Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!uunet!snjsn1!sjs!storkamp
From: storkamp@sjs.sj.ate.slb.com (Mark Storkamp, 408-998-0123 (ex 2079))
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Message-ID: <561@sjs.sj.ate.slb.com>
Date: 19 Jul 89 13:24:52 GMT
References: <4363@merlin.usc.edu> <3868@phri.UUCP> <5841@stiatl.UUCP>
Organization: Schlumberger Technologies Inc., ATE Division
Lines: 31

In article <5841@stiatl.UUCP>, john@stiatl.UUCP (John DeArmond) writes:
> In article <3868@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>>
>>	The problem is that TTL can sink current, but not source it.  With
>>TTL you really have to think current sinks instead of voltage sources.  For
>>example, the typical circuit to light an LED:
> 
> To clear up a small point.  Only open collector devices do not supply 
> current in the high state.  All other TTL supplies relatively high
> current through a totem-pole output arrangement - it must in order to 
> charge circuit capacitance and achieve high slew rates.  My TTL book from 
> National shows that a typical gate (7404 hex inverter to be exact) can 
> supply 35 ma in the high state.  Not a lot but still enough to work with.

My TI data book shows, for the 7404, maximum high-level output current as
-400uA. Somebody has sugested using a 74Hxx part, this has only -500uA.
If you realy must source current, the 74S will give you -1mA. A pull-up
on the base of the transistor to 5V should give you more drive, but the
maximum low-level output current on a 7404 is 16mA, so don't use anything
smaller than a 330. A 2N2222 has a minimum hfe of 100 @ 150mA, so this 
should give you at most 160mA for the bulb. For anything larger, try a
darlington configuration.
 
> 
> -- 
> John De Armond, WD4OQC                     | Manual? ... What manual ?!? 
> Sales Technologies, Inc.    Atlanta, GA    | This is Unix, My son, You 
> ....!gatech!stiatl!john    **I am the NRA** | just GOTTA Know!!! 

Mark Storkamp
Schlumberger Technologies