On Jul 12, 1:55*am, Tony wrote:
On Fri, 10 Jul 2009 21:38:51 -0700, Mycelium





wrote:
On Sat, 11 Jul 2009 12:54:49 +1000, Tony wrote:

but MOVs can have vastly higher
ratings, and can also provide good lifetime when de-rated in lower energy circuits, so
they shouldn't be dismissed without proper consideration.

They certainly should if the originally designed in devices are
transzorbs, which every evidence says they are.

You also should examine the application. There will be a maximum
expected spike potential, and energy. *The energy component of which is
pretty low. So big, monster MOVs is overkill, if you want to look at it
that way. *Still points toward Transzorbs being the right choice.

*Do you need to see it on a Silver Platter?

Not at all. I don't dispute that Transzorbs are a better choice for high repetition low
energy spikes - I use them like that wherever possible, and if I needed to go out buy
something, I would have used them for the OP's application as well. I only disagreed (and
still do) with the suggestions that (paraphrasing):
*- the original devices were transzorbs (unlikely, given the disc / fin construction),
*- MOVs wear out fast even when hit infrequently with a tiny fraction of their rating.
*- using an adequate but imperfect solution is always stupid and never acceptable,
*- the concept that ng posters always need to be told they're wrong, even when their
proposed (and often already implemented) solutions are quite adequate.

Cheers,
Tony- Hide quoted text -

- Show quoted text -

The joules that kick back will usually be less than or equal to the
energy put in, so any spike must be less than or equal to the what
went into the motor. That implies that the designer 'could' have
taken into account the MOV's ability to handle lesser energy than its
full rating for a much longer number of hits.

But then again I've never seen an MOV with fins. Plus, symbol for MOV
I'm used to is a long rectangle [like a consumer] and a diagonal line
drawn across it with a line drawn parallel to the rectangle at each
end of the diagonal line. Tranzorbs I've always seen 'nose to nose'
diodes with diagonal short lines off each end of the cathode line
[very open 'z'] which sounded like the original symbol.

Robert