Relays for Switching Sense Leads on Power Supplies

[EdV]

I have been asked to select relays that will be used to switch power
supply sense leads between remote and local sense. My impression is
that mercury wetted contacts will be the best bet for currents likely
to be less than 1ma. I say likely because the folks at the unnamed
power supply companies do not spec it and are researching the answer
for me.

Anyone care to share their experience in this or a similar
application.

Thanks much,
Ed V.

 

[Spehro Pefhany]

I have been asked to select relays that will be used to switch power
supply sense leads between remote and local sense. My impression is
that mercury wetted contacts will be the best bet for currents likely
to be less than 1ma. I say likely because the folks at the unnamed
power supply companies do not spec it and are researching the answer
for me.

Anyone care to share their experience in this or a similar
application.

Thanks much,
Ed V.

Mercury-wetted reeds should do the trick, but there are other kinds of
relays rated for dry circuit use that don't use mercury (eg. inert-gas
filled with precious metal contacts, regular reeds etc.).


Best regards,
Spehro Pefhany

 

[Don Lancaster]

Mercury-wetted reeds should do the trick, but there are other kinds of
relays rated for dry circuit use that don't use mercury (eg. inert-gas
filled with precious metal contacts, regular reeds etc.).


Best regards,
Spehro Pefhany

Most reed switches have wiping contacts.
And do not have the ROHS problems of mercury.

Curiously, my Pathfinder has this problem.
You have to blow the horn before turning on the cruise control.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Barry Lennox]

Most reed switches have wiping contacts.
And do not have the ROHS problems of mercury.

I seem to recall reading an EU directive that stated that
Mercury-wetted relays are RoHS exempt, as there is "no technical
alternative" There's several exemptions that have been declared in
that category.

Barry

 

[Joerg]

I have been asked to select relays that will be used to switch power
supply sense leads between remote and local sense. My impression is
that mercury wetted contacts will be the best bet for currents likely
to be less than 1ma. I say likely because the folks at the unnamed
power supply companies do not spec it and are researching the answer
for me.

Anyone care to share their experience in this or a similar
application.

Exemption or not, mercury can get you (or your client I guess) into
trouble in some markets because you never know when those bureaucrats
change their minds. Can't it be done via electronic multiplexing? Or
just little FETs?

 

[Genome]

I have been asked to select relays that will be used to switch power
supply sense leads between remote and local sense. My impression is
that mercury wetted contacts will be the best bet for currents likely
to be less than 1ma. I say likely because the folks at the unnamed
power supply companies do not spec it and are researching the answer
for me.

Anyone care to share their experience in this or a similar
application.

Thanks much,
Ed V.

As far as I know...... most power supplies maintain a permanent connection
between the main output terminals and the sense terminals using low value,
220R, resistors..... and incorporate some form of 'sense fault' protection
to turn the supply off in the event that the main power leads drop off and
you try and power the load through the sense leads. One method is to use PTC
thermistors which will self limit.

The assumption is made that if the end user wishes to use remote sensing
then they will have installed the wires to do so and that automagically
shorts out the resistors.

You do have to consider the relative DC error introduced by those resistors
when remote sensing is not used but it's sort of in the order of 0.2% so,
compared to the error introduced by the power leads, it might be ignored.

DNA

 

[Jon]

I have been asked to select relays that will be used to switch power
supply sense leads between remote and local sense. My impression is

Ed,
Mercury wetted relays are good if you are concerned with "contact
bounce". Otherwise, any relay with gold or gold/palladium contacts
should do the job nicely.

 

[Spehro Pefhany]

Ed,
Mercury wetted relays are good if you are concerned with "contact
bounce". Otherwise, any relay with gold or gold/palladium contacts
should do the job nicely.

I've used them (the palladium gold contacts, IIRC) quite successfully
to switch thermocouples, which are about as dry as things get*. JL,
OTOH, reported a less cheerful experience with them.

* Come to think of it, if there was an open, the break protection
circuitry would put a substantial voltage (volts, not mV) across the
open contact at << 1uA, so maybe not as dry as all that.


Best regards,
Spehro Pefhany

 

[Ross Herbert]

Most reed switches have wiping contacts.
And do not have the ROHS problems of mercury.

My experience with dry reed switches is that the contact resistance
will go quite high (~ 5 ohms) if they sit around doing nothing for
long periods of time. Indeed, I have measured such resistance in reed
elements I have in stock. As long as they are operated frequently the
wiping action of the contacts will keep the contact resistance down in
the milliohms range.

