HT Relay
 

"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.

HT Relay
 

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

HT Relay
 

The above has just reminded me of the joke in "Only Fools and Horses"
where
Trig (IIRC) was proudly telling everyone how long his broom had lasted.
Then to say, "...Of course I have had to replace the head X times... and
the handle Y times. But it is remarkable how long that broom has lasted."
:-)

Slainte,

Jim


Oh, c'mon! That joke started with George Washington's axe, that he
allegedly used to fell a cherry tree....

Regards,

GMacK

HT Relay
 

In article , Geoff Mackenzie
wrote:


The above has just reminded me of the joke in "Only Fools and Horses"
where Trig (IIRC) was proudly telling everyone how long his broom had
lasted. Then to say, "...Of course I have had to replace the head X
times... and the handle Y times. But it is remarkable how long that
broom has lasted."
:-)


Oh, c'mon! That joke started with George Washington's axe, that he
allegedly used to fell a cherry tree....

Jings! I'm younger than someone else here. :-)

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

HT Relay
 

"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.

HT Relay
 

On Sun, 27 Dec 2009 23:54:59 +0000, Ian Bell wrote:

snip

The PIC is an 8 pin DIL, costs a dollar and allows me to light a couple
of LEDs to show the HT state, handle the cable interlock and drive the
relay. It needs no external clock components and there's even a spare IO
line I could use for a push button emergency off switch.

snip


I tried something similar to this - using a PIC - a while ago, but never
actually put it into practice. The idea was to drive 2 relays. One
switched the HT on and the other shorted out a series resistor in the HT
line after a few seconds to give a soft-on characteristic. A single
button was used to control everything and a 2-colour LED for Off/Standby/
On indication (provisionally). Software was written in GCBASIC just
because it was a quick-n-dirty way to do it! The eventual idea was to
also monitor the cathode current of the output valve & shut down with an
error indication if it got too high. I never got to that bit because I
couldn't arrange a grounded DC supply for the PIC. The PIC is still
floating around somewhere... I like PICs... :-)

You must *never* depend on software for safety interlocks by the way.
It's very bad design.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

HT Relay
 

On Sun, 27 Dec 2009 23:16:05 +0000, David Looser wrote:

"mick" wrote

By the way, you may not find it too easy getting hold of a suitable
relay with a 5v coil. Manufacturers often list them down to 6v but
nobody stocks them! The nearest that you will get easily will be 12vdc
and the next one up is 24vdc. 5v coils are usually only on small pcb
relays, which don't have enough voltage rating on the contacts, even in
series (if you can ever find a complete data sheet!).


ISTM that 5V coils are now rather more common than 6V ones, and 5V coils
are just as likely to be available on power relays as any other coil
voltage. BTW why do you think PCB relays won't have enough voltage
rating?, I've just found a relay with a 10kV rating for the contact, and
that is a PCB mounting type.



You're correct that 5v coils are common on PCB relays, but there aren't
all that many PCB relays that could switch the HT line of an amplifier.
350vDC at 200mA can be difficult to break (and make). I originally
suggested plug-in devices because they come ready insulated and are easy
to use (the 2-pole ones fit a standard octal socket). Unfortunately I
don't think any manufacturer makes these with a 5v coil.

Having said all that, I've just found this: Tyco Electronics RT424005
(Farnell 162-9052). It's a 2-pole c/o PCB-mounting relay with a 5vdc
coil. Both n.o. contacts in series would switch about 400mA at 350v
(175vDC each). Using a single n.c. contact for the discharge function
would mean limiting the current to 100mA or so. It's a meaty beast and
it's cheap too! :-) Now, whether Ian would be happy with 350v floating
round these pins is something else.

A note on Isolation: For those considering using a solid-state device to
switch *off* the HT - you can't. SSRs, triacs etc. are not counted as
isolation devices. You should always have at least 2 air gaps, either as
2 "switch-type" contacts in series *on the same relay armature* or a
single "double-break" type contact. You can, of course, switch *on* the
HT using solid-state.

Someone suggested that a contact isn't needed. That's not strictly right
if you are protecting a plug and socket, unless the design is such that
the HT lead breaks cleanly before the earth connection and the HT socket
is suitably touch protected both during and after unplugging. You can
protect the user from a back-feed from the amp using a suitable diode, as
Jim has already mentioned. Protecting from the PSU is a bit harder. I fed
the heater supply back from the amp to the PSU and used that to close the
HT control relay. That way the plug has to be in to get any HT. I
switched that feed off on the amp using a miniature toggle switch (only
switching low voltage & current) to put it into standby.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

HT Relay
 

"mick" wrote in message
...
On Sun, 27 Dec 2009 23:16:05 +0000, David Looser wrote:

"mick" wrote

By the way, you may not find it too easy getting hold of a suitable
relay with a 5v coil. Manufacturers often list them down to 6v but
nobody stocks them! The nearest that you will get easily will be 12vdc
and the next one up is 24vdc. 5v coils are usually only on small pcb
relays, which don't have enough voltage rating on the contacts, even in
series (if you can ever find a complete data sheet!).


ISTM that 5V coils are now rather more common than 6V ones, and 5V coils
are just as likely to be available on power relays as any other coil
voltage. BTW why do you think PCB relays won't have enough voltage
rating?, I've just found a relay with a 10kV rating for the contact, and
that is a PCB mounting type.



