Improving a Holmes HAWF2043 window fan

Recently I got a window-mount fan with two separate fans and several operating modes. The device has a built in temperature sensor and supposedly is able to switch the fan based on the termperature in the room. It has one button that cycles through all 13 modes: off, fan on high, fan on low, fan on high with thermostat set to 60, 65, 70, 75, or 80 °F, and fan on low with thermostat set to 60, 65, 70, 75, or 80 °F. I didn't test the thermostat modes, but I do not think they would have worked well because temperature sensor was located in the case and would have read a different temperature than if it had been further into the room. Additionally, the fan would be unable to tell whether switching on the fan would have increased or decreased the temperature and might have made the room warmer even if it wanted to cool the room. However, I liked the ability to adjust the speed (and thus the noise) of the fans.

To make the fan easier to use, I first bypassed and removed the control circuit in the fan. When plugged in, the fan would now run at full speed. The fan was then plugged into an inline light dimmer so that the speed could be adjusted continuously, from off to full speed. Later, the dimmer was modified to allow the fan to run more quietly at full speed.

Operation of the fan control circuit

The fan circuit board and wiring have been reverse engineered. The schematic is shown below:

gEDA schematic file

The red box on the right contains the fan motors and the TRIAC that is used to control the speed of the fans. Each motor is represented by two coils in a green box. Depending on the setting of the direction switch, the mains voltage is applied either to the right or left coil. The other coil gets its power through a capacitor, which creates a phase shift (delay). Since the two coils are mounted at right angles in the motor, this creates a rotating magnetic field, which causes the fan to spin.

When a pulse is applied to the gate of the TRIAC, it conducts between its two main terminals until the current reaches zero. By adjusting the time between the zero-crossing of the input voltage and the trigger pulse to the TRIAC, the effective power of the fan can be controlled.

Operation of the dimmer circuit

A dimmer was purchased to replace the internal speed control and microcontroller. This dimmer allowed the fan to be switched off completely and allowed the speed of the fan to be continuously varied. The schematic of the dimmer is shown below:

gEDA schematic file | Simulation of circuit

Like in the fan control circuit, the dimmer also contains a TRIAC. C2, C3, R3, R4, and R5 form a delay network that delays the mains voltage by an amount controlled by the setting of R4 (the slider on the dimmer). Once the voltage on C3 reaches about 30 volts, the DIAC begins to conduct and the TRIAC switches on. The variable phase shift allows the amount of the sine wave passed to the load to be varied, which in this case controls the fan speed. Additionally, R1 and C1 form a snubber network that is able to absorb pulses of current when the TRIAC switches off.

Description of the modification

The control circuit was removed from the fan and the wires previously connected to the main terminals of the TRIAC were instead soldered together. The fan could now either be plugged straight into the wall or into a dimmer to allow the speed to be adjusted. I also noticed that when the dimmer was set to "full speed", the output of the dimmer was not a perfect sine wave, as a small amount of the sine wave was still chopped by the TRIAC. The edge that comes from the sudden turn-on of the TRIAC would cause the fan motors to emit noise while it was running. By replacing R3 in the dimmer with a short, I was able to reduce the noise when the fan was running at full speed. This is because the capacitor C3 could charge faster and the TRIAC would fire earlier, making the output waveform closer to a pure sine wave. However, the noise is still audible, especially when the fan is running at low speeds. (Upon testing the original circuit, I found it was not much quieter). The final schematic is shown below:

gEDA schematic file

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