At 05:59 PM 7/26/2003 -0600, you wrote:


> From what I understand about the back to back zeners is that this should
>keep the voltage on the top input between 3.3v and 5.1v, well into the
>guaranteed HI range for TTL, and even if there is any negative component
>in the +12V power supply. Although I don't expect any negative voltage
>because this is battery driven, I want to make sure the MUX/IN inputs
>are protected for potential spikes.

You misunderstand. The zeners will limit it to about +3.6V and -5.7V
(one Si diode drop + the rated reverse voltage in each direction),
ignoring the effect of the input protection diodes.

But you have a bigger problem, you say you are using a darlington
transistor for the drive transistor- it will have around 1V across it
when turned on with 2.4A flowing- that's too high for the TTL-level
inputs on a PIC (it should be < 400mV to allow for some noise
immunity). You'd have to add some series drop (a zener or perhaps
an LED) to get it to work with your circuit.

>The transistor I am using is a TIP120/125 which should be able to sink
>the current with not problem. The data sheet says they will handle  a
>max collector current of 5A and a max base current of 120mA. The gain on
>these Darlingtons are 1000.  Also, the buffer I am using is a Darlington
>relay/lamp driver (ULN2003) which is spec'd for 500mA. I'm not sure how
>to figure the value of the base biasing resistor and figured it to be
>only for current limiting. How would I calculate it?

The ULN2003 has an open-collector output.. there will be no current
flowing to the base the way you've drawn it. Also, *it* has a
darlington output so you have to consider the voltage drop when it's
on and how to keep from partially turning your power darlington on.
Something like this would work:

          +5
           |
          [1K]
       o   |     1N4148
---|>o----x-----|>|---------> darlington base (8K internal shunt)
  ULN2003A


But it's getting a bit ugly.


BTW, the MOSFET I suggested is a D2PAK SMT part that will require
no heatsink, and will in fact run pretty cold
(< 200mW dissipation at 2.5A)>

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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