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From: Fred Bloggs
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Subject: Re: Triac controlled holiday lights
References: <email@example.com> <firstname.lastname@example.org> <3DEB8034.email@example.com> <firstname.lastname@example.org>
Date: Tue, 03 Dec 2002 15:24:31 GMT
NNTP-Posting-Date: Tue, 03 Dec 2002 07:24:31 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Steve W.EE wrote:
> 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?
Is 8 Watt the actual power dissipation? If so then go with 1.1K at 20W,
these are available for about 30 cents, reasonable size, and called
"cement" power resistors in the trade. No heat sinks will be required.
Look in your yellow pages under electronic equip. & supls.- dlrs to find
a retailer who sells to electronics repair shops. They will have the NTE
line in cement power. You can place them on a small perf board that
mounts under/over your triac control board on standoffs.
> 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.
Right- but those were for bulbs rated for 120VAC. The half-wave
rectification cuts the RMS down to 70% of that or 84VAC. In your case,
you would have 84VAC across a string rated for 60VAC, and this will be
bad news. My dimensions were off in the original post, but if the bulbs
are rated at 700 hours on 60VAC, then 84VAC will reduce their lifetime
to 12 hours.
> 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.
Ahh- that's an idea.
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