HT Relay
 

"Ian Bell" wrote in message
...
David Looser wrote:
"Ian Bell" wrote in message
...
Pretty much but I was trying to avoid having a bleed resistor
permanently across the supply. The PSU has a lot of stored charge and a
bleed resistor that takes only a nominal 1% of the load current would
take about 5 minutes to discharge it. That in itself is not a problem as
the PSU is in an enclosure but when I am testing it, it is a pain to
have to wait that long.


If you switch off the supply to the HT rectifier before the heaters go
off then the load current itself will discharge the smoothing capacitors.



Normally that would be the case but this power supply has a lot more
capacitive smoothing than the average and there is still significant
stored charge once the heaters have cooled.


I think you missed my point. Switch off the supply to the HT rectifier
*before* you switch off the heaters. You simply leave the heaters on until
the HT capacitors have discharged. Having separate switches for LT and HT
also has the distinct advantage that you can leave the heaters on whilst
making modifications etc. No need to wait for the valves to warm up again
for the next test.

I'm currently restoring a mid '50s vintage broadcast TV transmitter. It has
a 4-stage power-up sequence:-

1/ Valve heaters and cooling fans,
2/ Auxiliary (350V and 560V) HT and bias supplies
3/ Main HT low
4/ Main HT full (1750V)

(Low main HT is obtained by connecting a bloody great resistor in series
with the primary of the Main HT transformer!)

The principle reason for this sequence, of course, was to ensure that valves
were fully up to temperature and that bias supplies were present and correct
before HT was applied during normal service switch-on, but being able to
switch off the HT supplies without having to switch off the heaters whilst
making adjustments is a real boon.

David.

HT Relay
 

In article , Ian Bell
wrote:
David Looser wrote:
"Ian Bell" wrote in message
...
Pretty much but I was trying to avoid having a bleed resistor
permanently across the supply. The PSU has a lot of stored charge
and a bleed resistor that takes only a nominal 1% of the load
current would take about 5 minutes to discharge it. That in itself
is not a problem as the PSU is in an enclosure but when I am testing
it, it is a pain to have to wait that long.


If you switch off the supply to the HT rectifier before the heaters go
off then the load current itself will discharge the smoothing
capacitors.



Normally that would be the case but this power supply has a lot more
capacitive smoothing than the average and there is still significant
stored charge once the heaters have cooled.

Must admit to being surprised by that. How long do the cathodes take to
cool down?

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

HT Relay
 

David Looser wrote:
"Ian Bell" wrote in message
...
David Looser wrote:
"Ian Bell" wrote in message
...
Pretty much but I was trying to avoid having a bleed resistor
permanently across the supply. The PSU has a lot of stored charge and a
bleed resistor that takes only a nominal 1% of the load current would
take about 5 minutes to discharge it. That in itself is not a problem as
the PSU is in an enclosure but when I am testing it, it is a pain to
have to wait that long.

If you switch off the supply to the HT rectifier before the heaters go
off then the load current itself will discharge the smoothing capacitors.


Normally that would be the case but this power supply has a lot more
capacitive smoothing than the average and there is still significant
stored charge once the heaters have cooled.


I think you missed my point. Switch off the supply to the HT rectifier
*before* you switch off the heaters. You simply leave the heaters on until
the HT capacitors have discharged. Having separate switches for LT and HT
also has the distinct advantage that you can leave the heaters on whilst
making modifications etc. No need to wait for the valves to warm up again
for the next test.


You are right, I had misunderstood. I now see what you meant.


I'm currently restoring a mid '50s vintage broadcast TV transmitter. It has
a 4-stage power-up sequence:-

1/ Valve heaters and cooling fans,
2/ Auxiliary (350V and 560V) HT and bias supplies
3/ Main HT low
4/ Main HT full (1750V)

(Low main HT is obtained by connecting a bloody great resistor in series
with the primary of the Main HT transformer!)


I think I have seen that used elsewhere with a relay across the resistor.

The principle reason for this sequence, of course, was to ensure that valves
were fully up to temperature and that bias supplies were present and correct
before HT was applied during normal service switch-on, but being able to
switch off the HT supplies without having to switch off the heaters whilst
making adjustments is a real boon.

And what do they use to switch the HT?

Cheers

ian

David.

