MEASURING RESISTANCE

MEASURING RESISTANCE
Turn a circuit off before measuring resistance.
If any voltage is present, the value of resistance will be incorrect.
In most cases you cannot measure a component while it is in-circuit. This is because
the meter is actually measuring a voltage across a component and calling it a
"resistance." The voltage comes from the battery inside the meter. If any other voltage
is present, the meter will produce a false reading.
If you are measuring the resistance of a component while still "in circuit," (with the

power off) the reading will be lower than the true reading.

Measuring resistance

Do not measure the "Resistance of a Battery"

Internal impedance) is not measured as shown in the diagrams above. It is measured

by creating a current-flow and measuring the voltage across the battery. Placing a

multimeter set to resistance (across a battery) will destroy the meter.

2. Do not try to measure the resistance of any voltage or any "supply."

Resistance is measured in OHMs.

The resistance of a 1cm x 1cm bar, one metre long is 1 ohm.

If the bar is thinner, the resistance is higher. If the bar is longer, the resistance is

higher.

If the material of the bar is changed, the resistance is higher.

When carbon is mixed with other elements, its resistance increases and this knowledge

is used to make RESISTORS.

Resistors have RESISTANCE and the main purpose of a resistor is to reduce the

CURRENT FLOW.

It's a bit like standing on a hose. The flow reduces.

When current flow is reduced, the output voltage is also reduced and that why the

water does not spray up so high. Resistors are simple devices but they produce many

different effects in a circuit.

A resistor of nearly pure carbon may be 1 ohm, but when non-conducting "impurities"

are added, the same-size resistor may be 100 ohms,

1,000 ohms or 1 million ohms.

Circuits use values of less than 1 ohm to more than 22

million ohms.

Resistors are identified on a circuit with numbers and

letters to show the exact value of resistance - such as 1k 2k2 4M7

The letter  (omega - a Greek symbol) is used to identify the word "Ohm."

but this symbol is not available on some word-processors, so the letter "R" is used.

The letter "E" is also sometimes used and both mean "Ohms."

A one-ohm resistor is written "1R" or "1E." It can also be written "1R0" or "1E0."

A resistor of one-tenth of an ohm is written "0R1" or "0E1." The letter takes the

place of the decimal point.

10 ohms = 10R

100 ohms = 100R

1,000 ohms = 1k (k= kilo = one thousand)

10,000 ohms = 10k

100,000 ohms = 100k

1,000,000 ohms = 1M (M = MEG = one million)

The size of a resistor has nothing to do with its resistance. The size determines the

wattage of the resistor - how much heat it can dissipate without getting too hot.

Every resistor is identified by colour bands on the body, but when the resistor is a

surface-mount device, numbers are used and sometimes letters.

You MUST learn the colour code for resistors and the following table shows all the

colours for the most common resistors from 1/10th of an ohm to 22 Meg ohms for

resistors with 5% and 10% tolerance.

If 3rd band is gold, Divide by 10

If 3rd band is silver, Divide by 100

(to get 0.22ohms etc)

resistor color coding table