Minimum switch current

[John Fields]

Bell Labs invented the reed relay in 1932 for used in telephone
exchanges. Please explain what switch they used to protect the dry
reed relay switches from having to carry current when opening or
closing even though they had to carry some current when closed. Then
tell me what kind of switch they used to protect the protective
switch ...

http://en.wikipedia.org/wiki/Reed_switch

In fact, it looks as if we are both wrong - the only reference I could
google claims that "dry reed" relays are so called to distinguish them
from mercury-wetted reed relays (which really don't need any wetting
current).

http://www.answers.com/topic/dry-reed-relay?cat=technology

I happen to like mercury-wetted reed relays - they offer a lower and
more stable contact resistance than you can get from any other kind of
relay, and they don't bounce. The fact that they have to be mounted
within 15 degrees of being vertical can be a problem. People kept on
offering orientation-insenstive mercury-wetted reed relays, but nobody
evers seems to have mastered the art of manufacturing them reliably.

---
Well, Bill, how nice to hear from you again, even though your
commentary is flawed, as usual.

In the second place, manufacturing orientation-insensitive
mercury-wetted reed relays reliably isn't the problem, having them
_perform_ reliably in any orientation seems to be.

In the first place, what's being discussed are contacts used in a
dry _circuit_, not the difference between mercury-wetted and non
mercury-wetted contacts.

Had you had your wits about you when you decided to use Google, you
might have searched for "dry circuit" and found, excerpted from the
National Association of Relay Manufacturers' handbook:

"Dry circuit loads: No current is switched. The contacts carry
current only after they are closed or before they are opened. The
currents may be high, as long as they are not switched. Since there
is no arcing, contact resistance is kept low by using gold plating
or gold alloy contacts."

at:

http://www.leachintl2.com/english/english2/vol6/properties/how4.htm

 

[John Fields]

I, at least, would jump at the chance to congratulate you for getting
something right at last.

You do seem to have caught one of my (rare) misconceptions here, but
since your theory about the meaning of "dry" in this context is even
less plausible than mine you haven't exactly enhanced your own
reputation in the process.

---
Once again, I refer you to:

http://www.leachintl2.com/english/english2/vol6/properties/how4.htm

and, more particularly, to:

"Dry circuit loads: No current is switched. The contacts carry
current only after they are closed or before they are opened. The
currents may be high, as long as they are not switched. Since there
is no arcing, contact resistance is kept low by using gold plating
or gold alloy contacts."
---

Not that I'm not claiming that you are wrong all the time, but one
could probably get equally reliable advice by consulting a random
number generator. After all, a stopped clock is still right twice a
day.
---
Really?

http://www.grainger.com/Grainger/items/6NN63
---


That is easy to explain. An expert is someone who knows how little
they know. You don't know how little you know, and interpret
corrections as personal attacks, rather than educational episodes.
Thus one more correction becomes one more attack from someone whom -
you chose to believe - hates you, rather than one more misconception
erased from your personal data bank (which probably ought to be
transcribed and sent to some kind of museum of primitive engineering,
for exhibition with the stone axes and the electronic circuits built
around the NE555).

---
LOL, that from someone with an axe to grind who would rather spend
tens of dollars on a non-555 solution to a trivial problem perfectly
suited for a 555!
---

You never learn, mores the pity. Obviously, you used to be able to
learn, but you seem to have started losing the capacity a few decades
ago and the process seems to have pretty much gone to completion.

---
LOL again, since I, at least, manage to stay fairly well abreast of
things and still function in the marketplace.

You, OTOH... Well, let's just leave it at that, shall we? No
reason to be unkind to the unemployed.

 

[Eeyore]

The switches I'm referring to are ones that would be suitable for the OP's application.

So far you have offered him no useful advice. Why do you prefer to bicker instead of
helping the OP ?

Graham

 

[Eeyore]

John Fields wrote:

"Dry switching" occurs when there is no voltage across or current
through the contacts and the action, one on the other, is strictly
mechanical.

Thank you for your opinion. It's not the only one.

