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Date: 3 Dec 2002 02:51:06 -0000
From: Steve W.EE
Author-Address: stevew-ee nym alias net
Subject: Triac controlled holiday lights
References: <email@example.com> <firstname.lastname@example.org> <3DEB8034.email@example.com>
In comp.home.automation Fred Bloggs wrote:
: Steve W.EE wrote:
: [...snip extraneous stuff...]
: Given your capability, the surest route to a finished project is:
: 1) power up the string of 50 bulbs at 120VAC and measure the AC current Iac;
: 2) compute resistor R= 60VAC/Iac ohms
: 3) compute resistor power rating P=R x (Iac)^2
: 4) select standard resistor value Rs closest to computed R and standard
: power rating no less than 1.5 x P
: 5) insert resistor Rs in series with string of 25 bulbs and power of 120VAC.
: example: string of 50 bulbs is 50W then you have Iac will be measured at
: 50W/120Vac=0.42 Amps. This makes R=60/0.42= 144 ohms, and power= 25
: Watts. Place a 150 ohm 50W power resistor in series with each string of
: 25 bulbs, or use 2x 75 ohm 20W in series with string etc...
I did the calculation, what I need is a 1100 ohm, 8 watt resistor.
Actually, 14 of them. These are not exactly off-the-shelf items. It
has to be something which is properly heat-sinked, and suitable to
use outdoors. Any ideas on where to get them?
Actually, I did find a place where I could get them for about 30
cents apiece, including water-resistant heat sinks, a mounting
bracket and even some quality connecting wire already attached.
There is some type of unwanted radiation they might give off but
it will not be difficult to fabricate a shield.
Care to take a guess where I plan to get these resistors?
: Your idea with the diode(s) will pop the bulbs in about 10 minutes.
I beg to disagree on this. It does not harm incandescent bulbs to
drive them with half-wave AC. In fact it lengthens their life because
they are burning at a lower level. In fact a few years back
they were selling diodes in a small flat package which could be placed
in a lightbulb socket to dim the lights and save energy.
The reason I'm going to abandon the plan with the diodes, it's not
practical. If I took a string of bulbs it would have to be cut
to 71% of its length. Then when driven through a diode the bulbs
would be at full brightness. A 50 bulb string would be cut to 35
bulbs, this is not enough a reduction to we worthwhile.
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