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.
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.
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.
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.
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.
: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.
** 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.
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
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