Any one over the age of 40 probably knows, as you get older
your eyes just don’t work as well as they use to. So it was no surprise to me
when I stated to have trouble seeing while ridding the old FXR after dark. I
just figured my night vision was going. Then one night I saw a reflection of
the headlight in the garage window and realized it was looking kind of dim. Of
course my first thought was that the charging system has a problem. How could
this be, Cycle Electric Inc components are not supposed to do that. So I
whipped out my trusty voltmeter and checked the charging voltage at the
battery. It brought a smile to my face to see 14.2 volts, right where it should
be. So why is the headlight so dim. I took a voltage reading at the headlight
and found only 9 volts. The difference in voltage between the battery and
headlight was due to voltage drops. Excessive voltage drops result in a low
voltage condition that can cause problems for voltage sensitive electrical
components. If there is too much voltage drop between the voltage regulator and
the battery the battery will not charge properly. This leads the question, what
is a voltage drop? What causes it and how do you track one down?
A voltage drop occurs
whenever electrical current passes threw a point of resistance. To recap from
last months article we learned Voltage is the force that makes electrons flow
and Amperage is the actual flow of electrons. Resistance restricts the flow of
electrons. A good analogy that can help you understand this is to compare the
flow of electricity in a conductor to the flow water in a pipe. Voltage can be
compared to water pressure. Amperage is measured in electrons per second where
as water flow is measured in gallons per minuet. Electrical resistance restricts
the flow of electrons. This can be compared to a partially clogged pipe
restricting the flow of water. Imagine a 1” water pipe with a section in the
middle that is reduced down to ¼” then increased back to 1”. One side of the
pipe is connected to a water supply that delivers an unlimited amount of water
at 50 PSI (pounds per square inch). The other end of the pipe has a valve on
it. The ¼” pipe in the middle is the resistance. When the valve on the end is
closed and no water is flowing the pressure will be 50 PSI on both sides of the
¼” resistance. As the valve is opened water will start flow. As long as the
water supply can supply enough current the pressure on the up stream side of
the ¼” restriction will stay at 50 PSI. The pressure down stream of the ¼”
restriction will start to drop. The wider you open the valve the more water
current will increase and the pressure after the resistance will drop. When the
valve is closed and the flow of current stops the pressure will equalize.
Pressure on both sides will be the same and there will be no pressure drop.
What we learn from this is if there is no current flowing there will be no
voltage drop. As the current increases, so does the voltage drop.
The fact of the matter is that all conductors
have resistance. Some just have more than others. Resistance is measured in Ohms. The resistance of a
wire depends on the material the wire is made of, the size or gauge and length
of the wire. 12-gauge copper wire has 1.588 ohms per thousand feet. 16-gauge
wire has 4.016 ohms per thousand feet At 68° F. The 12-gauge wire has less than half the
resistance of the 16-gauge wire. For low current applications a small wire will
do. As amperage requirements increase it is necessary to use a larger wire to
keep the voltage drop within reason. Not to mention the energy lost when the
voltage is dropping is turned in to heat. An excessive voltage drop can make
enough heat to melt insulation and even burn up wires. The resistance of the
wire should not change over time. The main source of resistance in the wiring
system is at connections. Every place
there is a connection there is a place for a bad connection. This is where you
should look for excessive voltage drops. The resistance of a new clean connector
depends on the contact surface areas and the amount of force between the two
surfaces. Over time the resistance can increase due to corrosion and possibly
becoming lose. To calculate the effect of increasing resistance compared to
voltage drop we can use ohms law. Ohms law states that it takes one volt to
push one amp through a one-ohm resister. Using this formula, if you know two of
the components you can find the third. For instance if you have 10 amps flowing
through a 1 ohm resister there will be a 10 volt drop across the resister. A
vary small resistance can make an unacceptable voltage drop. A derivative of
ohms law states I X R=V. V is the voltage drop across the resistance. I stands
for intensity, which is another term for current. R is the resistance measured
in ohms. Lets look at some scenarios.
In the case of my FXR the headlight draws 5
amps.
5amps X .6 ohms = 3 Volt drop
This means the circuit had
0.6 ohm between the battery and the headlight. Some of this was between the
positive terminal and the light bulb and some was between the bulb and the
negative battery terminal.
In the case of an electric
starter that draws 100 amps with .05 ohms in the circuit
100 amps X .05ohms = 5
volt drop
That would leave 7 volts
at the starter motor, which would be unacceptable.
The average ohmmeter is
not accurate enough to read tenths of an ohm. This is why it is better to use a
voltmeter to find bad connections that cause unacceptable voltage drops.
Since Voltmeters measure a difference in
voltage the easiest way to track down a voltage drop is with a voltage meter.
