Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ukma!gatech!emory!stiatl!john
From: john@stiatl.UUCP (John DeArmond)
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Keywords: NPN, TTL, bulb, 12 volts, bias
Message-ID: <5841@stiatl.UUCP>
Date: 16 Jul 89 01:57:55 GMT
References: <4363@merlin.usc.edu> <3868@phri.UUCP>
Reply-To: john@stiatl.UUCP (John DeArmond)
Distribution: na
Organization: Sales Technologies Inc., "The Procedure IS the product"
Lines: 18

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.


-- 
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!!!