Dimmer Phase Angle Test

Elliott Sound Products

These results were obtained from a circuit simulator, which allowed me to capture all the data I needed, without having to use test equipment attached to the mains. The results are not quite the same as with a real lamp, because the filament actually changes its resistance with temperature. The table below shows the theoretical power, current and power factor, ignoring the changing resistance.

Phase Angle	Volts _RMS	Current _RMS	Power	Power Factor
18°	19.28 V	33.47mA	645.3 mW	0.08
36°	52.93 V	91.89 mA	4.86 W	0.22
54°	92.53 V	160.6 mA	14.86 W	0.39
72°	132.9 V	230.7mA	30.65 W	0.55
90°	169.7 V	294.6 mA	50.00 W	0.71
108°	199.9 V	347.0 mA	69.35 W	0.83
126°	221.4 V	384.5 mA	85.14 W	0.92
144°	234.1 V	406.4 mA	95.14 W	0.98
162°	339.2 V	415.3 mA	99.35 W	0.99
180°	240.0 V	416.7 mA	100.00 W	1.00

Power vs. Phase Angle For TRIAC Dimmer

For the simulation, I used a 100W load, based on a supply voltage of 240V. This gives a resistance of 576 ohms, which is 100W at 240V. The phase angle is a measure of how many degrees of each half-cycle the dimmer allows through, and is in 10 steps. The power factor is as shown in the table above, and at most usable settings, it's no worse than a typical CFL. Since those pushing for a ban of incandescent lamps have never looked at power factor anyway, to them it is presumably irrelevant. :-)

To explain the table, a cycle of mains power is traditionally divided into 360°, so a half-cycle is 180°. I used 10 steps of 18° for the table, but real dimmers can use any phase angle as set by the control - they are not limited to discrete steps.