Simply measure across two points in a circuit while current is flowing threw
it. The reading you get will be the voltage drop. You need a meter sensitive enough
to read small voltages. See the section on voltage meters later in this
article. My old FXR has been ridden hard and put away wet for 20 years. This
makes it the perfect bike to use as an example for voltage drops. It was ten
years ago when I had dim headlight problems. The figures in this example are as
it is now. This is how I tracked the voltage drops down.
In order to keep the
voltage closer to normal operating voltage without the motor running I attached
a battery charger to the battery. With the battery charger on a 5 amp charge
and the lights on, battery voltage measured 13.76. I took a voltage reading
directly across the headlight. I did this by removing the bulb from the head
light shell but left the wire socket plugged in to the bulb. With the high beam
on I took a voltage reading across the negative pin and the high beam pin. This
time I got an 11.72-volt reading. The 2.04 voltage drops is not as bad as I had
back in 98 but will work of this example.
Voltage drops can be on the positive and negative side of the circuit.
To check the negative side I measured from the negative pin on the headlight
and the negative terminal on the battery and got a reading of .7 volts. To
check the positive side I put the black lead on the high beam pin and the red
lead on the positive battery terminal and got a reading of 1.37 volts. Every place there is a connection there is a
place for a bad connection. So I started measuring each connection point. Any
connection with more than .1 volt drop I cleaned and tightened. The power
starts at the battery. It goes threw the main circuit breaker to the ignition
switch than back to the lighting circuit breaker. From there it goes threw a
connector at the headlight shell to the high/low beam switch then to the
headlight. Every connection had some drop. Some of the factory crimp on
terminals had higher than normal drop across them. I did not get a substantial
drop at any one point on the positive side but a couple tenths of a volt here
and there can add up. After cleaning up some connections I got the drop down to
about 1.2 volts and decided to live with that.
Most technicians and home mechanics will be measuring
voltage with a Multi meter. A multi meter is a meter that can perform many
functions. Most can measure voltage and ohms. Some can measure amperage. Meters
can be Analog or digital. Analog meters have a needle that swings and points to
a number. Digital meters display a reading on a screen similar to a calculator.
When measuring amperage and voltage you will need to determine if you will be
measuring alternating current (AC) or direct current (DC) and select the proper
setting. Some meters will only read AC or DC but most will do both. On most
analog and older digital meters you need to select the proper range for what
you want to measure. The choices may look like this DC 2V, 20V, and 200V. It is
important to choose a range that is higher than the voltage you will be
connecting it to or the meter may by damaged. If you are not sure what range
you need it is best to start on a higher scale and work and your way down. The
lower scales will give better resolution and accuracy. You cannot read
accurately to a tenth of a volt on the 200-volt scale. Most modern digital
meters are auto ranging. All you need to do is select the appropriate function
such as DC volts (DCV) and the meter will automatically select a range to give
the best resolution. The most important thing (VERY IMPORTANT) is to read the
indicator to see what scale the meter is reading in. This is usually in the
upper right hand corner of the meter.
When measuring the voltage of a household appliance, a volt
or two doses not matter. When trying to track down a .1-volt drop you will need
a meter with better resolution. It is kind of like trying to time out 30
seconds on a watch with no second hand. In order to accurately measure tenths
you need to measure hundredths.
Some meters are
temperature compensated and calibrated to national standards and some are just
accurate to within a volt or two. For some jobs the accuracy and resolution of
the meter dose not matter, for other jobs it dose. At Cycle Electric Inc we
have many different meters. We mostly use fluke 73 and fluke 79 meters. They
are a good quality made in the USA middle of the road meter. We also have a
Fluke 7-600, which is a low-end meter we use for shop maintenance. This meter
is good to check for a blown fuse on the air compressor but has many
shortcomings. The voltage range starts at 4 volts so it is not good for low
voltage readings. The ohms scale only goes up to 400 ohms. When put it across a
500-ohm resister it will read OL or open. Before I know this I thought I had an
open circuit when the meter was just out of range. This steered me in the wrong
direction on my diagnosis. Low-end meters are also venerable to interference
from RFI or radio frequency radiation. This is the noise you can hear on your
radio. It can come from many places such as arcing brushes or solid ignition
wires. This interference can give you a faulty reading on your meter. The
classic case is when someone calls me and said they have something like 50
volts at the battery. The first thing I ask them these days is “did the head
light blow out”? When they say no I tell them they do not have 50 volts at the
battery. Most of them will insist the meter works just fine on other bikes.
This is usually traced to solid ignition wires emitting RFI and interfering
with their digital meter. In order to put significance to the reading you get
on your meter it is important know and understand its resolution and accuracy.
Let the road you choose to ride bring you happiness.
KARL S FAHRINGER
President
CYCLE ELECTRIC INC