experiments_with_resistors

8/7/2019 Experiments_With_Resistors

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Experiments With Resistors

Measuring Resistors

The Resistor Color Code

A Simple Resistor Experiment

Measuring Resistor Accuracy

Resistor CombinationsSeries Resistors

Parallel Resistors


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Measuring Resistances

Let's start by making one resistance

measurement.

Get a resistor. A one k-ohm resistor

would be good. You can tell it is a one

k-ohm resistor by the stripes. Theyshould be brown-black-red in that order

from the end, as shown at the right.


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You are now ready to measure the

resistance. Connect your resistor to an

ohmmeter as shown in figure. Don't care

about the lead on which end of the

resistor. It doesn't latter. (The resistor is a

bilateral element and should be the sameeither way), here are the connections you

make.


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The Resistor Color Code

You probably wondered about those

stripes on the resistor. There is a color

code that lets you tell what value theresistor has. Here's what's important.

This resistor is 1000 ohms = 10x102


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0 Black Black

1 Brown Brown2 Red Red

3 Orange Orange

4 Yellow Yellow

5 Green Green6 Blue Blue

7 Violet Violet

8 Gray Gray

9 White White 10% Silver Silver

5% Gold Gold

20% Black Black


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Tolerance band

The tolerance band is typically either gold orsilver. A gold tolerance band indicates that the

measured value will be within 5% of the nominal

value. A silver band indicates 10% tolerance.

For example a resistor with color code brown-black-red-silverindicates a nominal value of 1k.

The first two bands (brown-black) produce the

mantissa (10) and the third band (red) is the

exponent of ten (x10). Since the tolerance bandis silver, we can expect the measured value of

the resistor to be between (900 and 1100)ohm .


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To calculate the value of the

resistance you use the three

stripes. (If there are four stripes,just use the first three. The last

stripe tells you how accurate the

resistance value is). Here is thealgorithm.


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Algorithm

The first stripe is the most significant digit,

X, in XY x 10N

The second stripe is the next digit,Y, in XY x 10N

The third stripe is the exponent in

XY x 10N

Where N represents the Code of the color


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A Simple Resistor

Any conducting material can be usedto make a resistor. Any metal or

metallic alloy can be used. Other

conducting materials, like carbon,germanium or silicon can be used,

even if the material does not conduct

as well as a metal. In this exerciseyou are going to construct some

resistors made of carbon.


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The carbon you will use is the

carbon found in a pencil lead. On a

clean sheet of paper draw a shapelike the one below. It can be

smaller but drawn to scale. You

have just constructed and

measured a resistor. There is a

special symbol for a resistor thatyou need to become familiar

with. Here it is.


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The diagonal lines are intended to

suggest some resistance to the flowof current. This symbol can be used

in any orientation, and you will often

see this symbol rotated as shown atthe right.


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When you measured your resistor the

ohmmeter that you used actuallyapplied a small voltage across yourresistor and current flowed throughit. Often symbols are attached to the

resistor symbol to indicate how thevoltage is applied across the resistorand to define a positive direction forcurrent flow. In a case where both a

current and a voltage symbol wereused the situation would look like whatwe have shown above.


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The second resistor should be the samephysical size as the original resistor. Since it is

the same size and made of the same material it

should have the same resistance. There are

physical reasons why that is so, and there is amathematical expression that relates the

resistance to length, cross sectional area and a

property of the material called resistivity.

For resistors that have a constant cross section,A, and a length, L, the resistance is:

R = rL/A


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Now that you have the two same

resistors to measure, go ahead andmeasure the series combination.

When you have that measurement

taken and you think you are closeenough to the correct measured

value you can go on.


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