Serge Auckland wrote:

"Ian Bell" wrote in message
...
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.

So, I am guessing that this problem has been faced before and there
are relays that are known to work fine in this application. Any
recommendations?

By the way I prefer a 5V coil.

Cheers

Ian

Use a Triac. The gate can then be triggered by a low voltage relay like
a reed. I built such a delay into a valve amp I designed many years ago,
which used a SS bridge rectifier, and I wanted to delay the HT until the
heaters had warmed up. Used a Unijunction transistor timer to create a
30 second delay, then that closed the reed relay which fired the
Triac. 1000 volt 1 amp or more Triacs are cheap.


That sounds like a good idea.

Thanks

ian
S.

"Ian Bell" wrote

Yes, but unless it consumes a significant fraction of the load then its
decay time will be rather long.


True, but it's normal. How many bits of valve kit have relay connected
discharge resistors? - almost none. Is there any particular reason why you
need a very short discharge time?

David.

"Ian Bell" wrote

Except its discharge time would them be rather too long.

Too long for what?

David.

"Ian Bell" wrote

That sounds like a good idea.

It *is* a good idea. But it won't provide the discharge that seems to be so
essential unless you add a second triac.
(Actually, since this is DC, wouldn't a thyristor be more appropriate?)

David.

David Looser wrote:
"Ian Bell" wrote
Except its discharge time would them be rather too long.

Too long for what?


Safety. One of the functions of the relay is to disconnect the HT if the
PSU connector is unplugged. If I just switch the ac and bleed the HT
there will be a significant HT voltage on the PSU connector for long
enough for someone to get a shock from it. That's one reason why I
prefer to switch the dc HT.

Cheers

Ian

David.

David Looser wrote:
"Ian Bell" wrote
Yes, but unless it consumes a significant fraction of the load then its
decay time will be rather long.


True, but it's normal. How many bits of valve kit have relay connected
discharge resistors? - almost none. Is there any particular reason why you
need a very short discharge time?


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.

Cheers

ian
David.

"Ian Bell"


Safety. One of the functions of the relay is to disconnect the HT if the
PSU connector is unplugged. If I just switch the ac and bleed the HT there
will be a significant HT voltage on the PSU connector for long enough for
someone to get a shock from it. That's one reason why I prefer to switch
the dc HT.


** Bell must be the biggest idiot on the face of the planet !!!

What sort of UTTER MORON uses a connector with exposed and ** touchable
pins** for the output of a high voltage PSU ??

Has the pommy ASS never noticed how AC supply leads ALWAYS use
connectors with female pins on the end that goes to the equipment ????????

Unbelievable.


..... Phil

If you really want to go the relay route, you'll probably need to go to a
vacuum relay

See

http://www.jenningstech.com/ps/jen/p...?action=relays

I've been using one for a number of years for switching 1200V dc @ 0.5A and
have had no problems.

Coil voltages are normally 12 or 24VDC but I don't think they are available
in 5V



"Ian Bell" wrote in message
...
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.

So, I am guessing that this problem has been faced before and there are
relays that are known to work fine in this application. Any
recommendations?

By the way I prefer a 5V coil.

Cheers

Ian

"David Looser" wrote in message
...
"Ian Bell" wrote

That sounds like a good idea.

It *is* a good idea. But it won't provide the discharge that seems to be
so essential unless you add a second triac.
(Actually, since this is DC, wouldn't a thyristor be more appropriate?)

David.



I can't now remember why I used a triac rather than a thyristor. A thyristor
will certainly do the job. It's possible that I just happened to have a
triac lying around! Long time ago.

S.

In article , Ian Bell
wrote:
David Looser wrote:
"Ian Bell" wrote
Yes, but unless it consumes a significant fraction of the load then
its decay time will be rather long.


True, but it's normal. How many bits of valve kit have relay connected
discharge resistors? - almost none. Is there any particular reason
why you need a very short discharge time?


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.

For the HT source side I assume you use female connectors, so physically
preventing accidental contact with fingers, etc.

For the HT sink side you can use a silicon diode following a shunt resistor
to ground. Once the plug is pulled this isolates the HT from the prongs of
the male plug.

Note this means the cable is *not* reversable as it has a male at one end
and a female at the other to ensure that whichever end is opened you can
apply the above correctly. The shunt resistor before the diode will also
help remove the charge from any cable capacitance if the disconnection is
at the source end.

Afraid I don't recall you saying where the smoothing/reservoir caps are
located. So I'm assuming both/either ends of the link.

Also use a bleed resistor on the caps.

You could use something like a crowbar which trips if a loop via two
conductors of the cable is broken. However though neat in some ways it can
fall into the trap of being over-complex so designing in extra failure
modes. Hence if you are very safety conscious I'd still do the above.

My experience is that almost any type of conventional relay will end up
causing problems if the unit is used for many years. Avoid if at all
possible. Solid state relays would be much better if you can find one that
suits your purpose.

Slainte,

Jim

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