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.