"mick" wrote

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.

You are being very specific here, limiting your scope to plug-in relays.
Most relays don't plug in, though clearly they are the type you are most
familiar with.

I wouldn't like to say "the vast majority, are PCB mount types".

If you look in the catalogues you'll find many PCB relays for every plug-in
one..

Likewise I'd disagree
with "their voltage and current ratings are well in excess of what might
be needed for this application

Yes, I was wrong to say voltage. Few currently available GP relays have an
adequate voltage rating for this purpose. However PCB relays are no worse
than plug-in types in this respect.


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.


Is it single pole?, I didn't read the write up very carefully I'm afraid. I
just noticed that it had a 440V ac rating, wheras most relays have only a
250V ac rating.


That's fair enough. I'd originally understood (perhaps wrongly) that the
PSU had the HT going through the plug.

That was my understanding too. But from a safety POV it's the wrong approach
IMO.

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

Of course if you are building an amp and separate PSU for your own use you
don't *have* to do anything in particular in that respect. Just bear in mind
that you are responsible if anybody gets hurt.

David.

"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.

On Thu, 24 Dec 2009 14:43:30 +0000, Ian Bell wrote:

I am trying to select a relay for a delayed HT switch (which will also
discharge the HT when off). Most relays I can find have contacts rated
at 250VAC which translates into a peak of about 350V. However, data is
scarce on what dc voltage these relays can switch. So far I have found
only one that gives a dc current versus voltage curve and that stops at
210V dc (and 200mA) and I really want to be able to switch up to 350V at
up to 200mA. The rest just give a dc voltage at max current value.

The reason DC switching ratings are lower than AC ratings is that, when
breaking a circuit, AC limits the duration of arcing as the current
through the contacts drops to zero twice a cycle thus helping to
extinguish the arc. With DC the contacts have to open wide enough to
extinguish the arc on their own.

You say:

The PSU is remote and the relay has an interlock to turn off the HT if
the PSU HT output lead is disconnected. I need to either disconnect the
HT or bleed it very quickly to avoid a possible shock hazard.

I assume we have the following setup in the PSU:

relay
HT+
to -------------o COM
LOAD /
o o----------- HT+ SUPPLY
NC | NO
|
-----
| R | - discharge resistor
-----
|
GROUND -----------+---------------- GROUND

The relay contacts change from NC to NO a little while after mains has
been applied to the PSU (allowing valves time to warm up) and from NO
back to NC when either the system is switched off or the HT output lead
is disconnected.

In the first instance there is no current flowing until the contacts
close, so no arcing to bother with and I wouldn't worry about the relay's
DC switching rating being lower than the HT voltage: as long as the relay
is capable of holding off the voltage when the contacts are open and no
current is flowing (and of handling the expected supply current) it
should be OK.

In the second and third cases the HT current will try to arc across from
the NO to the COM contact until COM closes on NC at which point the
discharge resistor should attenuate the voltage from the supply and
extinguish the arc. In the second case - when the power to the PSU is
switched off normally - if you can allow a certain delay before operating
the relay then most the the HT energy should have been dissipated in the
load electronics, so the relay should have little current-breaking work
to do and the contacts would not be strained.

The third case - when the HT lead is disconnected - seems to pose the
most stressful conditions. The interlock that operates the relay must
also cut mains power into the PSU, and it is then a case of whether the
relay can handle the energy stored in the transformer and smoothing
capacitor(s) at the moment of switching. However this situation should be
relatively infrequent so as long as the relay contacts don't melt down
instantaneously it shouldn't be a problem.

I think your plan to choose a relay with contacts rated at higher current
than that you expect to switch is good: the relay will in any case need
to be rated to supply the current from the supply (with fully charged HT
smoothing capacitor(s)) into the discharge resistor. A bit of suck it and
see with deliberate disconnection of the PSU from the load and
observation of the relay as it operates (assuming it has a transparent
cover) would be the order of the day.

