Relay Throw Question

Dear All,

I have a circuit with a 12V, SPDT relay (250V 10A) and the load is ac
resistive, 220V, 8A.
When I apply 12V to the coils from a dc power supply, I can hear that
the relay gives a "click" sound, but when I measure the proper output
legs, I found that the relay actually didn't throw (i.e. opens the NC
contact and makes the NO contact closed).

I tried several times and suddenly the relay "click" sound became much
louder and then it worked properly.

Any clues? Poor contact or anything else?

Thank you very much!
Billy

 

The contacts may have been oxidized. By operating the relay a few
times, you may have "scrubbed" the contacts clean, by the normal wiping
action.

Another possibility could be that some soft object was stuck in between
the pole and the armature and was loosened by operating the relay a few
times.

It still may be wise to remove the cover and clean the relay contacts.

Tom

 

Thank you very much!

Somehow the relay is a sealed one and so I cannot easily remove the
cover.

Is it true that if my load is a bit inductive (e.g. cos theta = 0.9),
then without a load snubber (R-C), the relay contacts may have higher
oxidation/weld possibility? How do I know when I should add a snubber?

Thanks.
Billy

 

Are you sure that the 12V power supply is working properly and is able to
supply enough current to drive the relay coil?

Personally, I don't agree with Tom's suggestion that the contacts may have
been oxidized: if that were the case, the NC contacts would have opened,
even if the NO contacts didn't close. From your post I *think* you're
saying that the NC contacts stayed closed. And anyway, I've never heard of
oxidized contacts changing the sound of a relay.

But Tom's suggestion of some soft obstruction (or perhaps a small piece of
grit in the works somewhere) is very plausible. That's probably where I'd
bet my money. And if it's a sealed relay, I wonder if that piece of
material is still inside, waiting to cause problems again in the future.

 

Sounds like your relay was stuck open and then broke free.
Could be stuck NC contacts or an armature cocked due to physical abuse.
Tom

 

Walter Harley 寫�：

Yes Yes.. the NC still stays NC... can other be any possibilities? The
12V is clean and the current agrees with the datasheet.

Somehow from around 50 relays I have, few of them really have this kind
of "random stuck". The phenomenon sometimes occurred and somehow I
"activated" it again then the problem went away. But maybe of course
it will occur again in the future, I don't know.

Any other possibilities? Thanks!
Billy

 

Sorry, what does "stuck open" mean? In my case the NC stays NC and the
NO stays NO, somehow it sounds like the relay throws but bounced back
(but the 12V was still applied there!)

Thanks.
BIlly

 

How long had it sat without operating before this happened? There could
be some kind of lube grease on the armature that gets gummy with time,
and you loosened it up. Or if they're like that out of the box, it could
be crappy manufacturing.

But it definitely sounds mechanical in nature.

Good Luck!
Rich

 

It means that.

Cheers!
Rich

 

It means, that the contacts have been fused together.

 

if a used relay possibly the NC contacts were welded. and exercising it
unstuck them.

Bye.
Jasen

 

If it is mechanical... then what can I do about it? Actually is it
common to drive a 12V relay with, say, 15V to ensure proper driving?

Thanks.
Billy

 

Then, to avoid the same problem occuring again, should I add R-C
snubber to the relay, especially when I use it with inductive loads?

Thanks.
Billy

 

No. A 12V relay should be operated, ideally, by 12V. If you look at the
datasheets for some relays you'll see that they are specified in terms of a
number of voltages, including a max rated voltage (more than this will fry
the coil), and a minimum pull-in voltage (it takes at least this much to
make the relay turn on). A 12V relay might have, say, 9V pull-in and be
rated for up to 18V maximum. There's also a drop-out voltage; as long as
you maintain the coil voltage above the dropout voltage, after the relay is
energized, it will stay energized. The dropout voltage is usually a bit
less than the pull-in voltage.

If you've got 50 relays that claim to be 12V relays, and a handful of those
are not reliably activated by 12V, then what you have is a defective batch
of relays. If you purchased these from a manufacturer or legitimate
distributor, you should be contacting their technical support for
guaranteed-good replacements. If you purchased them from a surplus
supplier, or over eBay or some such... well, now you know why you got such a
good deal on them.

 

No. Relays generally will operate below their rated voltage.
It is not usual to overdrive them to ensure they transfer.
What you want to do is ensure that the path from the supply
to the relay coil does not reduce the current below what the
relay needs.

As far as old relays that may be sticking mechanically, you
can rig up a 555 with a driver transistor to "exercise"
them. Set the timer for something like 1 second on, one second
off. Let the circuit operate the relay for a few hours.
That may help to overcome mechanical binding.

+12 --------------------+-----+
| |k
[Rly] [Diode]
| |
+-----+
|
555 /c
Timer pin3 ---[470]---| PN2222A
\e
|
Ground

The transistor is not absolutely mandatory, as
many relays can be driven directly by the 555,
but it's worth adding. The diode is mandatory.

Ed

 

Yes, but this is a different issue than your original question. I
had the impression that the relay was just stuck, not fused.

Thanks,
Rich

 

Thank you... then I will keep the 12V. Let me try to find more
information from my supplier.

Thanks.
Billy

 

How do I determine which situation I have now?

Thanks.
Billy

 

I have no idea, other than to open the relay and look at it.
Is it really "sealed", or is it one of those ones with the
plastic case that you need two little screwdrivers to get
off?

Thanks,
Rich