Quoting Philip Pemberton <piclist@philpem.me.uk>:

> 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.

...snippage..
>
> Thanks,


Lose the diodes. Connect a 47V zener between COM on the ULN2003A's and ground.


Best regards,
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
s...@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com


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