--
John Stumbles

When I die, I want to go peacefully in my sleep like my father did,
not screaming in terror like his passengers.

John Stumbles wrote:
On Thu, 24 Dec 2009 14:43:30 +0000, Ian Bell wrote:

I am trying to select a relay for a delayed HT switch (which will also
discharge the HT when off). Most relays I can find have contacts rated
at 250VAC which translates into a peak of about 350V. However, data is
scarce on what dc voltage these relays can switch. So far I have found
only one that gives a dc current versus voltage curve and that stops at
210V dc (and 200mA) and I really want to be able to switch up to 350V at
up to 200mA. The rest just give a dc voltage at max current value.

The reason DC switching ratings are lower than AC ratings is that, when
breaking a circuit, AC limits the duration of arcing as the current
through the contacts drops to zero twice a cycle thus helping to
extinguish the arc. With DC the contacts have to open wide enough to
extinguish the arc on their own.


Indeed, I have found a very informative application note by Tyco that
explains wvery well what happens when contacts make and when they break.

You say:

The PSU is remote and the relay has an interlock to turn off the HT if
the PSU HT output lead is disconnected. I need to either disconnect the
HT or bleed it very quickly to avoid a possible shock hazard.

I assume we have the following setup in the PSU:

relay
HT+
to -------------o COM
LOAD /
o o----------- HT+ SUPPLY
NC | NO
|
-----
| R | - discharge resistor
-----
|
GROUND -----------+---------------- GROUND

Not quite, swap the LOAD and SUPPLY and it is correct - in other words
the HT supply goes to the common and is switched either to the load or
the discharge resistor because nearly all the charge storage is in the
HT supply not the load.

That somewhat negates the discussion below (which is still useful) so
perhaps you would be kind enough to repeat it with the actual circuit??

Cheers

Ian


The relay contacts change from NC to NO a little while after mains has
been applied to the PSU (allowing valves time to warm up) and from NO
back to NC when either the system is switched off or the HT output lead
is disconnected.

In the first instance there is no current flowing until the contacts
close, so no arcing to bother with and I wouldn't worry about the relay's
DC switching rating being lower than the HT voltage: as long as the relay
is capable of holding off the voltage when the contacts are open and no
current is flowing (and of handling the expected supply current) it
should be OK.

In the second and third cases the HT current will try to arc across from
the NO to the COM contact until COM closes on NC at which point the
discharge resistor should attenuate the voltage from the supply and
extinguish the arc. In the second case - when the power to the PSU is
switched off normally - if you can allow a certain delay before operating
the relay then most the the HT energy should have been dissipated in the
load electronics, so the relay should have little current-breaking work
to do and the contacts would not be strained.

The third case - when the HT lead is disconnected - seems to pose the
most stressful conditions. The interlock that operates the relay must
also cut mains power into the PSU, and it is then a case of whether the
relay can handle the energy stored in the transformer and smoothing
capacitor(s) at the moment of switching. However this situation should be
relatively infrequent so as long as the relay contacts don't melt down
instantaneously it shouldn't be a problem.

I think your plan to choose a relay with contacts rated at higher current
than that you expect to switch is good: the relay will in any case need
to be rated to supply the current from the supply (with fully charged HT
smoothing capacitor(s)) into the discharge resistor. A bit of suck it and
see with deliberate disconnection of the PSU from the load and
observation of the relay as it operates (assuming it has a transparent
cover) would be the order of the day.

On Wed, 30 Dec 2009 23:59:58 +0000, Ian Bell wrote:

Nice ASCII art John!
snip

relay
HT+
to -------------o COM
LOAD /
o o----------- HT+ SUPPLY
NC | NO
|
-----
| R | - discharge resistor
-----
|
GROUND -----------+---------------- GROUND

Not quite, swap the LOAD and SUPPLY and it is correct - in other words
the HT supply goes to the common and is switched either to the load or
the discharge resistor because nearly all the charge storage is in the
HT supply not the load.


