Page 233 - Robot Builder's Bonanza

202 ROBOT POWER SYSTEMS

• Zener diodes clamp the voltage to a specific level and won’t let it get any higher.
• Linear voltage regulators, the most common variety, are cheap but relatively inefficient.
In effect, they “step down” voltage from one level to another; the difference in voltage is
dissipated as heat.
• Switching voltage regulators are more efficient— some boast up to 80 percent. They’re
recommended over linear voltage regulators, but they may require more external compo-
nents to implement in your designs. Many switching regulators can increase voltage— boost
3 volts to 5 volts, for example— as well as produce negative voltages from a positive volt-
age source.
• Modular DC- DC converters are self- contained voltage changers. Internally they use one
or more switching regulators, and they also include all the additional components required.
They’re more expensive.

DROPPING VOLTAGE WITH SILICON DIODES

Diodes are the simplest of all semiconductors. A common use of diodes is to prevent current
from flowing a certain direction in a circuit. Current will flow only in the “allowed” direction,
but in doing this, there’s an inherent drop of voltage through the diode. For silicon diodes, the
cheapest and most plentiful diode type, the voltage is reduced by about 0.7 volts.

Diodes don’t really “regulate” voltage; they only drop it by approximate amounts. Not all
circuits need exact voltage regulation, but if yours does, one of the other methods is
G recommended instead.
What’s more, the actual voltage drop needs to be be measured when the diode is connected
in line with the circuit. The drop increases as the current load increases. The highest drop is at
the rated current for the diode.

You can use this effect to easily drop a voltage in increments of about 0.7 volts. Using one
diode, a 6- volt power supply is reduced to about 5.3 volts (see Figure 19- 9). That’s close
enough to the +5 volts needed by most electronics. Using two diodes drops it down even
further, to around 4.6 volts, and so on.

5.3v
Diode

+6 vdc To circuit

4.6v
Diodes
Figure 19- 9 Diodes
can be used to reduce
+6 vdc To circuit the voltage to a circuit.
Assuming silicon diodes,
the voltage drop is about
0.7 volts. Combine diodes
Note: The actual voltage drop
measured under load of circuit. for higher voltage drops.

19-chapter-19.indd 202 4/21/11 11:49 AM