Tech-Talk
Part 15
Welcome to Part 15 of our series.
Well, I've been rambling on about antennas, feedline, and
related topics for quite a while. I'm going to depart from that for
now, but I'll get back to it. I've noticed that quite a few folks have
questions about DC Power and wiring, so we'll spend some time exploring
that topic. I'll try to boil things down so that you can get an idea of
the considerations and arrive at appropriate choices. Standard
disclaimer -- I'm not an electrical engineer. I'll probably
over-simplify and over-generalize, and some of my thoughts may not be
"Mil-Spec" or exactly by the book. Nit-picking, whining, and carping
will be ignored; but if I make a major error I'd appreciate hearing
about it.
And the number one question about DC Power I get at Hamfests and online is...
"What size wire should I use to do this or that?"
In the US, wire sizes are most often referred to as "##
gauge", or sometimes "##AWG", which stands for American Wire Gauge. In
the metric world, wire is measured in square millimeters, referring to
the area of a cross-section. We'll stick with AWG for this discussion.
It's important to note that the higher the gauge number, the smaller
the wire is. Doubling, or halving, the size of the wire means
approximately 3 gauge numbers down or up. In other words, two #14 wires
in parallel roughly equals a #11 wire.
Factors involved in selecting wire include cost, size,
weight, flexibility, voltage rating, current capacity (ampacity), and
voltage drop. The first four are self-explanatory and not really
pertinent to our discussion. We'll focus on the other three.
(1) Voltage rating. At some voltage level, any insulation
will break down. Sometimes that's good -- when the current arcs across
the gap in your car's spark plugs, that ignites the air/gasoline mix in
the cylinder and your car moves. Most times, though, that's bad.
Fortunately, most of the wire and connectors we use for DC power are
rated for 600 volts or better, so in general we don't need to be
concerned with the voltage rating of our wire. But if you're working
with high voltage circuits, like those for vacuum tubes, you need to be
aware of it.
(2) Current capacity (ampacity). Here's where things start
to matter. All wire has some resistance. Current flowing through a
resistance generates some heat. The more current we draw through a
wire, the more heat is generated. Lower the resistance, and less heat is
generated. Larger diameter wire has less resistance, and thus can
safely carry more current. You can look online for tables giving the
resistance per foot of various sized wire.
If we pull enough current through a wire, it will heat to
the point that the insulation melts and possibly catches fire. At an
even higher level of current, such as a short circuit, the wire itself
will actually melt! Now you may be thinking "Hey, that sounds like a
fuse". And you'd be exactly right. A fuse is just a wire designed to
melt at a certain temperature when a given current attempts to flow
through it.
The National Electrical Code specifies #10 wire for 30 Amps,
#12 for 20 Amps, and #14 for 15 Amps. This assumes proper fuses or
circuit breakers are installed. You won't go wrong following these
guidelines. But keep in mind that they are for typical home or
commercial wiring enclosed in a plastic or metal sheath or in a conduit
-- and so with no cooling airflow around them. And they are for an AC
signal/voltage, where due to the skin effect, most of the current flows
on the outside of the wire. How are these ratings determined? By
determining a safe rise in wire temperature, given the type of
insulation used -- with a generous safety margin, of course.
There's no electrical penalty if you use larger wire than
necessary for the expected current. However, it will cost more, weigh
more, and be less flexible. You may need larger and more expensive
connectors, as well. But sometimes that's what's needed to get the job
done. Why? If you guessed "voltage drop", pat yourself on the back!
(3) The last, and sometimes forgotten, factor we'll
consider is voltage drop. As we noted above, all wire has some
resistance. And according to our old friend Mr. Ohm and his Laws, the
voltage drop across a load is directly proportional to both the
resistance and the current. Remember E=IR?
Let's take a simple circuit for an example -- a transceiver
powered by a 13.8V DC power supply. You can think of it as 3 resistors
connected in series. The positive supply wire, the radio, and the
negative return to the power supply. Basic theory tells us that the
voltage will divide among the 3 resistors, in proportion to their
value. Keep in mind that if you have a 10 foot run of wire from the
supply to the radio, you have 2 wires (+ and -) for a total of 20 feet
and thus two voltage drops in the wire.
Why does voltage drop matter? Electronic devices are
designed to operate at a specified voltage. Ham Radio transceivers are
quite often specified to require 13.8V +/- 15%, or 11.73 to 15.87V.
Check your radio's manual to be sure! If the voltage drops below the
minimum, you may get reports of distortion on your signal, or the radio
may shut itself off unexpectedly. Since many power supplies have
adjustable voltage, you could, in theory, boost the voltage. But at
higher voltages your electronics will run hotter. Heat is the enemy of
electronics! Not an approach I would recommend if you're looking for
long time service from your radio.
There's not a lot we can do to reduce the current that a
particular device draws, or to lessen its resistance -- and thus reduce
its voltage drop. A typical 100 Watt HF transceiver draws roughly 20
amps of current transmitting at full power. Sure, we can reduce the
output power, but think about it for a minute. Power (Watts) = Current
times Voltage. So to produce 100W of RF, our radio is using 20 Amps x
13.8 Volts or 276 Watts. Nearly 2/3 of the power is used by the radio's
internal circuitry, or is dissipated as heat. Cutting our power down
by half might save a few amps, but not very many.
And so we arrive at the obvious solution. Use wire heavy enough to reduce the voltage drop to an acceptable level.
That's it for this month. Next time, we'll look at some real-world examples and I'll explain how to determine what you need.
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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