Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!uxc.cso.uiuc.edu!garcon!tuna.cso.uiuc.edu!kline
From: kline@tuna.cso.uiuc.edu (Charley Kline)
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Keywords: NPN, TTL, bulb, 12 volts, bias
Message-ID: <1561@garcon.cso.uiuc.edu>
Date: 25 Jul 89 20:59:01 GMT
References: <1383@bnlux0.bnl.gov> <4363@merlin.usc.edu>
Sender: news@garcon.cso.uiuc.edu
Organization: University of Illinois at Urbana-Champaign
Lines: 32

In article <1383@bnlux0.bnl.gov>, geller@bnlux0.bnl.gov (joseph geller) 
and others speak in detail of how to build slightly higher-power drivers
for TTL outputs.


I just finished a project, which works fine, where a 74198 shift register is
driving the LED side of an optoisolator through the following circuit:
[sorry about the words, i don't have the patience for ASCII graphics]

  TTL output of 74198 to a 10k resistor to B of a 2N2222-style transistor. E
  of transistor to ground. C of transistor through 15 feet of ribbon cable
  to another enclosure, to a 270 ohm resistor, to the cathode of the LED.
  Anode of LED to +5.

Now as I mentioned, this all seems to be working perfectly. My only
reservation is that in operation, the 74198 gets a little warm. Not
alarmingly so, not even warm enough to bother me really, but when I had this
thing breadboarded on my bench and was just driving an LED through a 270 ohm
resistor from the 74198, with none of this transistor driver jazz, the 74198
ran with no detectable (to my finger) increase in temperature.

I would have thought that the 10k resistor between the driver and the 74198
would be limiting the high output current to something much less than even
the simple LED load would be presenting. Am I wrong? What else could be
causing this thing to heat up?

_____
Charley Kline, Assistant TD, Celebration Company at the Station Theatre
c-kline@uiuc.edu
uunet!uiucuxc!kline

"Just another useless dead thing, I've been killed by love."