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


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.

Alas, commercial makers of domestic kit can't rely on all their customers
taking such an attitude. :-)

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"?

The thermal delay relays used were the order of a couple of quid *each* in
quantity, back in the 1970s. Compare that with the few pence per diode
bridge for better diodes that didn't need the delay. Plus of course the
savings to all involved when the change had a quite marked effect in
lowering the numbers being returned for repair because the thermal delay
was unreliable.


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?

Both. And with the alternative of the kind I chose. Simply modifying the
design so as to make the component redundant.

As per the suggestions I made to Ian - using a diode as a 'one way'
connector, etc, so as to avoid needing any mechanical relays. Why design in
future problems you can avoid?


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.

I'm agreeing with you. But also pointing out that experience with this
relay being unreliable was one of the examples I've encountered where
physical relays were an expensive way to design in increased unreliability.
:-)

I also pointed out that I didn't 'use it' that way. I found it was being so
used, and designed it out.

By the standards of electromagnetic relays contact pressures are low and
switching times slow (even with hysteresis).

What measurements did you do on the delay unit we are discussing? And how
many of them did you check, and over what period of time?

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.

In this application the need was to close a switch when the current level
was generally low. Not to start high currents. Nor to break a current.

I did try other mechanical types/arrangements at the time but decided they
simply weren't worth the cost and reliability worries. So fixed the problem
in another way.

This meant that repairs to old sets actually got cheaper as well since the
offending item was removed/bypassed and new diodes fitted.



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.

I note your personal opinions. Perhaps you missed some of the things I
wrote. :-)

Slainte,

Jim

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