| 1.0 |
Coil Resistance is normally specified in Ohms at +25°C. Almost all electromagnetic
relay coils are wound with copper magnet wire to a specified number of turns. It is the ampere turns, the product
of the magnet wire turns and the current that determines the sensitivity and operating characteristics of a relay.
Since coil resistance is proportional to the absolute temperature for copper windings, the following ration applies:
| Ro |
= |
(234.5 + To) |
|
|
|
| R1 |
(234.5 + T1) |
The 234.5 constant is the implied point of zero resistance on the Celsius scale.
"R" is the coil resistance at the applicable temperature "T" in degrees Celsius. As an example,
for a M83536/9 relay with a 28V dc coil, the coil resistance "Ro" is 320 Ohms at "To" equal
to +25°C. At "T1" equal to +125°C, the coil resistance "R1" would be:
| R1 |
= |
R0(234.5 + 125) |
= |
320(359.5) |
|
|
|
|
(234.5 + 25)
|
259.5
|
| |
|
|
|
|
| R1 |
= |
443.3 Ohms |
|
|
|
| 2.0 |
Since the current required to perform a particular function (pick-up/pull-in current,
drop-out current) remains constant, assuming no instability, the power required is proportional to the absolute
temperature. The ampere-turn sensitivity, assuming no instability, is unaffected by temperature.
While the pull-in (pick-up) and drop-out current remains relatively constant, the pull-in (pick-up) and drop-out
voltage will vary directly with temperature as follows:
| Eo |
= |
(234.5 + To) |
|
|
|
| E1 |
(234.5 + T1) |
The pull-in (pick-up) voltage "Eo" for the M83536/9 relay is 13.0 V
dc at "To" equal to +25°C. The pull-in (pick-up) voltage "E1" at +125°C is as follows:
| E1 |
= |
E0(234.5 + 125) |
= |
13(359) |
|
|
|
|
(234.5 + 25)
|
259.5
|
| |
|
|
|
|
| E1 |
= |
18 Vdc |
|
|
|