HT Relay
 

Jim Lesurf wrote:
In article , Ian Bell
wrote:
David Looser wrote:
"Ian Bell" wrote in message
...
Pretty much but I was trying to avoid having a bleed resistor
permanently across the supply. The PSU has a lot of stored charge
and a bleed resistor that takes only a nominal 1% of the load
current would take about 5 minutes to discharge it. That in itself
is not a problem as the PSU is in an enclosure but when I am testing
it, it is a pain to have to wait that long.

If you switch off the supply to the HT rectifier before the heaters go
off then the load current itself will discharge the smoothing
capacitors.



Normally that would be the case but this power supply has a lot more
capacitive smoothing than the average and there is still significant
stored charge once the heaters have cooled.

Must admit to being surprised by that. How long do the cathodes take to
cool down?


30 seconds or thereabouts.

Cheers

Ian

Slainte,

Jim

HT Relay
 

In article , Ian Bell
wrote:
Jim Lesurf wrote:
In article , Ian Bell


Normally that would be the case but this power supply has a lot more
capacitive smoothing than the average and there is still significant
stored charge once the heaters have cooled.

Must admit to being surprised by that. How long do the cathodes take
to cool down?


30 seconds or thereabouts.

OK, that sounds reasonably long. So it makes me curious as to what fraction
of the cap voltage remains after 30 sec or 1min. And why you need so much
capacitance, I guess...

FWIW One of the advantages of 'capacity multiplier' PSU methods is that
the apparent working capacitance isn't the real storage. So although
you get the multiplied value for smoothing and output impedance
purposes when powered, the supply dies quickly when switched off as
the charge is soon sucked out when the multiplier effect is lost.

That said, if you are still a fan of relays you could use another
to drop another bleed resistor across when the input rectified
power vanishes. So cutting down the HT quickly when the mains
is removed.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

HT Relay
 

"Ian Bell" wrote

And what do they use to switch the HT?


The switching is all done in the primaries of the various mains
transformers, of which there are about 20 altogether. As originally designed
in the 1950s manual switches were used for steps 1, 2 and 4, and a relay
operated by a push-button for step 3. The relay could not be operated until
the bias supplies were present, and would be released if the main HT current
exceed a set level. Then in the late 1960s it was modified to allow the
transmitter to be controlled from a remote location.

In the modified system relays are used for all 4 steps and a logic board
using Mullard "Norbit" logic modules was added. In 'remote' operation this
steps through the four steps in turn allowing sufficient time between each
step for valves to warm up and H.T. supplies to settle in response to a
remote 'on' command.

In local operation a rotary switch enables each step in turn, though the
logic will not allow the relays to be operated before a minimum time has
elapsed since the step before. Again step 3 is conditional on bias supplies
being present, but the HT overload system is more complex than before. An
overload sets the logic back to step 2 for three seconds, after which steps
3 and then 4 are enabled as normal. If, however, three overloads occur
within a space of 30 seconds the transmitter is powered down completely and
requires manual intervention to restart it.

David.

HT Relay
 

Jim Lesurf wrote:
In article , Ian Bell
wrote:
Jim Lesurf wrote:
In article , Ian Bell

Normally that would be the case but this power supply has a lot more
capacitive smoothing than the average and there is still significant
stored charge once the heaters have cooled.
Must admit to being surprised by that. How long do the cathodes take
to cool down?


30 seconds or thereabouts.

OK, that sounds reasonably long.

Basically if you drive the circuit fairly hard (20V rms output),leave
the HT on and disconnect power to the heaters, the signal remains at the
set level for nearly 10 seconds and then gradually decays over the next 20.

So it makes me curious as to what fraction
of the cap voltage remains after 30 sec or 1min. And why you need so much
capacitance, I guess...


The time constant of a 1% load bleed resistor is about 5 minutes. There
is a total nearly 2000uF of smoothing capacitance. Yes it is a lot and
may be more than I need. The application is an all valve mixer which
essentially contains 40 class A stages each drawing about 5 mA at 350V
i.e about 70W. As many of these are sensitive microphone pre amps I
cannot afford the kind of ripple you would tolerate on the HT supply of
a 50W push pull power amp for example.

