28176 Parallax Inc, 28176 Datasheet - Page 240
28176
Manufacturer Part Number
28176
Description
KIT PARTS PROCESS CONTROL W/TEXT
Manufacturer
Parallax Inc
Datasheet
1.122-28176.pdf
(330 pages)
Specifications of 28176
Accessory Type
Process Control
Product
Microcontroller Accessories
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
For Use With/related Products
BASIC Stamp® 2 and Board of Education
Lead Free Status / RoHS Status
Lead free / RoHS Compliant, Contains lead / RoHS non-compliant
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Additional Information
The op-amp can be employed here to control an on-off system without the expense of a
microcontroller, ADC, and all the additional hardware. While we won't ask you tear
down your circuit for this, you may wish to try it at a later time. Consider the schematic
in Figure 7-6.
Recall the output of the op-amp attempts to drive the inverting input (−) towards the non-
inverting input (+). In this configuration, if the temperature is lower than the setpoint
(1.015 V = 101.5 °F), the op-amp output will go high in an attempt to drive the inverting
input down to the non-inverting. But the output is NOT connected to the inverting input,
so the op-amp will drive fully to the positive supply-voltage rail in a futile attempt at
affecting the inverting terminal. This causes the transistor to go into saturation, turning
on the heater.
Once temperature rises above the setpoint, the op-amp will drive to the other rail voltage
(ground/Vss) in an attempt to lower the inverting input towards the non-inverting input.
This will have the effect of turning off the heater. Thus, the heater will cycle around the
setpoint.
This op-amp configuration is called a comparator because its output is based on a
comparison of the two inputs. A digital signal output is based on comparing two analog
voltages.
Figure 7-6
Op-Amp On-Off
Control Circuit
DO NOT BUILD
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