Page 234 - Robot Builder's Bonanza

REGULATING VOLTAGE 203

When selecting a diode for dropping voltage you need to also consider the current con-
sumed by the circuit. Diodes are rated in amperes; pick a diode that can handle the current
drawn by your circuit. The very common 1N4001 silicon diode can handle up to 1 amp, at
50 volts. The 1N5401 silicon diode handles up to 3 amps at 100 volts.

These types of diodes are often called rectifiers. Not all rectifier diodes are silicon. When in
doubt, check the specs for the diode. Look at the cut- in or forward voltage drop specification.
G The voltage drop in most silicon diodes is about 0.7 volts; for Schottky and germanium diodes
it’s around 0.3 volts. LEDs, a type of diode, exhibit even higher forward voltage drops. But these
generally cannot handle much current, so they’re impractical as circuit voltage droppers.

ZENER DIODE VOLTAGE REGULATION
A quick and inexpensive method for providing a semiregulated voltage is to use zener diodes.
A typical hookup diagram is shown in Figure 19- 10. You can use zener regulation for circuits
that don’t consume a lot of power— say, under an amp or two.
With a zener diode, current does not begin to flow through the device until the voltage
exceeds a certain level. This level is called the breakdown voltage. Any voltage over this level
is then “shunted” through the zener diode, effectively limiting the voltage to the rest of the
circuit. Zener diodes are available in a variety of voltages, such as 3.3 volts, 5.1 volts, 6.2
volts, and others. A 5.1 zener is well suited for use on circuits needing a +5 volt supply.
Zener diodes are rated by their tolerance—1 percent and 5 percent are common. If you
need tighter regulation, get the 1 percent kind.
They’re also specified by their power rating, expressed in watts. For low- current applica-
tions, a 0.25- or 0.5- watt zener should be sufficient; higher currents require larger 1- , 5- , and
even 10- watt zeners. Note the resistor shown in Figure 19- 10. It limits the current through the
zener.
To calculate the value of this resistor, you need to know the maximum current draw of your
circuit. You then do a bit of math:

1. Calculate the difference between the input voltage and the voltage rating of the zener
diode. For example, suppose the input voltage is 7.2 volts, and you want to use a
5.1 volt zener:
7.2 5.1 = 2.1 volts

2. Determine the current draw of your circuit. You want to add an overhead margin of
about 200 percent. If, for example, the circuit draws 100 mA (milliamps), then

0.1 2 = 0.2
In the preceding equation, 0.1 is 100 milliamps.

3. Determine the value of the resistor by dividing the current draw by the dropped- down
voltage:

2.1 / 0.2 = 10.5 ohms

19-chapter-19.indd 203 4/21/11 11:49 AM