Hello,

I just came across this post, while looking for solutions to a similar
problem:

On a snare drum playing robot, I'm driving a number of continuous duty
solenoids.  They're running at 48VDC, being triggered by logic-level signals
sent from a microcontroller through ULN2004A Darlington arrays.  At 48V,
each solenoid draws approx. 350 mA, so they're well within the per-channel
current limits of the darlington arrays.

The combination of fast switching speeds (I can get some of the arms to roll
at ca. 25 Hz) and multiple solenoids triggered simultaneously through a
single array have resulted in heat up and failure of several arrays.  Oddly,
on another device (a plucked string robot), I've been triggering the same
solenoids at similar speeds through the same arrays for months without any
problems...so I'm a bit baffled by this.

As of now, I don't have 1N4001 (or similar) snubbing diodes in parallel to
each solenoid (I thought that since the darlington arrays have internal
diodes for protection against kickback this wasn't necessary).  I might try
this, but even if it works, as I learned from the contents of this thread,
this will apparently cause a 9+ ms delay in release time for the solenoid,
which might be noticeable in this application.  And I'm not sure it'll help
with the thermal buildup that fries the darlingtons.  

So the other viable options seem to include putting a zener diode in series
with the snubber diode, which does away with the latency at the expense of
slightly less kickback suppression, or to instead put a varistor in parallel
with the solenoid (which apparently takes care of thermal and latency
issues; and if this is best, what value is appropriate given my supply
voltage of 48V--will a 50V varistor work here?).  Or there's the suggestion
that came up on this thread to run a zener from the ULN2004's COM line to
ground.  I've tried simply piggy-backing up to 3 of the 2004A's to get more
headroom, but this hasn't proven effective.

So I'm curious, Philip, if you found one or more of these options to be
effective, or if anyone else has suggestions for the quickest, cheapest,
least space-consuming option for solving this problem.  The primary concern
is of course to stop burning out my ULN2004A's, but limiting release latency
would be nice as well.

Thanks to anyone for any assistance with this!

t



Philip Pemberton-2 wrote:
> 
> 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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