122-28176 Parallax Inc, 122-28176 Datasheet - Page 167

• 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

Does the voltage eventually start to decrease?
After the PWM has been applied, the BASIC Stamp output is high impedance, limiting
the loss of electrons on the charged capacitor. The charge is also lost through the ADC
sampling the voltage and leakage of the capacitor. Both of these are minimal because of
the very high impedance of the ADC input and the dielectric, or insulation, used in the
capacitor.
Can this average voltage be used to drive the fan even when the BASIC Stamp is busy
performing other tasks? Unfortunately not. The high current draw (90 mA) will quickly
bleed the charge off the capacitor. Let's test this using a 1 MΩ load, which at 5 V would
have a current draw of 0.005 mA or 5 µA – MUCH less than the current draw of the fan.
What occurs to the voltage with R
decreases as shown in Figure 5-11. The rate at which the voltage decreases can be
calculated. The time-constant (τ) of an RC network is approximately 1.1 times the
resistor value and the capacitor value or:
In this case,
While we won't get much deeper into the math, the charge and discharge of a capacitor
requires 5 time constants (5 τ) for the capacitor to nearly fully charge or discharge.
For this circuit:
√
√
Connect the 1 MΩ resistor, R
Repeat the stepped PWM test with a sample number of 25.
τ = 1.1 RC
τ = 1.1(1 MΩ)(0.68 µF) = 0.748 seconds.
5 τ = 5 (0.748 s) = 3.74 seconds.
L
L
, as shown earlier in Figure 5-11.
in place? The voltage on the capacitor rapidly