By necessity this book covers DC circuit theory and has no practical
application. Once you have been introduced to DC theory, AC theory and
semiconductor devices we can start to delve into some practical
application. However, just to make things a little more interesting,
let's jump ahead and see one way we can apply what we have learned so
far to part of a real circuit.
We are going to choose the proper resistor to work with a zener diode.
All we need to know about a zener diode at this time is that it is a
special device that maintains the same voltage difference regardless of
the current flowing through it. Let's say you have a zener diode and
its specifications say it maintains 6 volts and wants a nominal current
of 40 mA. Your circuit is powered by an 18 volt battery. There is a
resistor in series with the zener diode to control its current.
We need to calculate the value of the resistor in series with the zener diode.
The question is, what value do we need for the current-controlling
resistor in this circuit? To solve the problem we first need to
know how much voltage will be developed across the resistor under the
specified conditions. Let's use Kirchhoff's Voltage Law to figure that
out. The whole series circuit has 18 volts across it. The zener diode
has 6 volts across it. How much voltage does that leave for the
resistor? The answer is 12 volts. We now want 40 mA through the diode.
Since the resistor is in series with the diode it will have the same
current. Now the question is, how much resistance will conduct 40 mA of
current with 12 volts across it. We know the voltage so we divide into
it. That's 12 V 40 mA. On the calculator that is...
1
2
.
0
4
=
300
...for 300 ohms. We need a 300 ohm resistor.
Now let's complicate it a bit. Let's say the zener diode is controlling
the voltage across a light bulb. This light bulb conducts 1 ampere
at 6 volts. What resistor do we need now?
Now what value resistor do we need?
Now we not only have 40 mA going through the diode but we have 1 ampere
going through the light bulb. How much current is flowing through the
resistor now?
We now have a series-parallel circuit, so Kirchhoff's current law
applies. What resistance will develop 12 volts with a current of 1.04
amperes?
Here is the same circuit in the layout we are more familiar with. It's exactly the same circuit only laid out differently.
We now have a series-parallel circuit. The zener diode and light bulb
are in parallel with each other and together they are in series with
the resistor. The total current flows through the resistor. After
the resistor the current splits to the diode and the light bulb.
According to Kirchhoff's Current Law, the currents through the diode
and light bulb must add up to the current through the resistor. The
resistor now has 1.04 amperes flowing through it. The voltages haven't
changed so we just use this new current to calculate the resistor. That
is 12V 1.04 A. On the calculator that is...
1
2
1
.
0
4
=
11.64
...for about 11.6 ohms.
Let's take it one step further. How much power is the resistor
dissipating? That's 12 V times 1.04 A. On the calculator that is...
1
2
X
1
.
0
4
=
12.48
...for about 12.5 watts.
That's a pretty hefty resistor.
40 mA. On the calculator that is...