Ah. Just as I thought.
As David has pointed out though, moving the rectifier(s) and reservoir
caps etc. onto the amp chassis removes most of the problem. It has
several advantages:
You don't need a relay.
Only AC goes through the plug & socket.
The rectifier protects the socket against a back feed from the HT.
The plug & socket are "dead" as soon as the mains is switched off.
All you need to do is to make sure that the plug & socket can't be
disconnected while the mains is on, or rig up something so that a tool is
needed.

Incidentally, I used a flying lead from the PSU and a socket on the amp.
Fixed socket: RS 487-378
Cable plug: RS 487-384
These have a wider pin spacing than octal, so I didn't have any qualms
about using them, even though Bulgin don't give a DC rating for them.
The HT was switched by a relay as you described above, with an electrical
interlock so that the relay couldn't pull in until the plug & socket had
made.

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

mick wrote:
On Wed, 30 Dec 2009 23:59:58 +0000, Ian Bell wrote:

Nice ASCII art John!
snip
relay
HT+
to -------------o COM
LOAD /
o o----------- HT+ SUPPLY
NC | NO
|
-----
| R | - discharge resistor
-----
|
GROUND -----------+---------------- GROUND
Not quite, swap the LOAD and SUPPLY and it is correct - in other words
the HT supply goes to the common and is switched either to the load or
the discharge resistor because nearly all the charge storage is in the
HT supply not the load.


Ah. Just as I thought.
As David has pointed out though, moving the rectifier(s) and reservoir
caps etc. onto the amp chassis removes most of the problem. It has
several advantages:
You don't need a relay.
Only AC goes through the plug & socket.
The rectifier protects the socket against a back feed from the HT.
The plug & socket are "dead" as soon as the mains is switched off.
All you need to do is to make sure that the plug & socket can't be
disconnected while the mains is on, or rig up something so that a tool is
needed.


It also has some serious drawbacks.

1. Some god awful current spikes going up and down a longish cable
radiating crap all over the place.

2. AC inside a box where I am trying to keep it out by having an
external PSU

Incidentally, I used a flying lead from the PSU and a socket on the amp.
Fixed socket: RS 487-378
Cable plug: RS 487-384
These have a wider pin spacing than octal, so I didn't have any qualms
about using them, even though Bulgin don't give a DC rating for them.
The HT was switched by a relay as you described above, with an electrical
interlock so that the relay couldn't pull in until the plug & socket had
made.


Funnily enough, those are precisely what I was planning to use and I
even bought a few pairs. Trouble is they do not lock and can very easily
be pulled out.

I am now planning on using an 8 way Speakon locking connector - a tad
more expensive but you won't be able to pull it out.

Cheers

Ian

On Thu, 31 Dec 2009 14:42:24 +0000, Ian Bell wrote:

snip
It also has some serious drawbacks.

1. Some god awful current spikes going up and down a longish cable
radiating crap all over the place.


I hadn't considered that - good point.

2. AC inside a box where I am trying to keep it out by having an
external PSU


That makes sense too.

Incidentally, I used a flying lead from the PSU and a socket on the
amp. Fixed socket: RS 487-378
Cable plug: RS 487-384
These have a wider pin spacing than octal, so I didn't have any qualms
about using them, even though Bulgin don't give a DC rating for them.
The HT was switched by a relay as you described above, with an
electrical interlock so that the relay couldn't pull in until the plug
& socket had made.


Funnily enough, those are precisely what I was planning to use and I
even bought a few pairs. Trouble is they do not lock and can very easily
be pulled out.

I am now planning on using an 8 way Speakon locking connector - a tad
more expensive but you won't be able to pull it out.


