Hardware Reference
In-Depth Information
One side effect of the reverse-biased diode is that it slows down the decay of the
magnetic field. As current flows in the reverse direction, the magnetic collapse is resisted.
This results in magnetic forces inside the motor, which affect its speed. (This same effect
also slows the release of relay contacts.) When greater speed is required, a resistor is
sometimes used in series with the diode to limit its effect.
So what about voltage safety of our Raspberry Pi GPIO pins? Reexamine the
Darlington pair
Q
2
and
Q
1
. Pins COM and 1C can be as high as 50 V. But current cannot
leak through
D
3
(from COM) because the diode junction is reverse-biased. Current
cannot leak from the collectors of
Q
2
and
Q
1
(from 1C) into the base circuits because
those base-collector junctions are behaving as reverse-biased diodes. The main point of
caution is that
Q
2
and
Q
1
must remain intact
.
If
Q
1
were allowed to overheat, for example, it might break down. If
Q
1
or
Q
2
breaks
down, current will be allowed to flow from its collector into the base circuit, which is
connected to the GPIO. Therefore, the breakdown of the driver transistors must be strictly
prevented!
Driving the Driver
In this section, we look at the Raspberry Pi (GPIO) interface to the ULN2003A. There are
two things we are most interested in here:
•
The usual input logic-level interface
•
Power-on reset and boot conditions
Input Levels
The output current of the ULN2003A Darlington pair rises as the input voltage rises
above the
turn-on
level. We know from the Darlington configuration that there are two
base-emitter junctions in the path from input to ground. Therefore, the ULN2003A
driver requires a 2 ×
V
BE
voltage to forward-bias the pair. If we assume
V
BE
= 0.6
V
, we can
compute a
V
IL
for the driver as follows:
VV
=´
=´
=
2
206
12
IL
BE
.
.
V
