HT Relay
 

"Jim Lesurf" wrote in message
...
In article , David Looser
wrote:
"Jim Lesurf" wrote

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.


I don't share your jaundiced view of relays. Whilst they are not
suitable for every switching requirement, there are many applications
for which they are the most logical choice. I suspect your negative
view is based on the effects of cost-cutting by equipment
manufacturers.

Based also on kit I designed myself and where I chose relays. The problems
may take decades, but in my experience eventually turn up. If you think
about it, physical contacts all eventually wear or corrode, springs
weaken,
etc.


Yes physical contacts will wear, but nothing lasts forever. Electronic
components have their own failure mechanisms. I remember reading only a few
months ago about concern from the avionics industry about the short MTBFs of
modern high-density ICs.


However the problem Ian is converned with is a safety issue, and for all I
know might affect users in some decades time from now.

Indeed. But that statement carries an unspoken assumption that using a relay
will be more of a safety issue in years to come than using electronic
components instead. No components are 100% reliable, and correctly specified
relays have as good a record as any.

And FWIW I had to change the controls for our central heating some time
ago.

Was that change required because of relay failure? Anyway I wasn't talking
about "controls", I was talking about the boiler control unit, which
sequences the operation of the blower motor, the ignition transformer and
the oil valve at start-up. It also shuts down the boiler in the event of no
flame being detected, which *is* a safety issue. Over the years the boiler
has had to have new baffle plates, a new blower motor and a new drive
coupling to the oil pump, but the 30 year-old relays in the control box are
working as well as ever.

David.

HT Relay
 

On Thu, 24 Dec 2009 14:43:30 +0000, Ian Bell wrote:

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?




Did you get this sorted out Ian? The thread in rat seems to have
degenerated into the usual slanging match with Phil. :-(

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!).

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

HT Relay
 

In article , mick
wrote:
On Fri, 25 Dec 2009 09:45:32 +0000, Jim Lesurf wrote:

snip

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.


For goodness sake don't tell the thousands of small generating stations
that have been using relays for generator control continuously for the
last 40 years or so... They might get suspicious if they find out that
their technology is so unreliable. ;-)

When that is coupled with the very simple plug- n-play maintenance
(which is often all the systems get) it makes them ideal for use in
remote generation plant.

The above has just reminded me of the joke in "Only Fools and Horses" where
Trig (IIRC) was proudly telling everyone how long his broom had lasted.
Then to say, "...Of course I have had to replace the head X times... and
the handle Y times. But it is remarkable how long that broom has lasted."
:-)

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
 

mick wrote:
On Thu, 24 Dec 2009 14:43:30 +0000, Ian Bell wrote:

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?




Did you get this sorted out Ian? The thread in rat seems to have
degenerated into the usual slanging match with Phil. :-(

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!).


I'm getting there. Despite Phil's rantings there's quite a bit of useful
info in there if you take the time to sift it out. A bright lad but
sadly disabled.

Anyway, I have found a number of 5V relays that should do the trick,
along with some careful bits at both ends of the cable to ensure the
voltages broken are manageable. I can use 6V relays if necessary - the
current plan is to make a local dc supply from the heater 6.3VAC via a
5V low drop out regulator for the little PIC that decides when the relay
should operate. The relay could either work from the regulated 5V or the
raw smoothed dc. Next step is to make a simple test rig and try a life test.

Cheers

Ian

HT Relay
 

"Ian Bell" wrote


PIC that decides when the relay should operate.

A PIC? good lord you are making a meal out of this one aren't you? Anyway I
thought you had decided to go for Don's Triac option?

Had you thought about simply hard-wiring the two units together? then the
problem vanishes. Or if you *must* use a plug & socket make it impossible to
remove it without unplugging the mains first. I really do think that you are
likely to end up with an electrical interlock system that is so complex, and
based on potentially unreliable components, as to fail to provide the safety
that is supposedly your aim.

David.

HT Relay
 

"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.

David.

HT Relay
 

David Looser wrote:
"Ian Bell" wrote

PIC that decides when the relay should operate.

A PIC? good lord you are making a meal out of this one aren't you?

No, the initial reason for the relay was to delay the switch on of the
HT by 30 seconds or so to allow the heaters to warm up. That was all.
Then it occurred to me that as it would drop out when the mains went off
it could also be used to more rapidly discharge the HT smoothing caps
than by using a simple bleed resistor (I reckon there's abnout 150
joules of energy stored in them)

Once the relay is in there, it is then simple to arrange that it also
drops out if someone pulls the HT cable.

The PIC is an 8 pin DIL, costs a dollar and allows me to light a couple
of LEDs to show the HT state, handle the cable interlock and drive the
relay. It needs no external clock components and there's even a spare IO
line I could use for a push button emergency off switch.

Cheers

Ian

Anyway I
thought you had decided to go for Don's Triac option?

Had you thought about simply hard-wiring the two units together? then the
problem vanishes. Or if you *must* use a plug & socket make it impossible to
remove it without unplugging the mains first. I really do think that you are
likely to end up with an electrical interlock system that is so complex, and
based on potentially unreliable components, as to fail to provide the safety
that is supposedly your aim.

David.

HT Relay
 

"David Looser has Roos loose in his top paddock "


BTW why do you think PCB relays won't have enough voltage rating?

** The laws of physics ??


I've just found a relay with a 10kV rating for the contact,
and that is a PCB mounting type.


** One of these - right ?

http://www.farnell.com/datasheets/56123.pdf

It's a reed relay, so switching is done in a vacuum.

The 50 watt max rating means it will switch 5mA at 10kV.

Or 140mA at 350 VDC .............

Same as many other types mentioned already.

Wot a pathetic bull**** artist you are.


...... Phil

The ANSWER !!
 

"Ian Bell"

Once the relay is in there, it is then simple to arrange that it also
drops out if someone pulls the HT cable.


** Seems we have all be sucked into another of Bell's insane
misunderstandings of electronics.

IF the relay only has to disconnect the HT pin on the PSU connecter
AFTER the plug has been pulled - then it is switching off

NO CURRENT AT ALL !!!


ANY relay at all can be used for that.




..... Phil

HT Relay
 

In article , Ian Bell
wrote:
David Looser wrote:
"Ian Bell" wrote

PIC that decides when the relay should operate.

A PIC? good lord you are making a meal out of this one aren't you?

No, the initial reason for the relay was to delay the switch on of the
HT by 30 seconds or so to allow the heaters to warm up. That was all.

During the first years of manufacture the ampliers/recievers in the
Armstrong 600 range used a thermal delay relay to avoid a surge when the
unit was switched on.

These were expensive. And they were one of the main failure modes which
brought sets back for repair. One of the reasons I avoid physical relays. I
replaced them in later sets with a better PSU that simply didn't need a
mechanical relay. Over the following few years it was quite clear that this
had a big impact on the reliability of the sets.

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