🚀 go-pugleaf

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.
>John De Armond, WD4OQC                     | Manual? ... What manual ?!? 

There is either a typo in your databook, or you are mistakenly reading 
the ICCH rating. This is the current supplied through Vcc to the gate. 
This is NOT the TTL output current. The correct place to look at 
is the I_OL and I_OH  ratings. As the previous poster stated, TTL outputs 
can sink current, but cannot source as much. My databook from TI 
specifies recommended maximum LOW output current as 16 ma (sink), 
and HIGH output current as 0.4 ma (source). 
Back to the original question: 

                              12 volts
                                :
                                :
                              BULB
                                :---------
                 2.2K     C-----:        :
      TTL------<><><>----B      C        <
                          E----B         > R
                                E        <
                                :        :
                               GND      GND

A darlington transistor pair with a collective beta of 500 or more 
is sufficient enough to drive a 200ma bulb.  R is a warmup resistor 
for the light bulb to prevent cold rush current (Just for the sake 
of completeness. I don't think it is really necessary.)
-=-
Bulent Abali
Ohio State Univ., Dept.of Electrical Eng.
2015 Neil Av. Columbus, Ohio 43210
abali@baloo.eng.ohio-state.edu