This is not for heat pumps, which are a bit more complicated). This system uses a low voltage/low power control circuit, using a 24VAC control transformer and a set of relays for the Fan, Heating Device, and Cooling Device.
Such thermostats are designed for controlling LOW POWER devices like relay coils at 24VAC. Any operation outside of that range will likely smoke the unit.

Your CHT is probably very similar to the right-hand side of the block diagram, below. The box marked "Electronics" is exactly that- the microprocessor, display, and relay coils (not shown) that pull in the "Heat Call" and "Cool Call" contacts.

Basically, it goes like this... Whenever the electronics decide it's time to cool things off, it will close the relay contact labelled "cool call", This completes a circuit that energizes another relay coil in the HVAC system, which causes it to do whatever it does to make Cold. So it also goes for warming things up, except the "Heat Call" contact is used.

The Fan may be forced on to run at all times by moving the Fan switch to "ON." When it is in Auto mode, the Fan relay coil will piggyback on top of the Cooling or Heating relay coil, depending on how the Heat/Cool mode switch is set.
Note- the Heat/Cool switch also has signal contact (not shown) that is tied to the electronics.

The R, Rh and Rc terminals are usually at the same potential, and come from a controls transformer inside of the HVAC system. Separate terminals for Rc and Rh are provided because some HVAC systems contain separate transformers for the Heat and Cool apparatus, so removable links are provided. In most cases, they are left in place.

Hope this helps!