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.

These were expensive.

Maybe, but it was still cheap & nasty.

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

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. :-)

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.

Your original PSU didn't need it either IME. :-)

As I said, you were lucky. I probably saw more of the sets than you did.
:-) Without the relay the surge peak applied was well above the rating of
the original bridge diodes. ...And the orginal PSU wasn't actually mine. It
was designed by Ted Rule. I just changed it a few years later to be cheaper
and far more reliable.

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.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

"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.

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

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

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

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.

You'd expect thermal delay units to be expensive compared to diodes. But a
couple of quid each is cheap for thermal relays.

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?

Well clearly I made no measurements. I only had one unit to examine and it
had already failed by the time I examined it. But I do understand how the
things work, and have met a number used in different bits of kit over the
years. So I do know the limitations of the device.

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

I don't think I missed any, though you seem to have missed the point that
making judgments on the reliability of electromagnetic relays based on your
experience of an *entirely different* device seems, at best, hasty!

David.

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

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.

You'd expect thermal delay units to be expensive compared to diodes. But
a couple of quid each is cheap for thermal relays.

But still expensive when the - far more reliable - alternative is a few
pence for diodes.


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?

Well clearly I made no measurements. I only had one unit to examine and
it had already failed by the time I examined it. But I do understand
how the things work, and have met a number used in different bits of
kit over the years.

So I do know the limitations of the device.

Actually, no, you don't. You saw one and from your own admission judged it
simply on appearance and your general theories about the class of such
devices.

Whereas I at the time had experience of many of the specific devices *and*
did tests to compare them with alternatives, of a range of types/methods.

However I agree with you that such devices do tend to be far less reliable
than designing them out of the system. That is my experimental experience
for a range of types of mechanical switches. Mostly conventional relays,
but also thermal and other forms of sensor driven types. Hence my quoting
the delay as an example of a device relying on mechanical contacts that was
unreliable - and easily replaced by something cheaper and better.

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

I don't think I missed any, though you seem to have missed the point
that making judgments on the reliability of electromagnetic relays
based on your experience of an *entirely different* device seems, at
best, hasty!

I agree. Hence my noting that you made no measurements on the specific
device I used as an example, and just judged it on the basis of appearance
and your own general opinions. :-)

You may however have still missed the point that I have also experience
with conventional electrically operated relays in a range of applications.

So I'll stay with my views based on a mix of my experimental experience and
having tried various alternatives. if you prefer electromechanical switches
to solid state alternatives you are welcome to do so. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

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


So I do know the limitations of the device.

Actually, no, you don't. You saw one and from your own admission judged it
simply on appearance and your general theories about the class of such
devices.

There's something odd going on here. :-) *You* can group all
electromechanical devices together and damn the lot of them, but if I try to
talk about a class of devices known as "thermal relays" you instantly try to
restrict the discussion to the particular unit that you used on the 600. I
wonder why that is? Just to make it clear, by "the device" I mean the device
known as a thermal relay, not specifically the particular one you chose to
use.

However I agree with you that such devices do tend to be far less reliable
than designing them out of the system.

Designing out any unecessary complexity improves reliability, and that
includes semiconductor devices. High-voltage semiconductors, including SSRs,
do not have a particularly good reliability record.


I don't think I missed any, though you seem to have missed the point
that making judgments on the reliability of electromagnetic relays
based on your experience of an *entirely different* device seems, at
best, hasty!

I agree. Hence my noting that you made no measurements on the specific
device I used as an example, and just judged it on the basis of appearance
and your own general opinions. :-)

You are so keen to keep coming back to my judgement of that specific device
aren't you? I have extensive experience of relays and their uses. Whilst
electromagnetic relays can be very reliable the thermal relay isn't (That's
thermal relays in general, not just your specific one!)

You may however have still missed the point that I have also experience
with conventional electrically operated relays in a range of applications.

And so do I!

So I'll stay with my views based on a mix of my experimental experience
and
having tried various alternatives. if you prefer electromechanical
switches
to solid state alternatives you are welcome to do so. :-)


That entirely depends on the application. For signal switching, particularly
low-level unwetted analogue audio/video I'd usually prefer electronic
switching. Loudspeaker relays in high-powered amps can be a problem too
because the contact material needs to be able to handle high currents yet
also work well with very low signal currents. But then SSRs are no use in
that application either.

But the particular virtues of the relay - very low on resistance,
negligable off-state leakage, ability to offer a wide range of contact types
and number, galvanic isolation between coil and contacts etc. can make it a
very attractive option in many applications.

At it's peak in the 1960s the Strowger-based PSTN network in the UK had in
excess of 10 million relays in use, and by then many of them were over 30
years old. Several hundred might well be involved in any one call. Unlike
selector mechanisms relays generally did not receive routine maintenance,
nor were they (or selectors for that matter) replaced as a standard repair
technique. Individual relays were wired into relay sets and replacing them
was a fiddly job and rarely done. When exchanges were retired after a
typical life of 40 years almost all of the original relays and selectors
would still be in use. Yet relays were rarely the cause of faults, selector
mechanisms, contact banks, selector jacks or indeed shelf wiring were in
fact the usual cause of faults in Strowger exchanges. Of course these were
mostly PO3000 type relays, bulky and expensive by modern standards, but
very reliable.

David.