122-28176 Parallax Inc, 122-28176 Datasheet - Page 152

• Manufacturer Part Number: 122-28176
• Description: GUIDE STUDENT PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 122-28176

• Accessory Type: Manual
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Propeller Education (PE) Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

“Duty cycle” compares the percentage of time the output is high, or on, to the total
period, or cycle time. If the period is 10 ms, and the output is high for 5 ms:
Figure 5-6a is a waveform with a 50% duty cycle. The average voltage supplied is 50%
of the maximum voltage, or 6 V in this case.
The fan will be running at 50% of rated speed, or around 4000 RPM.
Next, consider the duty cycle in Figure 5-6b. At what speed would the fan effectively be
running for this duty cycle? 25% rated speed or 2000 RPM.
Notice that the time of the wave did not change, only how long the output was high
during that time. The lower the duty cycle, the lower the average voltage. The higher the
duty cycle, the higher the average voltage.
When running the Fast On-Off Cycling challenge in Activity #1, did the LED appear to
dim with lower duty cycles? In fact, the LED did not dim. When the output is high the
LED is just as bright as it normally is. Your eye averaged out the light you were
receiving between on and off to make it appear dimmer. Try this – darken the room
while running the Fast On-Off Cycling challenge and wave the board up and down while
watching the LED. You should see the LED at normal brightness as a series of dots as it
blinks on and off while being moved.
Infrared LEDs used in TV remotes have a continuous current rating of 100 mA. This is
based on how much power the LED can safely handle due to heat dissipation. If a very
small duty cycle is used to drive the LED with a much higher current, the average power
will be very low but the LED will be much 'brighter' during the on-time. In fact, the
LEDs in TV remotes can handle 1 amp for very short bursts! This allows a strong signal
to reach the TV receiver without damaging the LED.
Duty Cycle = On-Time/Period = 5 ms/10 ms = 0.5 x 100 = 50%
V
avg
= V
max
x Duty Cycle = 12 V x 50% = 6 V