Due to the need for consistently reliable readings over the lifetime
of the equipment indicated by the OP, I would hesitate to use dry reed
switches. I would lean towards a mercury wetted type such as
http://www.celduc-relais.com/all/pdfcelduc/f81ax500.pdf or similar.

 

[John Larkin]

I have been asked to select relays that will be used to switch power
supply sense leads between remote and local sense. My impression is
that mercury wetted contacts will be the best bet for currents likely
to be less than 1ma. I say likely because the folks at the unnamed
power supply companies do not spec it and are researching the answer
for me.

Anyone care to share their experience in this or a similar
application.

Thanks much,
Ed V.

Take a look at Fujitsu type FTR-B3 relays. We use them all the time
for low-level switching. Several mfrs have drop-in equivalents.

Hey, I just googled "Fujitsu type FTR-B3" and got a very interesting
first hit! Very strange.

John

 

[Tony Williams]

I have been asked to select relays that will be used to switch
power supply sense leads between remote and local sense. My
impression is that mercury wetted contacts will be the best bet
for currents likely to be less than 1ma. I say likely because
the folks at the unnamed power supply companies do not spec it
and are researching the answer for me.

One of the big problems of switching sense-leads
is that any fault or glitch causes the supply to
go to an uncontrollable output.... perhaps causing
damage to A.N Other's equipment. It is really a
designed-in accident waiting to happen.

DNA's suggestion looks the safer way to go.

 

[Spehro Pefhany]

One of the big problems of switching sense-leads
is that any fault or glitch causes the supply to
go to an uncontrollable output.... perhaps causing
damage to A.N Other's equipment. It is really a
designed-in accident waiting to happen.

DNA's suggestion looks the safer way to go.

Surely they'd run a few nA or uA through the leads to avoid that
situation?


Best regards,
Spehro Pefhany

 

[EdV]

Take a look at Fujitsu type FTR-B3 relays. We use them all the time
for low-level switching. Several mfrs have drop-in equivalents.

Hey, I just googled "Fujitsu type FTR-B3" and got a very interesting
first hit! Very strange.

John

Thanks for the suggestion I order a bunch of the SMT types that Mouser
had in stock. ExpressPCB.com here I come.

Ed V.

 

[EdV]

One of the big problems of switching sense-leads
is that any fault or glitch causes the supply to
go to an uncontrollable output.... perhaps causing
damage to A.N Other's equipment. It is really a
designed-in accident waiting to happen.

DNA's suggestion looks the safer way to go.

I have recommended the needed data acquistion resources be used to
ensure that the proper sense lead configuration is in place prior to
switching the power supply output on.
Whether or not my collegues will do this is a matter of debate.

All of the equipment that will powered from these power supplies are
equipped to withstand a wide variety of minsconnections and report
faults over an RS 485 or other com to the test system. Operators are
protected from exposure to all electrical connections by interlocked
shields. Which is all great when it all works but oh boy the one time
it doesn't.

Thanks for keeping me on the safety track.

Ed V.

 

[John Larkin]

I have recommended the needed data acquistion resources be used to
ensure that the proper sense lead configuration is in place prior to
switching the power supply output on.
Whether or not my collegues will do this is a matter of debate.

All of the equipment that will powered from these power supplies are
equipped to withstand a wide variety of minsconnections and report
faults over an RS 485 or other com to the test system. Operators are
protected from exposure to all electrical connections by interlocked
shields. Which is all great when it all works but oh boy the one time
it doesn't.

Thanks for keeping me on the safety track.

Ed V.

Most supplies have internal resistors across the sense paths, so they
won't go open-loop if the remote sense circuit opens up.

John

 

[ehsjr]

Take a look at Fujitsu type FTR-B3 relays. We use them all the time
for low-level switching. Several mfrs have drop-in equivalents.

Hey, I just googled "Fujitsu type FTR-B3" and got a very interesting
first hit! Very strange.

John

 

[Tony Williams]

Tony Williams wrote:

Surely they'd run a few nA or uA through the leads to
avoid that situation?

I've just looked at a 600W proprietry system supply.
There is a permanent resistive connection between
Vout and +Sense, but it is 10k. DNA's 470 ohm or
so looked much more reasonable.

Vout -----O--+--------+--------->Load
| |
\ |
470/ _ o
\ /|
| /
+Sense ----O--+-----P o------>Remote sense.