28128 Parallax Inc, 28128 Datasheet - Page 134

• Manufacturer Part Number: 28128
• Description: KIT BASIC ANALOG/DIGI PARTS ONLY
• Manufacturer: Parallax Inc

Specifications of 28128

• Accessory Type: ADC, DAC
• Product: Microcontroller Accessories
• For Use With/related Products: BASIC Stamp® 2 and Board of Education
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

You could also disconnect the battery from the capacitor, and it would maintain its
voltage. Typically, there is a small amount of current that does make it through the
dielectric material that separates the plates. It's called leakage current. Leakage current on
a charged capacitor will cause the voltage to slowly dissipate.
The RC circuit shown can charge to within 1% of 5 volts very quickly. Assume that a
single pulse is applied to the circuit in Figure 7-1 instead of the many pulses sent via a
PWM signal. If the values of the components are exact, it would take about 6.93
milliseconds for the capacitor to charge up to 2.5 volts, and would take 46.1 milliseconds
for the output to get all the way to 4.95 volts, which is 99% of the full 5 volts.
For advanced math enthusiasts, let's take a look at the proof. The modeling equation for
the RC circuit from Figure 7-1 responding to a pulse that goes from 0 to 5 volts is:
The modeling equation is rearranged to isolate the exponential term:
The natural log of both sides is taken to remove the exponential term, and the terms are
rearranged:
1
V
−
Output
+
_
V
V
Output
Input
=
V
Input
=
e
R
×
−
×
t
C



1
−
e
R
−
×
t
C



Figure 7-2
Parallel Plate Capacitor Charged by a
Battery
The opposed charges congregate at
their respective plates. They want to
get across the gap between the two
metal plates, but there is an insulating
material called a dielectric in-between
the plates, so they can't get across.