Dry switching
The relay contact switches no or a very small electrical load (< 1mA, <100mV).
http://relays.tycoelectronics.com/lexicon/Contact.asp

It is any supply supplying more than about 10ma. closed circuit. Or applying more than a
few volts open circuit. Currents/voltages less than this are considered "dry switching"
because there is insufficient energy to punch through the oxide films common to for example
tin and silver contacts.
http://www.control.com/thread/1026231012


If you can offer an authoritative source that says dry switching switches *no current* I'd
be obliged. And in that case what do you call switching currents in the ~ 50uA to 10mA
range ? In any case, it's clear that term is widely used exactly as I stated.


Graham

 

[[email protected]]

Or so you'd like to think. You'd be better off accomodating yourself
to the idea that you too might have got it wrong, but developing that
level of self-confidence does seem to depend on having got it right
sometime or other.

In the second place, manufacturing orientation-insensitive
mercury-wetted reed relays reliably isn't the problem, having them
_perform_ reliably in any orientation seems to be.

No matter how you want to word it, my impression was that those people
who had claimed to be able to manufacture orientation-insensitive
mercury-wetted reed relays don't persist with the claim, which put me
off even trying the next offering when it came on the market.

In the first place, what's being discussed are contacts used in a
dry _circuit_, not the difference between mercury-wetted and non
mercury-wetted contacts.

No - the discussion had moved on to what the "dry" in the name "dry
reed relays" actually referred to.

I had though it referred to the capacity of the reed to functon in the
absence of significant wetting current, that is when switching
currents less than a milliamp. I wasn't able to find any evidence to
support this opinion, and the evidence I did find (and posted) claims
that "dry reed relays" were so called to distinguish them from
"mercury-wetted reed relays" which is equally plausible.

You seem to have got it into your head that the word "dry" in the name
"dry reed relay" is related to the use of the word "dry" in dry
contact switching, where the relays contacts are carrying no current
at all when opened, and see aren't sustaining any potential difference
when closed. You haven't advanced any evidence to suggest that this -
more rigorous interpretation - of the term "dry" in the name "dry reed
relay" has anything to do with dry switching. In 1932, when Bell Labs
developed the dry reed relay, the technology didn't exist to let them
operate relays as totally dry switches within the telephone switching
system.

Had you had your wits about you when you decided to use Google, you
might have searched for "dry circuit" and found, excerpted from the
National Association of Relay Manufacturers' handbook:

"Dry circuit loads: No current is switched. The contacts carry
current only after they are closed or before they are opened. The
currents may be high, as long as they are not switched. Since there
is no arcing, contact resistance is kept low by using gold plating
or gold alloy contacts."

at:

http://www.leachintl2.com/english/english2/vol6/properties/how4.htm

If you had the wit to understand how to construct an argument, you
would also have devoted some time to searching for some evidence to
connect the use of the word "dry" in this context to the use of the
word "dry" in the phrase "dry reed relay". Without that connection the
information you have adduced is irrelevant.

 

[[email protected]]

---
Once again, I refer you to:

http://www.leachintl2.com/english/english2/vol6/properties/how4.htm

and, more particularly, to:

"Dry circuit loads: No current is switched. The contacts carry
current only after they are closed or before they are opened. The
currents may be high, as long as they are not switched. Since there
is no arcing, contact resistance is kept low by using gold plating
or gold alloy contacts."
---


---
Really?

http://www.grainger.com/Grainger/items/6NN63
---

Nice try. Ever seen one in real life?

---
LOL, that from someone with an axe to grind who would rather spend
tens of dollars on a non-555 solution to a trivial problem perfectly
suited for a 555!
---

To a man with a hammer, everything looks like a nail.

Find the right customers, and you can make a living casting
horoscopes. If you are sufficiently ill-informed you can even think
that casting horoscopes is doing something useful for the customer,
rather than defrauding them.

You, OTOH... Well, let's just leave it at that, shall we? No
reason to be unkind to the unemployed.

I suppose that I too could make a living selling obsolete technology
to the unsophisticated, but since I happen know something about
current developments in electronic technology I'd have to tell them
that they could do better elsewhere.

 

[John Fields]

Nice try. Ever seen one in real life?

---
Sure. A couple of times a day, at least.

Here's the one in my shop:

---

To a man with a hammer, everything looks like a nail.

---
If that's true, then you haven't obviously learned which hammer goes
with which nail.
---

Find the right customers, and you can make a living casting
horoscopes.

---
Sure, but since that's not what I do, so what?

