SN55107A, SN75107A, SN75107B, SN75108A

DUAL LINE RECEIVERS

SLLS069D – JANUARY 1977 – REVISED APRIL 1998

15

POST OFFICE BOX 655303

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