Trouble with second-source ULN2068 motor driver IC's.

[Geoffster]

Allegro no longer make their quad darlington drivers ULN2068 amongst others. We use these to drive unipolar stepper motors driven from INTEL 8255 ports in full stepping mode with a 24v zener from the catch diodes to the 12V supply which is also connected to the centre of each motor winding. Trouble is, I tried a Thomson brand and it does not work correctly. It's hard to explain without diagrams but here goes. The waveforms on coil "2" look perfect ie. each end is monitored and they are 180 deg out of phase with 50% duty cycle. The waveforms on coil "1" are not ideal. They have a glitch in the on period which turns the driver off for about 1 ms and similar in the on period. I am running the motors at about 70 ms waveform period. If I hold the motor shaft or try and help speed it up the width of the glitch changes slightly. This leads me to suspect that it is not a logic oreiented problem but possibly a crosstalk problem or similar. I am sure my input waveforms have no artifacts. The design is 15 years old and fault free until I tried to change brands of IC. The local sales rep said that many complain about non-Allegro brand IC's not working but I would like to know why and design around the difference.

 

[jbartos]

Suggestion: Check with

http://www.allegromicro.com/datafile/archive/2064.pdf

for an official substitutes for the discontinued ULN2068 motor driver IC

 

[Geoffster]

This link is useless as Allegro are cutting most of their range. They say this is due to cost of second source manufacturers.

 

[jbartos]

Suggestion: Visit

http://www.miami.muohio.edu/president/personal/w8zr/images/eztunerpartslist.pdf

for more info, e.g.
U201ULN2068 quad 1.5A darlington switch w. protective diode.
They may give you some leads how to obtain it.

 

[Geoffster]

Thank you jbartos for the link, I'm sure it could get me some more parts but this is not what I need right now. I have 7000 parts in stock but I want a new design that will last for many years of production.

 

[jbartos]

Comments: Then, it may be a good idea to approach to manufacturers who have inhouse design capabilities. This would then be more on an industrial or manufacturing engineering side.