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
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