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- HPM timer - "interesting" instruction sheet
June 16, 2005, 1:59 pm
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Recently got a HPM digital timer (that replaces a standard wall switch
plate) and had a rather interesting part in the instructions that
sounds a bit strange to me
Note the part at bottom right - that has been circled in black:
"Power factor correction capacitors, (if fitted) must be either
removed, or placed on the line side of your HPM product. if this is
not possible to do, then a 36w fluorescent ballast must be connected
as a dummy load in parallel wiith the fluroescent lamps"
This is a 2 wire device (in series with the lamp) has no neutral
connection, and I assume that the leakage current through the lamp
(when turned off) is used to charge the battery in the unit,
I also assume that the power factor capacitor would provide a surge
through the triac in the unit when it switches on, and possibly short
it out, which is why the cap has to be removed or put before the swit
ch? However i fail to see how a 36w ballast put in parallel with the
output is going to stop this from happening, as well as being a waste
of power when the lights are turned on ? I would imagine that a 36w
ballast straight across the mains is going to dissipate about 50w or
more and is hardly a safe situation ?
Finally, isn't the moving of a power factor capacitor from the
switched side of a fluro, to the mains supply going to upset the power
factor (admittedly very slightly) when the fluro light isn't turned on
Re: HPM timer - "interesting" instruction sheet
** The parallel combination of the ballast's inductance and the correction
capacitor forms a resonant circuit at approx 50 Hz. Being a parallel
resonance means the impedance is high and inrush surge current relatively
low compared to a simple capacitor.
** Dangerous business imagining figures like that - you need to measure
the rms current draw with 240 AC applied plus the resistance of the winding
and compute the power. I tried this just now with a 15 watt rated ballast
(from an old desk lamp) and got a current of 340mA and a resistance of 51
ohms giving a dissipation figure of 5.9 watts.
** A modest amount of capacitance added permanently across the AC supply
can only improve the overall power factor which is nearly always lagging due
to loads like fridge motors, fans, A/C units etc.