Telecom relay coil polarity

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Bamboozled for a while by a Telecom relay from Altronics, S4130B (12VDC
coil). Turns out that the coil has a polarity, the diagram in the catalogue
has a small + on one side of the coil. Can anyone explain why? The google
refs I've checked don't have an explanation.

It isn't a suppression diode across the coil, because the resistance
measures the same either way. If it's powered the wrong way round, it just
doesn't switch.



Re: Telecom relay coil polarity
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It has to be a suppression diode across the coil then, and also the
reason the resistance
measures the same each way is likely because the coil resistance would
be lower than that of the conducting diode ?


If you were to put a power source of a few volts and low current on it
(say 2x 1.5v batteries in series) and try it in
both directions, one way will result in a voltage across the coil of
about 0.6v (diode) and the other
it probably won't change that much, depending on the resistance of the
coil.

(The full 12v might not be a good idea, as the diode will conduct in
one
direction, and if the 12v supply delivers any sort of current (IE: SLA
battery) the diode might short out.)

It is also possible that there is an LED across the coil. I have seen
that with a batch of 4PDT cradle relays,
that also had the terminal polarity markings marked opposite to what
they actually are.
The LED was described as a diode in the literature, so that was a 3rd
way to get caught, as I have my doubts
as to how well an LED would suppress back EMF.

An external diode was soon added.  The LED was hard to spot as it was
green, moulded into the internal
plastic support, and very dim when lit.


Re: Telecom relay coil polarity

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It has to be a suppression diode across the coil then, and also the
reason the resistance
measures the same each way is likely because the coil resistance would
be lower than that of the conducting diode ?

Nuh, R is identical either way (about 1030 ohms), and current draw from a
12V supply is also the same either way.

If you were to put a power source of a few volts and low current on it
(say 2x 1.5v batteries in series) and try it in
both directions, one way will result in a voltage across the coil of
about 0.6v (diode) and the other
it probably won't change that much, depending on the resistance of the
coil.

(The full 12v might not be a good idea, as the diode will conduct in
one
direction, and if the 12v supply delivers any sort of current (IE: SLA
battery) the diode might short out.)

It is also possible that there is an LED across the coil. I have seen
that with a batch of 4PDT cradle relays,
that also had the terminal polarity markings marked opposite to what
they actually are.
The LED was described as a diode in the literature, so that was a 3rd
way to get caught, as I have my doubts
as to how well an LED would suppress back EMF.

An external diode was soon added.  The LED was hard to spot as it was
green, moulded into the internal
plastic support, and very dim when lit.



Re: Telecom relay coil polarity

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Nuh, R is identical either way (about 1030 ohms), and current draw
from a 12V supply is also the same either way.

The mutimeter does not bias the diode.


Re: Telecom relay coil polarity

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** DMMs have been designed that way since the first available models I can
remember.

A blessing and curse at the same time.

Means that service techs ( like me) need to still keep at least one
functioning analogue multimeter on hand for "in circuit" testing of diodes
and BJT junctions.



....  Phil

 



Re: Telecom relay coil polarity
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I still my first brown leather covered dvm  issued when I joined well
before krone tools existed
  I still bear scars from  winding those bastard metal posts

--
X-No-Archive: Yes


Re: Telecom relay coil polarity

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I wonder if these relays have a magnetic/magnetised armature.

Typical data sheet does not give too many clues (to me anyway!)

http://www.image.micros.com.pl/_dane_techniczne_auto/p%20p12.pdf



Re: Telecom relay coil polarity

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Hints of it here:

http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t60%991&view=next



Re: Telecom relay coil polarity

"Bruce Varley"
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** The spec sheet is here:

http://www.altronics.com.au/download/ndb/specifications/S/S4128B.pdf

From the very low release voltage ( 1.2 volts) the relay appears to be
pre-biased by a permanent magnet.  The low operating power of 140mW suggests
this too.

Beware of the spec in the Altronics cat that indicate the relay can switch
220 VDC and 250VAC at 1 amp.

The real specs are 30VDC at 1 amp and 125VAC at 0.5 amp.

At 220VDC, the switching capacity is probably only a few mA.


.....   Phil



Re: Telecom relay coil polarity

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Thanks for the caution Phil. I for one wouldn't be brave enough to put
anything near 250 on those relays. The pic in the catalogue doesn't do
justice to how small they really are, just a bit wider than a DIP, with 0.1"
spacing on the pins and with the contacts all adjacent and no excess spacing
between the coil and the contact pins. Surely prudent design practice
totally excludes 250V with something like this.


 



Re: Telecom relay coil polarity

"Bruce Varley"
 "Phil Allison"
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** Prudent design went out with the last load of bath water.

I am still fixing Perreaux mosfet amplifiers ( PMF 6000, 8000 and 9000
series ) that were designed with parallel traces separated by as little as
0.25mm and with 220 volts DC between them. The PCB is not coated in any way
and gets grime, dust and moisture blown all over it by a un-filtered fan.

Once the inevitable " tracking " begins between the parallel traces  -
expensive mosfets and fuses blow, big time.

The Chinese are famous for running traces in between the rows of
opto-isolators with the AC mains supply on one side too.


....  Phil





Re: Telecom relay coil polarity

:Bamboozled for a while by a Telecom relay from Altronics, S4130B (12VDC
:coil). Turns out that the coil has a polarity, the diagram in the catalogue
:has a small + on one side of the coil. Can anyone explain why? The google
:refs I've checked don't have an explanation.
:
:It isn't a suppression diode across the coil, because the resistance
:measures the same either way. If it's powered the wrong way round, it just
:doesn't switch.
:

As Phil has suggested this relay is polarised by a permanent magnet. With relays
designated as signal for telecommunications purposes, they must withstand shock
and vibration. The polarisation magnet ensures that contacts remain "in contact"
under such conditions.

A typical high quality telecommunications relay which has been around since the
1980's is the Panasonic DS. Note the reference to the magnetic polarisation in
the list of features.
http://pewa.panasonic.com/assets/pcsd/catalog/ds-catalog.pdf

Re: Telecom relay coil polarity
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Thank you for that.  I never knew of this before :).

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