SN55107A, SN75107A, SN75107B, SN75108A
DUAL LINE RECEIVERS
SLLS069D – JANUARY 1977 – REVISED APRIL 1998
15
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
furnace control using the SN75108A
The furnace control circuit in Figure 18 is an example of the possible use of the SN75108A series in areas other
than what would normally be considered electronic systems. A description of the operation of this control
follows. When the room temperature is below the desired level, the resistance of the room temperature sensor
is high and channel 1 noninverting input is below (less positive than) the reference level set on the input
differential amplifier. This situation causes a low output, operating the heat-on relay and turning on the heat.
The channel 2 noninverting input is below the reference level when the bonnet temperature of the furnace
reaches the desired level. This causes a low output, thus operating the blower relay. Normally the furnace is
shut down when the room temperature reaches the desired level and the channel 1 output goes high, turning
the heat off. The blower remains on as long as the bonnet temperature is high, even after the heat-on relay is
off. There is also a safety switch in the bonnet that shuts down the furnace if the temperature there exceeds
desired limits. The types of temperature-sensing devices and bias-resistor values used are determined by the
particular operating conditions encountered.
1 Y
+ T
Room
Temp.
Sensor
– T
Blower on Control
Room
Temp.
Setting
A
B
2A
2B
2 Y
To Heat-on
Relay Return
To Blower
Relay Return
Bonnet Upper
Limit Switch
Channel 1
Channel 2
Bonnet
Temp.
Sensor
5 V
Figure 18. Furnace Control Using SN75108A
