Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!zephyr!tektronix!tekig5!brianr
From: brianr@tekig5.PEN.TEK.COM (Brian Rhodefer)
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Keywords: NPN, TTL, bulb, 12 volts, bias
Message-ID: <4516@tekig5.PEN.TEK.COM>
Date: 14 Jul 89 18:42:56 GMT
References: <4363@merlin.usc.edu> <5769@stiatl.UUCP>
Reply-To: brianr@tekig5.PEN.TEK.COM (Brian Rhodefer)
Distribution: na
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 37

When comiserating with my co-workers after committing some engineering
boner, I used to lament that "I'm not competent to control a ##@$#%$ing
light bulb".  I *thought* the expression was hyperbole.

In article <5769@stiatl.UUCP> john@stiatl.UUCP (John DeArmond) writes:

>may only be a 6 watt bulb (indicating about 500 ma), it will probably 
>momentarily draw 10 amps or so when you apply power.

A quick scan through our component database turns up four 12V, bayonet-base
incandescent lamps.  The highest operating current in the bunch is 200mA,
and the others are in the 50 to 60 mA range.

I don't have a manufacturer's catalog in front of me, but if we're
talking about T1 3/4 - sized "pilot lamps", 5W seems way out of line
for power dissipation.

>Normally this is not a problem.  Most circuits have enough reserve, either
>from adequate capacity, stray capacitance or inductance to provide this boost.

Stray capacitance, which is seldom larger than a few tens of picofarads,
is NOT going to help light up an incandescent lamp.  And NO amount of
inductance will help in this application.

>Even driven hard, a small signal
>transistor probably cannot handle the initial or inrush current due to
>collector saturation.  In your circuit, a 1k resistor with 4.3 volts
>(5 volts - .7 volt base-emitter drop) passes 4.3 MA.  This translates

Fine, except that the problem is the transistor coming OUT of saturation,
and into its constant-current mode.  Also, TTL gates can't pull all the way
to +5V while sourcing useful current.  My old TI databook has VoutHigh
dropping to 2.5V at an output current of 10mA for vanilla TTL.

Fiat Lux!

Brian Rhodefer