28176 Parallax Inc, 28176 Datasheet - Page 239

• Manufacturer Part Number: 28176
• Description: KIT PARTS PROCESS CONTROL W/TEXT
• Manufacturer: Parallax Inc

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

Chapter 7: Closed Loop Process Control · Page 229
The slow-moving data that is cycling through the setpoint has a high-frequency noise
component riding on it. As you can see, the coupled effects of the noise result in the data
passing above and below the setpoint several times. The microcontroller would attempt to
turn the heating element on and off accordingly. In an actual incubator application where
larger amounts of power are controlled, this rapid switching could cause unwanted RF
noise. This rapid cycling could also be damaging to electromechanical output elements
such as motors, relays, and solenoids. Do you observe the rapid cycling of the LED as
the temperature approaches the setpoint?
You may note the temperature fluctuating slightly as the heater cycles as shown in Figure
7-5. The heater draws significant current, dropping the supply voltage. This in turns
affects the PWM voltages controlling the ADC. As the heater cycles on and off, the data
from the ADC representing temperature varies slightly. This is another form of noise
caused by cycling of high-current loads.
Figure 7-5 Temperature Cycling Around Setpoint with On-Off Control