reed switch fails........open or closed?

[Lee Carkenord]

When a reed switch fails, does it usually go into an "open" condition
at time of failure, or does most often show a "closed" condition at
time of failure, and there-after?

Thank you..... Lee Carkenord

 

[Winfield Hill]

Lee Carkenord wrote...

When a reed switch fails, does it usually go into an "open" condition
at time of failure, or does most often show a "closed" condition at
time of failure, and there-after?

Q. Reed switch fails........open or closed? A. Yes, correct.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[Ken Smith]

When a reed switch fails, does it usually go into an "open" condition
at time of failure, or does most often show a "closed" condition at
time of failure, and there-after?

It depends a lot on the circuit it is used in.

It is very rare for the coil in a reed relay to go bad if it is not
abused but it can happen. This results in the always open case.

If the contacts are used for small signals only, they can fail as a higher
than specified resistance. This is a neither open nor closed failure.

At higher currents, the contacts can stick in the closed condition.

 

[Ken Smith]

Lee Carkenord wrote...

Q. Reed switch fails........open or closed? A. Yes, correct.

No, actually in my experience they can also fail betwix the two. The
contact resistance can exceed the spec.

 

[John Fields]

When a reed switch fails, does it usually go into an "open" condition
at time of failure, or does most often show a "closed" condition at
time of failure, and there-after?

---
Since the attraction between the contacts in a normally open
non-biased reed switch is magnetic and they want to close the gap
between them when in the presence of a magnetic field, it's unlikely
that they'll fail open because the only thing that's keeping them
apart is the springiness of the moving contact. OTOH, If the contacts
weld because they're improperly hot-switched, then when the magnetic
field is removed the springiness of the moving contact won't be great
enough to break the weld and the contacts will stay made.

However, if it's a reed _relay_, the coil could burn open, in which
case the failure would be and open, unmakeable relay.

So, as usual, it depends...

 

[Tony Williams]

When a reed switch fails, does it usually go into an "open"
condition at time of failure, or does most often show a "closed"
condition at time of failure, and there-after?

I've used quite a few reed relays. The most frequent
initial sign of failure is that it goes 'sticky', ie,
slow to open when the power is removed, progressing
to staying closed permanently.

A good way to extend the life of reed contacts is
to use a series resistor (at least a few kohms) wired
as close to the capsule as possible. The resistor
limits the current the contacts have to handle and
the close mounting minimises the capacitive current
at each switch closure.

 

[Terry Pinnell]

When a reed switch fails, does it usually go into an "open" condition
at time of failure, or does most often show a "closed" condition at
time of failure, and there-after?

Thank you..... Lee Carkenord

See also my recent thread:
Newsgroups: sci.electronics.design
Subject: Reed contacts sticking
Date: Tue, 08 Jun 2004 20:42:04 +0100
Message-ID: <[email protected]>

 

[Ian Stirling]

It depends a lot on the circuit it is used in.

It is very rare for the coil in a reed relay to go bad if it is not
abused but it can happen. This results in the always open case.

If the contacts are used for small signals only, they can fail as a higher
than specified resistance. This is a neither open nor closed failure.

At higher currents, the contacts can stick in the closed condition.

And at even higher currents, you can melt off the ends.
But you'r not supposed to use reed relays like that...

 

[Terry Given]

I've used quite a few reed relays. The most frequent
initial sign of failure is that it goes 'sticky', ie,
slow to open when the power is removed, progressing
to staying closed permanently.

A good way to extend the life of reed contacts is
to use a series resistor (at least a few kohms) wired
as close to the capsule as possible. The resistor
limits the current the contacts have to handle and
the close mounting minimises the capacitive current
at each switch closure.

I just did a design incorporating a little motion sensor. I likewise used a
resistor to limit peak current to about 1/2 the rated value, and got
harangued for doing so. When I pointed out the desired lifetime, along with
the 100nF cap across the switch, the lone haranguer was silenced (I have
been waiting years to use that phrase

Cheers
Terry