I've had a similar problem with some fast stepper motors.

I ended up using some zener diodes as snubbers and it greatly improved the 
response time.

I've noticed on a recent pair of scanners I've stripped they used the 
ULN2003 on the stepper drive stage but put a zener in series with the 
snubber pin on the ULN then fed this to +ve.

HTH

Dom

----- Original Message ----- 
From: "Philip Pemberton" <piclist@philpem.me.uk>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Monday, August 25, 2008 9:20 PM
Subject: [EE]: Back-EMF catch diodes slowing solenoid response


> Hi,
> Well, I'm two days into my summer holiday, and I've already run out of 
> things
> to do on my current projects. So I figured I'd resurrect an earlier 
> project
> that I put on the backburner ages ago.
>
> A good few years ago, I bought a pair of Epson M-180 series impact printer
> mechanisms. These are basically 24-character dot-matrix printers which use
> five horizontal solenoids instead of the more typical single 9-pin 
> vertical
> head. The idea being that it speeds up printing, and allows graphics 
> printing
> where necessary. It's actually pretty speedy... for a 1980s vintage piece 
> of
> kit. A lot more power hungry than my Seiko MTP-series thermal mechs, and a 
> bit
> of a pig to design hardware around. Typical Epson kit really, fussy and 
> finicky :)
>
> From my project notebook, I was having issues with the print head 
> solenoids
> holding down for too long:
>
> > Print quality still terrible. P/Head solenoids seem to be holding too 
> > long,
> > 1N4001 diode snubber slowing decay. Removed diode - some improvement, 
> > but
> > ULN2003 driver failed in short order, probably due to back-emf. Needs
> > further work.
>
> I've been digging through Horowitz & Hill, and scanning Google for about 
> 40
> minutes now, and haven't found any decent material on back-EMF suppression 
> for
> transistors used for driving inductive components. I've found tons of
> references that boil down to "just use a 1N4001", a few that actually 
> admit
> that a '4001 will slow a relay's switch-off response "significantly", but
> nothing on what to do to speed things up.
>
> Does anyone know of any good articles on this? I'd rather like to get the
> M180s doing something useful - maybe hook one up to a PIC and use it to 
> grab
> screendumps and trace data from some of my testgear.
>
> Of course the easy answer is "give up and dig out a thermal printer 
> module",
> but they're text only (7-pin head, not the 8-pin graphics head) and 
> frankly
> it's far easier to get paper and ribbons for the Epson modules than it is 
> to
> get 4" wide thermal paper for the MTPs...
>
> Thanks,
> -- 
> Phil.
> piclist@philpem.me.uk
> http://www.philpem.me.uk/
> -- 
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> 

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