You're correct that 5v coils are common on PCB relays,

I didn't say that. I said that 5V was more common than 6V on ALL relays.

but there aren't
all that many PCB relays that could switch the HT line of an amplifier.
350vDC at 200mA can be difficult to break (and make). I originally
suggested plug-in devices because they come ready insulated and are easy
to use (the 2-pole ones fit a standard octal socket). Unfortunately I
don't think any manufacturer makes these with a 5v coil.

The octal plug in ones mostly seem to use mains voltage coils. But most
relays these days, the vast majority, are PCB mount types, their voltage and
current ratings are well in excess of what might be needed for this
application (and no worse than those of the octal plug-in jobs)

Having said all that, I've just found this: Tyco Electronics RT424005
(Farnell 162-9052).

Or this one?

http://uk.rs-online.com/web/search/s...ct&R=198695 5


Someone suggested that a contact isn't needed.

I suggested that. It isn't, not if you don't unplug the PSU from the load
when it's powered up! IMO by far the best solution is to NOT do that. Either
keep people stupid enough to try doing so away from the kit, or if you can't
do that make sure that the mains has to be disconnected first. Of course
the "PSU" is only the transformer, there is no point in having the rectifier
and reservoir capacitors remote from the load, so as soon as the mains goes
off the PSU output is safe.

David.

HT Relay
 

On Mon, 28 Dec 2009 23:04:36 +0000, David Looser wrote:

"mick" wrote in message
...
On Sun, 27 Dec 2009 23:16:05 +0000, David Looser wrote:

"mick" wrote

By the way, you may not find it too easy getting hold of a suitable
relay with a 5v coil. Manufacturers often list them down to 6v but
nobody stocks them! The nearest that you will get easily will be
12vdc and the next one up is 24vdc. 5v coils are usually only on
small pcb relays, which don't have enough voltage rating on the
contacts, even in series (if you can ever find a complete data
sheet!).


ISTM that 5V coils are now rather more common than 6V ones, and 5V
coils are just as likely to be available on power relays as any other
coil voltage. BTW why do you think PCB relays won't have enough
voltage rating?, I've just found a relay with a 10kV rating for the
contact, and that is a PCB mounting type.



You're correct that 5v coils are common on PCB relays,

I didn't say that. I said that 5V was more common than 6V on ALL relays.



Probably 90% of plug-in relays used for general purpose control are 24v
(AC & DC), 110vAC and 220/230vAC. You also find 12vDC used occasionally,
particularly in fire alarm applications. You won't often find 5v relays
with 3 or more contacts either.


but there aren't
all that many PCB relays that could switch the HT line of an amplifier.
350vDC at 200mA can be difficult to break (and make). I originally
suggested plug-in devices because they come ready insulated and are
easy to use (the 2-pole ones fit a standard octal socket).
Unfortunately I don't think any manufacturer makes these with a 5v
coil.

The octal plug in ones mostly seem to use mains voltage coils. But most
relays these days, the vast majority, are PCB mount types, their voltage
and current ratings are well in excess of what might be needed for this
application (and no worse than those of the octal plug-in jobs)

I wouldn't like to say "the vast majority, are PCB mount types". I'd
agree that there is more choice of PCB types though (there are really
only 3 or 4 plug-in base-styles in common use). Likewise I'd disagree
with "their voltage and current ratings are well in excess of what might
be needed for this application (and no worse than those of the octal plug-
in jobs)" as this implies that they are suitable for DC switching above
30vDC, which is almost always not the case. There quite simply isn't
enough air gap between the contacts in most miniature packages. Many of
them are only rated up to 125vAC or less. There's a good reason for that.
Most control circuits used to be 48vDC (mostly telecoms), 110vDC or
240vAC. 48v and 110v were from batteries. Nowadays most control circuits
are at low voltage (24v max) and current, so there is far less need to
use such large relays. Generally, where AC switching is needed, relays
aren't used at all now of course.



Having said all that, I've just found this: Tyco Electronics RT424005
(Farnell 162-9052).

Or this one?

http://uk.rs-online.com/web/search/s...seAction.html?
method=getProduct&R=1986955


That might be ok. Running it with 350v takes it out of it's DC rating,
but you'ld probably get away with it if the load is about 200mA. It does
break the isolation rule of having 2 breaks in series though, as it's
only a single pole relay.



Someone suggested that a contact isn't needed.

I suggested that. It isn't, not if you don't unplug the PSU from the
load when it's powered up! IMO by far the best solution is to NOT do
that. Either keep people stupid enough to try doing so away from the
kit, or if you can't do that make sure that the mains has to be
disconnected first. Of course the "PSU" is only the transformer, there
is no point in having the rectifier and reservoir capacitors remote from
the load, so as soon as the mains goes off the PSU output is safe.


That's fair enough. I'd originally understood (perhaps wrongly) that the
PSU had the HT going through the plug. That's how I built mine because I
wanted a modular PSU that I could use with alternative amplifier chassis.

Perhaps a better way still is to have a bracket holding the plug in. The
bracket has to be removed by undoing a screw. That would be ok even for
"stupid people". :-) It always annoys me that we can install a lump of
live copperwork in a steel cupboard, stick big warning notices all over
the door and yet still have to shroud the copper to IP2x (with more
warning notices) just in case someone is daft enough to ignore the
notices, open the door and stick their hand in without looking...
sizzle

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

HT Relay
 

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