Path: utzoo!attcan!uunet!cs.utexas.edu!csd4.milw.wisc.edu!mailrus!sharkey!teemc!mibte!gamma!thumper!dk
From: dk@thumper.bellcore.com (Dan Kahn)
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Summary: current surge in incandescent bulbs not necessary
Message-ID: <1647@thumper.bellcore.com>
Date: 18 Jul 89 12:24:43 GMT
References: <4363@merlin.usc.edu>
Organization: Bell Communications Research
Lines: 66

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.  The net effect is that the bulb 
> draws much more current cold than after it is lit. While your dashboard bulb 
> may only be a 6 watt bulb (indicating about 500 ma), it will probably 
> momentarily draw 10 amps or so when you apply power.
> 
> Normally this is not a problem.  Most circuits have enough reserve, either
> from adequate capacity, stray capacitance or inductance to provide this boost

1. I can't think of a practical case where the additional inrush
current would be provided by "stray capacitance or inductance," but
that is not relevant to the main point.

2. The steady-state (rated) current of a 12-volt instrument panel
bulb is not likely to be anywhere near 500 ma, but let's assume it is.

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.

That this is not necessary can be established with either a real or a
"thought" experiment.  Connect the bulb rated at 12 volts and 500 ma
to 112 volts thru thru a 200-ohm resistor.  Note that I've chosen the
values such that if 500 ma flows thru this series circuit, there will
be 100 volts across the resistor and 12 volts across the bulb, for a
total of 112 volts, = the source.  With 12 volts and 500 ma, the lamp
would be at normal brilliance.  Will  this actually happen?

According to John's posting it will not, since when the bulb is
placed across the 112-V/200-ohm series combination, the most current
that could possibly flow thru it would be 112 V / 200 ohms = 560 ma
(obtained by assuming that the cold resistance of the bulb is 0),
only a bit more than its steady-state current, while John's claim is
that several amps is required to get the bulb going.

But without even hooking up any wires, think about the implication
of John's claim: if the bulb doesn't light up, its resistance is
very low, thus close to 560 ma is flowing thru it.  Alongside of it
we have an identical bulb connected to a stiff 12-volt battery,
brilliantly lit, with only 500 ma flowing thru it.  The bulb with
_less_ current is the bright one under this claim.  The light output
of incandescent bulbs is far from linear with current, but it sure
is monotonic!

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.

4. I've tried to illustrate the irrelevance of the "inrush"
phenomenon to cyamamot's problem without the complicating factor of
the transistor.  But the transistor doesn't change anything.  The bulb
should light if enough current is supplied to the base.  Several
postings have addressed the issue of the ability of TTL to source
adequate current.

Dan Kahn      Bell Communications Research      Morristown, NJ