What I do is predict the future by designing.
---

If you are sufficiently ill-informed you can even think
that casting horoscopes is doing something useful for the customer,
rather than defrauding them.

 

[Rich Grise]

If you can offer an authoritative source that says dry switching switches *no current*

Hmm. If you're switching "NO CURRENT", how would you know? ;-)

Cheers!
Rich

 

[Michael A. Terrell]

Nice try. Ever seen one in real life?

Hundreds. maybe over 1000. They are quite common on military bases,
some government offices, radio & TV stations, not to mention at ham
radio stations. Places that someone like you would never be allowed to
visit.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[John Fields]

Hmm. If you're switching "NO CURRENT", how would you know? ;-)

 

[John Fields]

The switches I'm referring to are ones that would be suitable for the OP's application.

So far you have offered him no useful advice. Why do you prefer to bicker instead of
helping the OP ?

Calling you to task for the improper use of a technical term isn't
bickering, it's correcting you.

The OP received useful advice from others which didn't bear
parroting and, as it turned out, the switch he wanted to use was
perfectly adequate for the task so he left happy.

 

[John Fields]

John Fields wrote:



Thank you for your opinion. It's not the only one.

---
It's not an opinion, it's an agreed-upon definition of "Dry
switching" which you'll eventually have to come to terms with.
---

Dry switching
The relay contact switches no or a very small electrical load (< 1mA, <100mV).
http://relays.tycoelectronics.com/lexicon/Contact.asp
---
Wrong.
---

It is any supply supplying more than about 10ma. closed circuit. Or applying more than a
few volts open circuit. Currents/voltages less than this are considered "dry switching"
because there is insufficient energy to punch through the oxide films common to for example
tin and silver contacts.
http://www.control.com/thread/1026231012

 

[[email protected]]

Hundreds. maybe over 1000. They are quite common on military bases,
some government offices, radio & TV stations, not to mention at ham
radio stations. Places that someone like you would never be allowed to
visit.

Didn't notice any when I went to US Army ECOM at Fort Monmouth, New
Jersey back in 1970.

Not that that had much to do with real life.

 

[[email protected]]

Yes, I suppose you'd have to if you didn't want them banging on your
door looking for their money back, hey?

Can't say that I'd be worried about that myself.

And the sort of people who buy horoscopes and antiquated electronics
don't seem to realise that they have been ripped off. (or may be too
embarassed to admit it, which comes to the same thing).

 

[[email protected]]

Check with a linguist sometime. Unfortunately, meaning is defined by
wide-spread use. A number of useful words - "gay" is an example - have
been redefined in recent years, despite the fulminations of the usual
pedants.

Your absolute definition of "dry switching" as applying only to a
switch which is totally unloaded doesn't match what I've read in trade
journals and application notes. Idiosyncratic academics do tend to
favour absolute definitions as easier to teach and justify, even if
the concept so defined isn't much use in real life. Who brain-washed
you?

 

[John Fields]

Didn't notice any when I went to US Army ECOM at Fort Monmouth, New
Jersey back in 1970.

Not that that had much to do with real life.

 

[John Fields]

Check with a linguist sometime. Unfortunately, meaning is defined by
wide-spread use. A number of useful words - "gay" is an example - have
been redefined in recent years, despite the fulminations of the usual
pedants.

---
The meaning of 'gay' hasn't been redefined, it's been elaborated on.
---

Your absolute definition of "dry switching" as applying only to a
switch which is totally unloaded doesn't match what I've read in trade
journals and application notes.

---
Then you either misunderstood what was written, the authors of the
articles used "dry switching' incorrectly, or their editors (like
you) were ignorant of the true meaning of the term and let the error
slip through.
----

Idiosyncratic academics do tend to
favour absolute definitions as easier to teach and justify, even if
the concept so defined isn't much use in real life.

---
I suspect the term "dry switching" was coined and its meaning
defined because of the need to tersely describe a switched circuit
through which charge was allowed to begin or cease flowing only when
the "dry" switch was closed.

Just off the top of my head, that concept would find use in the real
world in ATE, where millions of switches going through millions of
switching cycles aren't forced to endure the contact erosion
inherent in hot switching, even in the inert atmosphere reed
switches enjoy.
---

Who brain-washed you?

