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From: davidc@vlsisj.VLSI.COM (David Chapman)
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Summary: why didn't I think of that?
Keywords: TTL, incandescent lights
Message-ID: <15276@vlsisj.VLSI.COM>
Date: 1 Aug 89 04:32:04 GMT
References: <4363@merlin.usc.edu> <1647@thumper.bellcore.com>
Reply-To: davidc@vlsisj.UUCP (David Chapman)
Organization: VLSI Technology Inc., San Jose, CA
Lines: 55

In article <1647@thumper.bellcore.com> dk@thumper.bellcore.com (Dan Kahn) writes:
>cyamamot@nunki asks why he can't fully light an instrument-panel bulb
>in the collector of a 2N2222 driven by TTL.  john@stiatl suggests
>
>> Your ciruit would work if the load was not a bulb.   The problem is the 
>> characteristics of a tungsten bulb.  The resistance of tungsten varys widely 
>> between ambient and operating temperature.
>> 
>3. It's true that when such a bulb is placed across a stiff 12-volt
>source, it will at first draw much more than 500 ma.  The crucial
>error in John's explanation, however, is the assumption that a bulb
>_must_ go thru this high-current inrush stage in order to reach its
>rated-current steady state.
>
>[sensible explanations deleted]
>
>And for those unimpressed with my "thought" experiment, try it in
>your lab.  Precise voltages and resistors are not required.  For ex.,
>you can use 120 VAC, a 6- or 12-volt bulb, and whatever resistance you
>need to get close to the bulb's rated current.  Not only will the bulb
>light up, it'll be a lot happier not to have been put thru the shock
>of the inrush surge that it experiences when placed across a stiff
>source at its rated voltage.

Oh, yes, why didn't _I_ think of that?  I've been watching this discussion
from a distance (I personally wouldn't use incandescent lights for anything -
strictly semiconductors :-) and never once did I remember:

There's a market for "light bulb life extenders" that do precisely that -
limit the inrush current while the bulb warms up.  A surge is clearly not
necessary to light the bulb.

Personally, I'd use some sort of Darlington arrangement with much more gain
than necessary.  TTL chips, even from the same manufacturer, vary widely
in their output current capability.  If it says 400 uA, don't try to use
any more than 400 uA even though you "know" that number is conservative.

(Of course, I expect to light an LED with the output of a 4049 - CMOS buffer -
in series with a 330 ohm resistor :-)

>Dan Kahn      Bell Communications Research      Morristown, NJ


(space filler)



So who was the idiot who wrote inews?  Why should there be more of "my"
text than what I quoted if the article is short?  They should be _glad_
that I'm not contributing _more_ to net.noise!  Grrr.....
-- 
		David Chapman

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