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Chapter One

Basic circuit elements and
Methods of showing voltage and current on a schematic

Fig 1a shows the schematic symbols for the basic circuit elements.

Fig 1b shows current and voltage arrows which show polarity.
The head of the voltage arrow is positive and the tail is negative.
The small current arrow shows the direction of current flow, assumed
to flow from the positive to the negative.

Fig 1c shows alternate forms of showing current and voltage.
The first resistor has a small black arrow showing current
The second resistor has a ground terminal on the bottom of it,
and the voltage at the top of the resistor is indicated with the
letter 'V'. This is possible when the voltage is referenced to
The third resistor shows what the voltage from top to ground is,
and shows what the current is and where this current is measured
and what it's direction is.

Although this seems simple, these are very important facts about
circuit elements. In the future we'll always have to be very
careful where we place our arrows and exactly how they are oriented.

Ideal Circuit Elements and Real Life Circuit Elements Made Up Of
A Combination Of Ideal Circuit Elements

Voltage/Current Polarities
First step in circuit analysis

Kirchhoff's Current and Voltage Laws
The sum of the voltages around a closed current path equals zero.
The sum of the currents entering a node equals the sum of the
currents leaving a node.

How close an LED circuit is to a resistive circuit

Replacing an LED with a voltage source
Fig 6d is a simple parallel circuit.

Parallel resistors in series with an LED


Series R Tester Circuit (below)

This is just something to show what we will be looking at in
the future. This circuit is more advanced then we want to
deal with right now, but once we get going we'll find out
how easy this circuit really is!

This circuit can be used to measure the series resistance of
an Li-ion cell. The 'ground' just to the left of R2 is removed
for one reading and replaced for the second reading. When
this action produces twice the current 'I' through R6 when the
short is removed as when it is in place (as drawn) the resistor
R2 is exactly twice the value of the series resistance. The
Li-ion's series resistance is labeled R1 in this schematic.

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