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From: Spehro Pefhany
Subject: Re: Current draw of several light bulbs?
X-Newsreader: Forte Agent 1.92/32.572
Date: Wed, 11 Dec 2002 01:49:59 GMT
NNTP-Posting-Date: Tue, 10 Dec 2002 20:49:59 EST
On 11 Dec 2002 01:39:44 GMT, the renowned firstname.lastname@example.org
>I need a rough and dirty way of estimating how much current I will need to
>drive a set of light bulbs. The situation is that at some installations, there
>will be 6 light bulbs in parallel while at others, there will be as many as 42.
> These are #47 incandescent bulbs rated at 6.3 volts and 0.15 amps. I assume
>that this means it will draw 0.15 amps at 6.3 volts when illuminated (so each
>bulb is 42 ohms).
>My first guess would be to simply add the current draws (i.e. 6 bulbs in
>parallel would draw 6 x 0.15 or 0.90 amps). Can I assume that 42 bulbs will
>draw 6.30 amps (i.e. 42 x 0.15 amps)? I don't have any of the bulbs yet (so I
>can't do a bench test) and want to get started on designing the power supply.
Yes. The resistance value of 42 (!) ohms is only good when you have
6.3V across the bulb, it will have a significantly lower resistance
when cold. Other than that, they just add, the current for n bulbs in
parallel is n* the current of one at a given voltage.
>I also looked at the resulting resistance of a network of 42 bulbs in parallel
>assuming that all have a resistance of 42 ohms. This calculation also leads me
>to a total current draw of 6.3 ohms, but that calculation might be simply a
>re-arrangement of my first attempt since they are based on the same
>Another question, what is the difference between driving them with DC rather
>than AC? I know that the bulb doesn't care
True only to a first approximation. Check out these links:
Look at the excellent photos of filament "notching" in the Transport
> and I'd like to use AC because it
>simplifies the power supply. Are there any advantages or disadvantages between
Regulated voltage would give more predictable bulb life.
>the two?? Any safety issues?
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