I just relied on the fact that I was going to be in the room while it was
switched on. :-) At one point I even had bare HT terminals on the OPTs -
just for the melodrama, you understand... "It's got lit valves - dare I
touch it?" :-D I put the connector on top of the chassis, with the
umbilical going off backwards. It's easy to see and doesn't pull out
easily at all, as it requires pulling the plug upward. I wonder if you
could take out the centre pin and fit a retaining screw in that position
somehow? That would be neat.

The Speakons would be fine, but you can still undo them without a tool.

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

"Ian Bell" wrote

It also has some serious drawbacks.

1. Some god awful current spikes going up and down a longish cable
radiating crap all over the place.

There shouldn't be "god awful current spikes" unless the conduction angle is
excessively short, which isn't good for the transformer, the rectifiers or
the capacitors. Since both the a.c. conductors are, presumably, co-routed,
the magnetic fields will very largely cancel anyway.

2. AC inside a box where I am trying to keep it out by having an external
PSU

Are you using a.c. for the heaters? that has to go right up and into to the
valves, whilst the a.c. for the HT need go nowhere near them or any audio
wiring.

The reasons for keeping the transformer separate are to lose it's
weight/bulk and to remove leakage magnetic fields from the amp.

David.

mick wrote:
On Thu, 31 Dec 2009 14:42:24 +0000, Ian Bell wrote:

snip
It also has some serious drawbacks.

1. Some god awful current spikes going up and down a longish cable
radiating crap all over the place.


I hadn't considered that - good point.

2. AC inside a box where I am trying to keep it out by having an
external PSU


That makes sense too.

Incidentally, I used a flying lead from the PSU and a socket on the
amp. Fixed socket: RS 487-378
Cable plug: RS 487-384
These have a wider pin spacing than octal, so I didn't have any qualms
about using them, even though Bulgin don't give a DC rating for them.
The HT was switched by a relay as you described above, with an
electrical interlock so that the relay couldn't pull in until the plug
& socket had made.


Funnily enough, those are precisely what I was planning to use and I
even bought a few pairs. Trouble is they do not lock and can very easily
be pulled out.

I am now planning on using an 8 way Speakon locking connector - a tad
more expensive but you won't be able to pull it out.


I just relied on the fact that I was going to be in the room while it was
switched on. :-) At one point I even had bare HT terminals on the OPTs -
just for the melodrama, you understand... "It's got lit valves - dare I
touch it?" :-D I put the connector on top of the chassis, with the
umbilical going off backwards. It's easy to see and doesn't pull out
easily at all, as it requires pulling the plug upward. I wonder if you
could take out the centre pin and fit a retaining screw in that position
somehow? That would be neat.

The Speakons would be fine, but you can still undo them without a tool.


Yes, Speakons have a little slide thingy on top to release them so you
can twist and release them. It is not easy to do (good) but it can be
done (good).

When I was a lot younger (40+ years ago) I had built so many valve
projects that I was just about immune to 300V dc shocks. However, on day
our old B&W TV broke down so I took the back off to have a look.
Unfortunately the EHT had not completely discharged and I got a 12KV
belt that shot me across the room.

Cheers

ian

David Looser wrote:
"Ian Bell" wrote
It also has some serious drawbacks.

1. Some god awful current spikes going up and down a longish cable
radiating crap all over the place.

There shouldn't be "god awful current spikes" unless the conduction angle is
excessively short, which isn't good for the transformer, the rectifiers or
the capacitors. Since both the a.c. conductors are, presumably, co-routed,
the magnetic fields will very largely cancel anyway.

Well, if you aim initially for 10% ripple then that will be the
conduction angle to a first approximation and the current spikes will be
10 times the load current.

2. AC inside a box where I am trying to keep it out by having an external
PSU

Are you using a.c. for the heaters? that has to go right up and into to the
valves, whilst the a.c. for the HT need go nowhere near them or any audio
wiring.


No, the heaters are dc.

The reasons for keeping the transformer separate are to lose it's
weight/bulk and to remove leakage magnetic fields from the amp.

That's a couple of them, yes.

Cheers

ian

David.