CHAPTER 18 ■ ADDING GEARHEAD MOTORS
Most diodes physically resemble resistors, but without the multiple color bands (see Figure 18-6).
There is only one band, located at the end of the diode, which indicates the cathode. The other end, the end
without the band, is the anode. Current can only flow from the anode (positive) to the cathode (negative).
You can think of the band as a wall that prevents current from entering from that direction.
Like LEDs, diodes consume or drop some voltage as power passes through them. The voltage drop of
most diodes is between 0.4 V and 1 V, as opposed to 1.2 V or more for LEDs. In exchange for this voltage cost,
a diode acts as a one-way valve (a check valve).
Protecting the Transistor with a Flyback Diode
Wait a minute! Diode D1 is facing in the wrong direction in the schematic in Figure 18-5. The arrow is going
up toward positive, unlike the LEDs in that same schematic, whose arrows point toward the negative battery
terminal.
Diode D1 is purposely installed in this circuit in a direction that doesn’t allow power to flow through it
during normal circumstances. However, when power is disconnected from a motor, a large reverse voltage
suddenly appears (see the following note). The diode allows the reverse voltage to flow around and around
in the motor to be safely consumed.
If the diode wasn’t installed, the reverse voltage in the motor could damage transistor Q7. A diode
utilized to reroute reverse-induced voltage is called a flyback diode. The diode intentionally provides a short
circuit for the leftover motor juices.
■ Note The reverse-voltage phenomenon is not limited only to motors. It also appears in coils, switches, and
anywhere a current suddenly shuts off. If a large enough current flows through a motor, then the reverse voltage
may potentially damage connected electronics.
Think of the reverse-voltage effect as a washing machine or dishwasher that rapidly shuts off its water supply
and a loud jerk reverberates through the pipes. Suddenly the rushing water doesn’t have anywhere to go, so
pressure builds against the closed end of the pipe and is released backwards.
Picking the Schottky Barrier Diode
There are two specific characteristics required of a flyback diode.
First, a flyback diode should act as a barrier, not as a path, when current is going in the correct direction.
Some diodes are too leaky in reverse to be good-quality barriers. In this circuit, a leaky flyback diode would
waste some current that otherwise would zip through the motor and provide motion.
Second, when the motor generates current in the wrong direction, the flyback diode should be the
least-resistant path (lowest voltage drop). That way, the electricity will choose the path through the flyback
Figure 18-6. A 1N5817 diode, typical looking of many diodes