Second version of the PWM dimmer and test results
In this article I will give updates on the PWM dimmer I explained in a previous article. I will explain the changes that have been made and present a new schematic. I will also quantitatively compare the performance of the PWM dimmer to the performance of a phase control dimmer.
A better design would have an second copy of the MOSFET/rectifier/gate drive circuit between the output and neutral connected to another winding on the GDT, but with the opposite polarity. This would cause the MOSFETS to switch alternately, meaning there is always a path for current to flow in or out of the output, but never a path for current to flow from live to neutral directly.
It is also possible that the core of the inductor is saturating, which causes it to heat up as it dissipates energy rather than storing it in its magnetic field. If this is happening, I would need to replace the filter inductors with appropriately sized ones.
New schematic and explanation
The new schematic is shown below:Link to simulation (1Ω resistor and 500pf capacitor are only for the simulation, they are not part of the circuit)
I have made several changes to the design:
Gate drive circuit
The gate drive transformer was replaced with a handmade one made from a toroidal ferrite core and 9 turns of a single twisted pair from a CAT5 cable. An additional circuit was also inserted between the 555 timer and the gate drive transformer to amplify the current on the output. This circuit consists of an NPN and a PNP transistor, with both bases connected through separate resistors to the output of the 555 timer. The capacitors between the output and the bases help the transistors turn on and off quickly by compensating for the parasitic base-emitter capacitance of the transistors.Filter circuit
To improve the efficiency and make the MOSFET run cooler when the dimmer is used with a fan, a filter circuit was added to the output of the dimmer. This circuit consists of an inductor, two capacitors, and a common mode choke. The inductor and larger capacitor act as a filter on the output of the dimmer circuit. The smaller capacitor acts as a snubber to absorb the energy in the inductor when the MOSFET turns off. However, this circuit does not work very well as the inductor can get hot enough to boil water during operation, and was measured to be over 150°C with an infrared thermometer.A better design would have an second copy of the MOSFET/rectifier/gate drive circuit between the output and neutral connected to another winding on the GDT, but with the opposite polarity. This would cause the MOSFETS to switch alternately, meaning there is always a path for current to flow in or out of the output, but never a path for current to flow from live to neutral directly.
It is also possible that the core of the inductor is saturating, which causes it to heat up as it dissipates energy rather than storing it in its magnetic field. If this is happening, I would need to replace the filter inductors with appropriately sized ones.
Data collection
The current waveform, noise level, and rotational speed of the fan were measured, both with the PWM dimmer and the phase control dimmer. This would allow me to compare the performance of the two dimmers in a variety of metrics. Both dimmers were tested using a fan and an incandescent bulb. For both dimmers, the current was measured using a home-made current transformer consisting of a transformer from a power supply with a single turn of wire as the primary and the mains-voltage winding with a 316 ohm resistor soldered across it as the secondary.Measurements
For each dimmer and each increment, I measured the following variables:- The RMS voltage on the current transformer. (fan and bulb)
- The RMS voltage of the principal component of the output of the current transformer (fan and bulb). I used the FFT function of my scope to view the current waveform in the frequency domain and find the amplitude of the 60Hz componet of the current waveform.
- The overall noise level (in dB) produced by the fan, measured at a distance of around 8 inches. (fan)
- The frequency of the sound produced. This is directly proportional to the rotational speed of the fan. (fan)
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