"Jim Lesurf" wrote in message
...
In article , David Looser
wrote:
"Jim Lesurf" wrote in

During the first years of manufacture the ampliers/recievers in the
Armstrong 600 range used a thermal delay relay to avoid a surge when
the unit was switched on.


I remember it well as I had a 600 with one in. To me it appeared to be a
cheap & nasty little thing that failed very early on. I simply shorted
it out and the receiver continued to work for many years without it.

In that case you were fortunate to get away with not realising what other
changes needed to be made! Sometimes the Ghods forgive those who make
changes in ignorance of the likely consequences. The main point of the
delay relay was that the bridge diodes that had been used had too low a
surge current rating. So bypassing the relay without also changing to new
diodes was increasing the chance of PSU failure.

Maybe, but in my case it worked. Had the diodes subsequently failed I'd
simply have replaced them by more substantial types. But that didn't prove
necessary.

These were expensive.

Maybe, but it was still cheap & nasty.

Odd that you haven't noticed that 'expensive' and 'cheap' clash here. :-)

They do not necessarily clash. By what criterion were they "expensive"?

However I tested a number of them over a period of time, and compared them
with alternatives. I note your opinion based - presumably - on appearance,
though. :-)

Indeed, one can often get a fair idea of the quality of an item, and it's
likely reliability, from a physical inspection. When you say you compared
them with alternatives, do you mean alternative thermal relays?, or
alternative methods of providing a soft-start?

And they were one of the main failure modes which brought sets back
for repair. One of the reasons I avoid physical relays.

But a thermal relay is a totally different animal from a conventional
electromagnetic relay. They have very slow make & break of the contacts,
and the characteristics of the bi-metal strip changes with age.

You may need to think more carefully about the application in the example
before assuming that is relevant. You could also check to see if the relay
was such that it flipped state with some hysteresis.

Actually I'm suggesting that *any* thermal relay was inappropriate in that
application, or leastways used as you used it. By the standards of
electromagnetic relays contact pressures are low and switching times slow
(even with hysteresis). When thermal relays were commonly used as delay
timers it was normal practice to use their contact to operate a conventional
relay, which then did the real work.


The particular problem was easily solved simply by using high surge rated
diodes. So far as I was concerned any mechanical relay at all was
expensive
and nasty in such a situation, and easily avoided with far more reliable
solid state devices. Hence my mentioning it in this thread.


Experience with thermal relays is irrelevant when talking about
electromagnetic relays, the two are chalk and cheese. You might as well make
judgements on the reliability of polypropylene capacitors in fast rise-time
pulse circuits based on your experience with electrolytics.


David.