130-28176 Parallax Inc, 130-28176 Datasheet - Page 151

• Manufacturer Part Number: 130-28176
• Description: KIT PARTS PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 130-28176

• Accessory Type: Education Kit
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Board of Education Full Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

Challenge 5-1: Fast On-Off Cycling
What happens to the fan's speed and to the brightness of the LED? Can you determine
why it has these effects? This is the subject of Activity #2, Pulse Width Modulation.
ACTIVITY #2: PULSE WIDTH MODULATION
In the previous activity’s Fast On-Off Cycling challenge, the ON time and OFF time
were very short. What effect did it have on the fan and the LED? The fan ran at a slower
speed and the LED was dimmer. This activity explores Pulse Width Modulation (PWM)
in the control of devices.
Parts Required
Same circuit as Activity #1, using the BS170 MOSFET transisitor.
When the fan is running at 12 V, it draws 90 mA of current. This equates to a power
draw of 1.08 watts:
When the fan is off, it draws 0 watts. Over a short period of time, if the fan is on half the
time, and off half the time, on average how much power is the fan able to draw? It’s able
to draw one-half of the maximum power, or 0.54 watt. The fan will run at half speed if it
only receives one-half the power it requires.
Another way to look at it is in terms of voltage. If a DC motor is effectively supplied
only half the normal voltage, it will run at approximately one-half full voltage speed.
The average voltage over time is one-half the maximum when the output is high only
50% of the time.
√
√
√
Save the program FanOnOffControl.bs2 under a new name.
Change the
fast is the fan running compared to previously?
Test other values where the on and off times add up to 10, such as
and
12 V
OffTime
DC
x 90 mA = 1.08 W.
OnTime
at 3. Note: At low values of
and
OffTime
to 5 (5 ms) each and run the program. How
OnTime
(< 4) the fan may not run.
OnTime
at 7