Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-pcd!hplsla!tomb
From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Message-ID: <5170048@hplsla.HP.COM>
Date: 21 Jul 89 18:57:06 GMT
References: <4363@merlin.usc.edu>
Organization: HP Lake Stevens, WA
Lines: 30

john@stiatl.UUCP (John DeArmond) writes:
>>
>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.
>
...

>If you want to actually measure the cold resistance, you must use a low current
>ohmmeter. No, your Simpson 260 won't cut it.  The problem with VOMs is that the
>current from the meter will heat the filament.  The temperature vs resistance
>curve for tungsten is fairly steep so no heating is tolerated.  A DVM on the
>low current ranges (ranges designed not to turn a diode junction on) will
>do pretty well.  I've played around with this a bit.  I've measured the
>effect of self-heating by carefully breaking a bulb and measuring the
>resistance of the filament with a DVM while the filament is immersed in
>oil as a cooling media.  The difference is not great but is measurable.
>
>Hope this helps..
>
>John

These two paragraphs, separated by several others in the original,
seem to contradict eachother.  If "my" Simpson 260 will heat the
filament enough to significantly change its resistance, then I'm
quite sure the 2n2222 won't have much problem overcoming the cold-
resistance effect.