Applying 5V to the inputs (power supply is also 5V), I'm only getting
0.6V at the outputs. I thought it was a faulty chip, or perhaps damaged while soldering, but I've tried another one in a holder and got the same result. What could be causing this?
How is it connected ?
What voltage did you expect to see ?
Graham
"Ben"
** That is correct operation for a transistor in common emitter mode.Input high causes output to go low.
** What the hell did you expect ??The ULN 2803A darlington array is intended to drive relays, lamps, LEDs etc.
YOU connect one side of the load to + supply and the array will connect the other to 0 volts when driven on.
....... Phil
A ULN2803 is just a bunch of Darlington transistors, emitter to ground, and collector to the output pin. They are inverters, so a high (+5V) input will produce a low (near zero) output. Also, the things don't have anything to pull the outputs high, so, unless you supply an external pull-up (such as a resistor to +5V), a low input will also produce a low output.
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"Peter Bennett"
** It won't be near zero volts.The voltage drive for the base of second device in the darlington pair comes from its own collector - so it will be at least 0.6 volts even at very low current.
The data sheet gives saturation values for Vce up to 1.6 volts at 350 mA.
....... Phil
Thanks for the explanation, I think I'd misunderstood what a Darlington array actually is. I was expecting a high input to produce a high output, with enough current to drive relays. Does such an IC exist?
What value of pull-up resistor would you recommend for the Darlington array?
This is a perfect chip to drive your relays, the coil goes between the output and posative supply. You may find a circuit somewhere on the data sheet or do a search.
Do you know what a transistor does ?
You're fixated on 'ICs' aren't you ?
What do you need a pull-up for ? The load *is* the 'pull-up'.
Graham
========================================= Be nice. Or just dont say anything. Some patient person will explain politely.
I'd need 8 transistors, so a single chip that does the job seemed like a good idea.
Yeah, this worked great - thanks :-)
you don't need a pull up resistor.. use the Array outputs as the Common/ground source when on. they are simply what is called open collector outputs. you can supply the relay coil with what ever voltage is required by the relay up to the max stand off voltage of the array of course. the + side of the relay coil does not need to be the same voltage as the Vcc (+ rail) of the chip..
-- Real Programmers Do things like this. http://webpages.charter.net/jamie_5
Graham: the last time i looked at the name on this NG, it said "BASICS"!
where i come from that means something. maybe you should show a little
consideration for others that are at least trying, and not criticize
at every turn.
(Just an opinion from the peanut gallery)
-- Real Programmers Do things like this. http://webpages.charter.net/jamie_5
Spot on Ben, good work.
Don...
-- Don McKenzie E-Mail Contact Page:
Micro,TTL,USB to 1.5" color LCD
that's about right.
probably you're using it incorrectly. the load should be connected between the output and a positive supply.
-- Bye. Jasen
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