Lecture Text.
Ohms and Kirchhoff's Laws.
Today Kirchhoff's laws appear
fairly intuitive. If water in a pipe branched into two lines we would expect
that the sum of the water coming out of the two pipes is the same as the rate of
flow of the water in the single pipe. However, since current was observed
strictly in terms of magnetic field this infers that the magnetic fields are
directly proportional to the flow of electrons. Likewise the linear
relationship expressed by ohms law was not obvious to Ohm from fluid mechanics,
because pressure drop of fluids in a line is not linear with respect to flow
rate.
On a more practical note consider what it means if
Kirchhoff's law don't appear to be being obeyed. What if you had three wires
attached to a terminal and measurement indicated that the current going into the
terminal from one wire did not equal the current leaving the terminal in the
remaining two wires. This would indicate that the current was going to
ground or an other line via the terminal and traveling a path not shown on your
circuit schematic. This could of coarse be caused by a obvious solder splash or less obviously by a contaminant on surface of PC board.
In the case of multi-layer boards a breakdown of insulation between layers is
also possible.
Ohms law states that current in
amperes is equal to voltage divided by resistance. This form of ohms
law is most useful for technicians, because it is easy to measure voltage with a
single probe of a grounded voltmeter. Connect negative lead of your
voltmeter to ground, circuit common, or power source return line and you can
measure the voltage on as many component junctions (commonly called circuit
nodes) as are accessible to your meter probe. You can determine the
current through a resistor by measuring the voltage on both sides of the
resistor, then calculating the difference and dividing voltage difference by the resistance the
resistor. This resistance is written on the resistor in letters, or
frequently it is indicated by a color code. It is also available from the circuit
schematic. Current calculated in this manner tells the troubleshooter how
much current is in this branch of the circuit. Usually one current calculation and
several voltage measurement tell you all you need to know about transistor,
diode, operational amplifiers, or even vacuum tube circuits. Thus, you most
be able to use ohms law to analyze what is going on, not only in
resistor circuits but in the more complex circuits that you will studying in
future lessons.
Resistance of circuit can be measured with an
ohmmeter when the power to a circuit is off. Sometimes when you measure
resistance across any component you may also measure resistance of components in
parallel circuit branches. You can prevent this by lifting one end of the
resistor from the circuit board.
