This section will cover the DC power supply section of the
radio. It has
the job of supplying the radio with a useable D.C. power source for the
different circuit modules. There are two basic voltage supplies on the
rig. There is the unregulated D.C. supply and there is a regulated 8
First find the following components in your kit.
D13 1N4001 Diode
C112 220 microfarad electrolytic capacitor
C102 .01 microfarad ceramic capacitor
U2 78L08 three terminal voltage regulator
As stated in the manual, many components are polarized. In other words,
it matters which direction you install them. Diodes, I.C.'s and some
capacitors are examples of polarized components.
Install the above components. PLEASE double check polarity and position
before soldering. Reference page 11 for details on the diode install.
Connect a 2 pieces of hookup wire between the power supply connections
on the circuit board and your power source. The - supply goes to ground
(J4 pin 1) and the + supply goes to the +12 volts (J4 pin 2). You need
to be able to easily turn this on and off, so make sure these
connections are easily removed. (Don't build with the power on!!!!)
Voltages are measured with reference to ground unless otherwise
specified. This means that the black (-) lead of your volt meter should
be connected to the board ground, along with the power source -
connection. You can connect the meter black lead to the power supply -
connection to make the following measurements.
J4 pin 2 should be the same as the power source + terminal.
Cathode of D13 should measure the Power source voltage (Vps) - D13
dropping voltage (about 0.7 volts). The diode dropping voltage will vary
with device and with the amount of current passing through it.
Output of the U2 voltage regulator should be about 8 volts (between 7.7
and 8.3 volts). A convenient place to measure this is at pin 1 of J2.
If you get these readings, you have built the power supply section
correctly. Now on to the circuit description.
As you all know, a diode allows electrical current to pass in one
direction but not in the reverse direction. When a diode cathode is more
negative than the anode, it will conduct current. In this circuit, D13
is in series with the power source as it feeds the rest of the circuit.
It's function is to protect the board from damage if the power leads are
connected backwards. If the power leads are reversed the diode will be
reverse biased and no current will flow into the circuit, saving all
those little components from certain death ;-)
The disadvantage to using a series diode for polarity protection is that
you lose voltage (and power) in your power supply. There are other
methods of polarity protection that don't significantly affect the
supply voltage or power, but are significantly more complex or use a
fuse. For a simple circuit this is an excellent solution.
C102 and C112 provide decoupling on the power rail. They provide a low
impedance path for any AC on the internal power system. This keeps the
supply a clean D.C. voltage with a very small A.C. component.
U2 is a low power three terminal voltage regulator. It is there to
provide sensitive circuits a constant voltage. The input voltage to the
board is not regulated. It can be anywhere between 12 and 15 volts. Lets
just say we are using a battery for the power source. All power sources
have an internal resistance (usualy small). As more current is drawn
from the supply, the voltage drop across the internal resistance
increases, decreasing the output voltage. So when we key up the transmit
section and the current draw from the power source jumps, the voltage
provided drops. There are certain circuit components that require a very
stable voltage source. The VFO is one example of this. Imagine what the
radio would sound like if the VFO frequency changed depending on how
long the key was held down. (We call that chirp). This is a bad thing.
So we use a linear voltage regulator to isolate the sensitive circuits
from variations in the supply voltage.
The 78L08 device can take anywhere from 10.5 volts to 23 volts at its
input and provide an 8 volt output. It provides overcurrent protection,
short circuit protection and thermal protection (shut down if it gets
Remember that these devices are not perfect. We have looked at them up
till now as a "perfect" device. They can only supply 100mA maximum. They
can only dissipate a total of 700mW assuming the ambient temperature is
<25 degees C. Power dissipated by the device is equal to the voltage
dropped by the device (15-8= 7 volts) times the current delivered.
Notice that in this design that if the maximum current was drawn out of
the device (100mA) at the maximum supply voltage (15 volts), the max
power dissipation on the device is 700mW. Coincidence? I think not!
Also, variations on the input do appear at the output, although greatly
attenuated. The spec is 48dB at 120Hz (full wave rectified line ripple).
If my math is correct, a one volt change in input voltage will cause a
15.8 microvolt change in the output.
In addition, the device itself uses a certain amount of power. Bias
current is about 4mA. At 15 volts that equals 60 mW.
If you are curious, you can view the data sheet if your browser has a
PDF reader. http://www-s.ti.com/sc/psheets/slvs010e/slvs010e.pdf
Next, we build and discuss the VFO. It will probably be done in several
seperate sections. Stay tuned.