---
Me? LOL, I'm not the one buying into your dumbing down technical
English terminology for the convenience of the uninformed, while you
seem to be the "idiosyncratic academic" selling it, so that your
errors can be blamed on them.

Nice try, but no seegar!

 

[Jim Thompson]

On Fri, 03 Aug 2007 09:21:21 -0500, John Fields

[snip][snip]

"Cowboys and outlaws, right guys and south-paws
Good dogs and all kinds of cats
Dirt roads and white lined, and all kinds of stop signs
I'll stand right here where I'm at
'Cause I wear my own kind of hat
There's two kinds of lovers, two kinds of brothers
Two kinds of babies to hold
There's two kinds of cherries, two kinds of fairies
Two kinds of mothers I'm told...............I'm told"

...Jim Thompson

 

[[email protected]]

Wrong. It used to mean something close to cheerful. Now it means
homosexual. That's a redefinition, not an elaboration

---
Then you either misunderstood what was written, the authors of the
articles used "dry switching' incorrectly, or their editors (like
you) were ignorant of the true meaning of the term and let the error
slip through.
----

The third option is - of course - that your idea of the true meaning
is idiosyncratic. It certainly doesn't seem to be a majority opinion
on this user group.

---
I suspect the term "dry switching" was coined and its meaning
defined because of the need to tersely describe a switched circuit
through which charge was allowed to begin or cease flowing only when
the "dry" switch was closed.

Just off the top of my head, that concept would find use in the real
world in ATE, where millions of switches going through millions of
switching cycles aren't forced to endure the contact erosion
inherent in hot switching, even in the inert atmosphere reed
switches enjoy.
---

Fine. The popular usage recognises that switching at low currents
doesn't do any more damage to the contact surface than simply bashing
the contact surfaces together in the absence of any current flow or
voltage difference.

I'm sure that there are other doctrinaire obsessives around who don't
know this, but you are a tiny minority.

---
Me? LOL, I'm not the one buying into your dumbing down technical
English terminology for the convenience of the uninformed, while you
seem to be the "idiosyncratic academic" selling it, so that your
errors can be blamed on them.

Nice try, but no seegar!

Your ill-informed opinion is noted. You may also note that I don't
teach at a university or or any other sort of academy, which is what
defines an academic. If you ever had an academic education, you should
have known this - but it is fairly obvious that any academic training
you might have had failed to stick.

 

[John Fields]

Wrong. It used to mean something close to cheerful. Now it means
homosexual. That's a redefinition, not an elaboration

---
Hmmm...

"The sun shines bright on my old Kentucky home
Tis summer, the darkies are homosexual"? That hardly seems right.
---

The third option is - of course - that your idea of the true meaning
is idiosyncratic. It certainly doesn't seem to be a majority opinion
on this user group.

---
That's only because they've mostly seen the term used incorrectly
and have seldom, if ever, seen that usage contested.

Unlike you, however, once presented with factual evidence (in terms
of the definition given by Leach from the NARM engineers handbook)
most of the members of this group seem capable of accepting the
correct definition and moving on with their lives instead of
clinging to preconceptions which they are loath to give up for fear
of having to admit to earlier ignorance.

In the same vein, as far as bowing to group opinion goes, I daresay
that 90% of this group considers the 555 to be an OK chip for some
applications, while you steadfastly reject the majority opinion in
favor of your oft-professed belief that the 555 is a POS.

So it seems your position is that no matter what, anyone who or any
group which disagrees with you is wrong. LOL, so much for being
able to learn, eh?
---

Fine. The popular usage recognises that switching at low currents
doesn't do any more damage to the contact surface than simply bashing
the contact surfaces together in the absence of any current flow or
voltage difference.

---
The "popular usage" may recognize that, but it's false recognition.

If it were true, the extra expense involved in employing true dry
switching would make it impractical, yet we see it done all the time
in applications where contact erosion (metal transfer through a
plasma and the consequent degradation of the contact surface) must
be minimized.
---

I'm sure that there are other doctrinaire obsessives around who don't
know this, but you are a tiny minority.

---
Don't know what? That banging contacts around mechanically is as
damaging as banging them around mechanically while causing them to
switch current?

That's your daft claim, not mine, which clearly places you in the
ranks of the uninformed minority.
---

Your ill-informed opinion is noted. You may also note that I don't
teach at a university or or any other sort of academy,