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% .