[Rance]

Hi,

My question is concerning the Stepper controller UCN5804. This is for some very small steppers I'm using (12v, 33 ohm). Could someone tell me the reason for the resistor coming from the center tap of the motor coils and how to determine the values I might need?

Also, which configuration of diodes on the other motor leads should I use and why? I am assuming that most any old diode will work (4001, 4004, etc.). Thanks.

Rance

[fyffe555]

I've used these things, but I'm by no means an electronics expert so take this as you will;

Normally a resistor on the earth leg of the unipolar motor as shown in the allegro datasheet is used to limit the current to the device (5804) rated current, in the case of the 5804 that's 1.5a at 35v. Also note at that rating they get hot so some cooling is a good idea, if they get too hot they shutdown until below the shutoff temp - something like 160c..

Your motors would be under 0.5 a and 12v so you could go without resistors or cooling.

The diodes are to stop the current being generated by the motor forcing the voltage below ground and so cause a current from the motor which is a bad thing. If the diode is rated at more than 35v and 1.5a then , yes anything would do. However at the fairly low rates the motors will be switching Schottky diodes work better and smooth out the motors motion.

I had trouble driving the inputs... The minimum input is 3.5v, which was a bit high for TTL I was using to drive the 5804's inputs, the min voltage from the ttl could apparently be less than that and it didn't work too well until I put pull up resistors on all TTL driving the 5804 inputs.

[Rance]

Thanks for the reply fyffe, and for the heads-up on the ttl levels. I'll just leave the resistors out. So which diode configuration did you use on yours, Inline with the coil leads or from the leads to ground? Way back when I sortof worked with electrons, I never had the opportunity to use Schottky diodes. I'll go look them up to find the appropriate ones.

Rance

[fyffe555]

I did leads to ground, but as long as the diode acts to prevent current flowing back to the output pin either option works.. You're aware the 5804's are obsolete now and getting harder, more costly to find? Still work well for small projects though..

[Rance]

I figured the leads to ground would be better due to not having the voltage drop across the diodes inline with the coil but wanted a 2nd opinion. Yeah I realize they're obsolete. I've got 10 coming in for $4 a piece. I look forward to tinkering. Thanks for your input.

Rance