On Jul 8, 1:30*am, Tony wrote:
On Tue, 7 Jul 2009 07:48:36 -0700 (PDT), Robert Macy wrote:
On Jul 7, 12:58*am, Tony wrote:
On Mon, 6 Jul 2009 21:45:02 -0700, DaveC wrote:
Stud-mount diodes (four in a bridge configuration) are protected by MOVs: one
across the AC connections, one across the DC connections.

These MOVs are antique-looking finned affairs (a la selenium rectifiers).
Both sport PN S01AAA?AAA (?=illegible) with Westinghouse trademark. The
wiring diagram symbol for these looks like a zener diode with an extra arrow
head (both arrow points touch, with the familiar "z" shaped line between
them).

The AC supply voltage to the bridge is 75 rms. The bridge diodes are
originally 100V/1A devices; I'm replacing with 400V/35A ones.

This rectifier drives a DC motor via 2 contactors: one for direction (by
reversing polarity) and the other for "GO/STOP". The MOVs are to limit the
kick from opening contacts, I presume.

Someone suggested that with such stout diodes available today that maybe the
MOVs are not required.

If you think I need to keep the MOVs, how do I select new ones?

What say y'all?

Thanks,

To apply MOVs (and other clamp devices) there needs to be a decent difference between the
semiconductor voltage rating and the actual peak circuit operating voltage. In this case
the circuit operates at 75 Vrms or 106 V peak plus a margin for supply variation, say 117
V max, and the devices were rated at 100 V. There isn't a MOV voltage that would protect
the semis and not destroy itself in normal operation.

But luckily it survived (maybe the semis could actually take a lot more than 100 V?), and
you're using 400 V diodes, so you can safely choose a MOV that starts to break down around
150-250 Vdc or 100-150 Vac. And since MOVs are so cheap now you can just go with something
rated for, say, 100-150 Joules, which should be more than adequate.- Hide quoted text -

- Show quoted text -

Check the spec on MOV's *Mycelium is correct, after a number of uses
they are considered GONE.

Hard to believe anyone would design a circuit 'requiring' the presence
of MOV's to protect during 'normal' operation catching flyback
surges.

Yes, replace with tranzorbs. *And try to match the characteristics of
your new diodes to the old. *Higher current devices tend to be slower
and the current going the wrong way for that short time may heat
things up.

I have seen the turn off time of diodes matched to a circuit. Diodes
selected based upon allowing a slight amount of reverse current to
slowly die off during voltage reversal. *With the end result the
diodes turn off 'quietly'. *For example, I've seen linear power
supplies with faster diodes [the diodes made very high frequency shut
off noise] fail conducted RFI/EMI testing caused by this phenomenon.

Robert

Maybe you replied to the wrong post (I didn't see or comment on Mycelium's post)? Anyway,
usenet certainly supports both those who answer posters' questions directly and those who
point out other potential problems.

Yes MOVs eventually wear out, but they are cheap and readily available with high ratings.
In this case a few 10s of ms transient of a few 10s of Watts amounts to under a Joule. A
100+ Joule MOV as suggested would last way longer than the relays that this circuit
supplies. Transzorbs would be fine too - depends what's on hand or easy to get. If you
think about it, it's not so hard to believe that designers could do such a thing; it's
often just practical reality.

The original 100 V diodes on a 75 Vrms AC supply? well that's another matter.

I agree with you about the diode speed IF it's a SMPS. But it sounded to me like an
ordinary AC mains application, in which case the 35 A bridge is still quite an overkill,
but it's just NOT going to cause the problems you mentioned. I assumed it was a part the
OP had on hand, which would be fine.

Cheers,
Tony- Hide quoted text -

- Show quoted text -

Goggle access to Usenet implies a 'string' of responses that tack on
end to end. Thus, the appearance I was replying to you.

Good comments, too

Robert