I am using a smoothing technique first described by Scroggie around 60
years ago where you replace a single R C filter with N stages in series
each of R/N and C/N so you have the same series resistance (and voltage
drop) and total capacitance. However, you get orders of magnitude
improvement in ripple reduction. The four resistors and four 470uF caps
I am using give 60dB more ripple reduction than a single resistor and
2000uF capacitance. In total I get nearly 120dB of 100Hz attenuation
which means 10V ripple at the input capacitor is 10uV on the HT line. As
the pre amp stages have a PSRR of about 20dB there should be about 1uV
residual 100Hz hum at the stage output i.e -120dBV which is comfortably
below the pre amps own output noise of around -85dBV. I could get away
with the ripple being an order of magnitude higher but that translates
only into a small change in capacitance.

FWIW One of the advantages of 'capacity multiplier' PSU methods is that
the apparent working capacitance isn't the real storage. So although
you get the multiplied value for smoothing and output impedance
purposes when powered, the supply dies quickly when switched off as
the charge is soon sucked out when the multiplier effect is lost.

That said, if you are still a fan of relays you could use another
to drop another bleed resistor across when the input rectified
power vanishes. So cutting down the HT quickly when the mains
is removed.


That was pretty much the plan - the NC contact of the relay connects to
a bleed resistor equivalent to say 20% of the load, the NO contact is
the output and the common goes to the supply.

Cheers

Ian

Slainte,

Jim

HT Relay
 

On Thu, 07 Jan 2010 10:44:22 +0000, Ian Bell wrote:

I have not changed the brief. Originally it was simply to delay the
switch on of HT until the heaters had warmed up; that and nothing more.
It then occurred to me that when the relay dropped out a contact could
be used to discharge the PSU caps more rapidly than a bleed resistor. It
also occurred to me that I could include the relay coil in a loop in the
dc power cable so if it were disconnected the HT would be disconnected too.

With respect, Ian, although your original post talked about discharging
the HT, later the same day you said "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." What I proposed would seem to satisfy the
requirement by implementing the first option: "disconnect the HT".


--
John Stumbles

I am neither for nor against apathy

HT Relay
 

On Thu, 07 Jan 2010 21:07:00 +0000, Ian Bell wrote:
I am using a smoothing technique first described by Scroggie

Not in Foundations of Wireless, IIRC. Second Thoughts, perhaps? I never
really got my brain round that one as well as I did the former (which was
a pressie to me when I was a spotty 'erbert from an uncle who worked in the
beeb).


That was pretty much the plan - the NC contact of the relay connects to
a bleed resistor equivalent to say 20% of the load, the NO contact is
the output and the common goes to the supply.

Well even with that arrangement any arc drawn as the contacts interrupt
the 'normal' load current would then have somewhere to go - into the bleed
resistance - so shouldn't be as bad for them as trying to break the
current to nothing. Why would the bleed resistor be sized to draw only 20%
of the load current? Why not all of it? (assuming the mains input to the
PSU can be guaranteed to be off at this point and so the bleed R isn't
being asked to dissipate the 70W load continuously).


--
John Stumbles

Never believe anyone who claims to be a liar

HT Relay
 

John Stumbles wrote:
On Thu, 07 Jan 2010 10:44:22 +0000, Ian Bell wrote:

I have not changed the brief. Originally it was simply to delay the
switch on of HT until the heaters had warmed up; that and nothing more.
It then occurred to me that when the relay dropped out a contact could
be used to discharge the PSU caps more rapidly than a bleed resistor. It
also occurred to me that I could include the relay coil in a loop in the
dc power cable so if it were disconnected the HT would be disconnected too.

With respect, Ian, although your original post talked about discharging
the HT, later the same day you said "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." What I proposed would seem to satisfy the
requirement by implementing the first option: "disconnect the HT".




Yes, you are quite right. My thoughts were developing during that day.
IIRC someone said 'why not switch the ac' and perhaps I should have said
I would like the 'option' to disconnect or quickly bleed the HT - just
trying to keep as many design option doors open as I can.

My primary concern was, and still is, dc switching of high voltages with
relays. The specs as published gave me cause for concern, hence my
original post. As a result of several replies, yours included, and
reading some articles by relay manufacturers, it is clear to me that
managing the voltage across the contacts, especially when breaking them,
is the key.

Many thanks for your input.

Cheers

Ian