Electromechanical latching relay pull-in voltage_Technical issue under different environments

[vinodquilon]

We are using MIL standard Latching Electro-mechanical relays having dual ON/OFF coils. During screening process the relays are being subjected to pull-in voltage measurements for both ON & OFF coils in SRC (25 degree Celsius),COLD (8 degree Celsius) & HOT (80 degree Celsius) conditions. For pull-in measurement, the coil voltage starts at 0V and gradually increasing to 28V in continuous manner. Pull-in will occur in 6-16V range. Status monitoring (Open/Close) is done at relay poles.

The anomaly is that the pull-in voltage measured at Hot is showing values lesser than Cold and Cold value is higher than SRC. This observation is against the normal intuition of pull-in voltage HOT>SRC>COLD.
For example, The pull-in of RL-1 ON coil shows 11.496V (ISRC), 15.042V (Cold), 17.719V (Hot). That is SRC reading is 3.5V lesser than Cold.Also the difference voltage (3.5V) shows inconsistency under repeated tests of the same part.The curreny limit of 30V source is set at maximum coil current of relay as per data sheet.
The relay specifications are 450 Ohm coil, 15A/28V DC contact, 30V DC coil/500mA.

The deviation is observed only for 0.2% of total relays screened per batch.All others are operating well.

I hope for feedback.

 

[Externet]

Posted feedback in another forum.

 

[Reloadron]

I have to agree it makes no sense. Just as an example:

The copper wire used to wind reed relay coils increases its resistance
by 0.4% for every 1oC rise in temperature. Reed relays are currentsensitive
devices: their operate and release levels are based on the
current input to the coil. If a voltage source is used to drive the relays,
an increase in coil resistance causes less current to flow through
the coil. The voltage must be increased to compensate and maintain
current flow. From a voltage perspective, the relay has become less
sensitive. Industry standards define that relays are typically specified
at 25oC ambient, unless otherwise specifically defined in advance by
the user. If the relay will be used under higher ambient conditions
or near external sources of heat, this must be carefully considered.
Sometimes standard relay designs have to be customized to accommodate
high ambient temperature conditions.
Consider for example that a standard relay nominally rated at 5
VDC may have a 3.8 VDC maximum operate value at 25oC as
allowed by the specifications. If the relay is to be used in a 75oC
environment however, the 50 degree temperature rise increases the
operate voltage by 50 x .4%, or 20%. Thus the relay will be observed
to operate at 3.8 VDC + (3.8 VDC x 20%), or 4.56VDC. If there is
more than a 0.5 VDC drop in supply voltage due to a device driver
or sagging power supply, the relay may not operate. If there is sufficient
voltage to drive the relay, it should be noted that there will be
increases in operate and release timing to approximately the same
20% .

While the focus is Reed Relay coils the theory should also be true of any magnetic activated relay using a copper wire for the coil. Keep in mind here the relay is actually a current activated device. The only thing I can suggest is taking not only voltage but also current readings on the relay coils of the relays that are going against the grain. I would also contact the relay manufacturer and see what they have to say about what is happening. Do these relays by any chance have an internal flyback diode across the coil?

The deviation is observed only for 0.2% of total relays screened per batch.All others are operating well.

Granted a very small percentage but the question still begs why?

Ron

 

[audioguru]

I hate seeing the same threads on different forums.

 

[vinodquilon]

I have to agree it makes no sense. Just as an example:



Do these relays by any chance have an internal flyback diode across the coil?



Granted a very small percentage but the question still begs why?

Ron

External fly back diodes we are providing across the coil.

 

[Externet]

I have to agree it makes no sense. ... ... but the question still begs why? Ron

Other components of the relay (as springs, unlatching friction) that are also affected by temperature are being ignored in the evaluation, focusing only on the copper coil.

 

[Paul_L]

What is the Mil-Spec number? and what is the relay part number? If you do not know the Mil Spec I can look it up.

 

[alec_t]

Magnetic permeability varies with temperature. According to this reference, "For low inductions the magnetizing force necessary to produce a certain induction decreases with increase in temperature while for high inductions it increases". So if the relay induction qualifies as "low", this effect could perhaps more than compensate for the coil resistance increase with temperature?

 

[KeepItSimpleStupid]

Another issue is friction. That may actually significantly decrease with heat.

A lot depends on the relay construction,

 

[vinodquilon]

What is the Mil-Spec number? and what is the relay part number? If you do not know the Mil Spec I can look it up.

M402J2AHM10 LEACH Relay