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

"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.


The ratings of relays (indeed switches generally) is lower with DC than AC.
The usual general purpose relays available these days are rated to 250Vac,
but often only about 30Vdc at rated current. You could use a separate heater
transformer and use the relay to switch the primary of the HT transformer.

David.

David Looser 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.


The ratings of relays (indeed switches generally) is lower with DC than AC.
The usual general purpose relays available these days are rated to 250Vac,
but often only about 30Vdc at rated current.

Yes, I realise that, but what happens if a use a relay rated at a much
higher current than I am actually using - say a 20 amp relay for a
0.2amp supply.

You could use a separate heater
transformer and use the relay to switch the primary of the HT transformer.


The current (sic) design does have a separate heater transformer so that
is a viable alternative except I want to ensure the HT is discharged
when the mains is turned off.

Cheers

ian
David.

I imagine the crucial time is as the contacts mate and demate, as the gaps
involved would spark at that current I'd say.

Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"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

I got the impression he wanted to short the ht out to make it safe though.
Another thing to ask is is the load inductive? When I could see Ive welded
many a rely like this!

I guess for the crobar effect one could use a semiconductor, and maybe one
the other way around so to speak to do the soft rise in voltage. Its a
highish current, but not impossible.

Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"David Looser" wrote in message
...
"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.


The ratings of relays (indeed switches generally) is lower with DC than
AC. The usual general purpose relays available these days are rated to
250Vac, but often only about 30Vdc at rated current. You could use a
separate heater transformer and use the relay to switch the primary of the
HT transformer.

David.

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

You could use a separate heater
transformer and use the relay to switch the primary of the HT
transformer.


The current (sic) design does have a separate heater transformer so that
is a viable alternative

Then that is what I would do. It is a technique that was often used in
valve-based industrial equipment, such as broadcast transmitters.

except I want to ensure the HT is discharged when the mains is turned
off.


Bleeder resistor?

David.

"Brian Gaff" wrote in message
m...

I got the impression he wanted to short the ht out to make it safe though.

Shorting out the HT is a pretty drastic thing to do! Guaranteed to wreck the
relay that does it and the smoothing capacitors.

I suspect he intends to use the relay connect a resistor across the HT to
discharge the capacitors. But such a resistor can be left permanently
connected at the cost of a few wasted milliamps of HT current.

David.

"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.

S.

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

You could use a separate heater
transformer and use the relay to switch the primary of the HT
transformer.

The current (sic) design does have a separate heater transformer so that
is a viable alternative

Then that is what I would do. It is a technique that was often used in
valve-based industrial equipment, such as broadcast transmitters.

except I want to ensure the HT is discharged when the mains is turned
off.


Bleeder resistor?

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

Cheers

Ian

David.

David Looser wrote:
"Brian Gaff" wrote in message
m...

I got the impression he wanted to short the ht out to make it safe though.

Shorting out the HT is a pretty drastic thing to do! Guaranteed to wreck the
relay that does it and the smoothing capacitors.

I suspect he intends to use the relay connect a resistor across the HT to
discharge the capacitors. But such a resistor can be left permanently
connected at the cost of a few wasted milliamps of HT current.


Except its discharge time would them be rather too long.

Cheers

ian

David.