Hi there, i'm getting wrong somewhere (smoke ...), but cant figure out why.
I'm trying to control a small stepper (PM35S-048) with a darlington ULN2003A (wiring : http://arduino.cc/en/Reference/StepperUnipolarCircuit#.UxdTweBKd-t Four pin version).
My supply is a 18V, 1.1 Amp taken from from an old printer -- i guess it has a voltage regulator to keep the 18V (?).
To be compliant with the ULN2003A (it resists the 1,1 Amps, but i suppose it wont resist it for a long time), i'm trying to reduce the current to 120mA (cf. datasheet, recommended rate at 50% duty cycle ...).
To do this, i've added a resistance (potentiometer scraped on a board) and adjusted its resistance to 140 ohm (just used 2 pins), actually, that was tricky, it was a 6kohm pot, and i played with the wheel until i got that tiny 140 ohm (says the multimeter)
That should give me ~120mA (18v/140o).
But it doesn't.
Putting the Multimeter in the middle, shows that i've still got 1 amp
Stopped testing when smoke started getting out of the pot'
...
where am i getting wrong ?
Do printers (HP 940C) power supplies usually deliver constant current instead of voltage ?
Would playing with the pot wheel near the 0 ohm be a bad idea (140 ohm is 2% of 6kohm tried bumping it to see if it changes, that was ok, it changed by 2-3 ohms) ?
I guess there's a trivial way of controlling the current ?
Cheers !
Ram.
