"David Loser has Roos loose in his top paddock "


BTW why do you think PCB relays won't have enough voltage rating?

** The laws of physics ??


I've just found a relay with a 10kV rating for the contact,
and that is a PCB mounting type.


** One of these - right ?

http://www.farnell.com/datasheets/56123.pdf

It's a reed relay, so switching is done in a vacuum.

The 50 watt max rating means it will switch 5mA at 10kV.

Or 140mA at 350 VDC .............

Same as many other types mentioned already.

Wot a pathetic ****img bull**** artist you are.

FOAD.



...... Phil

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.

"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.

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.

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.