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From: John Woodgate
Subject: Re: Current draw of several light bulbs?
Date: Wed, 11 Dec 2002 20:52:55 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Wed, 11 Dec 2002 21:30:26 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Rileyesi
wrote (in <firstname.lastname@example.org>) about 'Current
draw of several light bulbs?', on Wed, 11 Dec 2002:
>>Yor calculations seems to be correct. What is the input power
>>source? AC line? Looks like a simple transformer is all you
>>need. Safety can be an issue in outdoors applications. In this
>>case it's always better to use the isolated supply - same
>>transformer. Making a fancy switching supply just to drive few
>>bulbs looks like an overkill to me.
>My intent is to simply use a step down transformer from the 110 VAC wall
>socket. Someone else mentioned in rush current. Anyway to calculate that?
Measure the cold resistance of a lamp and compare with rated
voltage/rated current. I just did that with a British 6.5 V 0.3 A lamp
at 1.7 ohms at room temperature, so the initial inrush current is 3.8 A.
Ten times the rated current is quite normal.
>other words, if I assume that I will need 6.3 amps to drive the lamps, what
>capacity of a transformer do I need to account for the inrush. 7.5 amps would
>be about 120% of the in service current. Is 120% enough???
You don't need to uprate the transformer, because the inrush last for
only a few milliseconds, but it kills lamps rather frequently. That is
why I recommend a thermistor.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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