How do you find the value of resistor?

Answer 1

This depends largely on the purpose of a specific resistor in the circuit and may be a trivial calculation or a very difficult calculation that must simultaneously determine the values of several other components (e.g. resistors, capacitors, inductors) in the circuit. Also such calculations rarely give one and only one usable value for the resistor, in which case you must consult the list of available resistor values manufactured and pick the most reasonable ones (also remembering to follow the rule given to Richard Feynman when he was designing mechanical analog computers for the military, before he moved to the Manhattan Project, for selecting gears: "always pick from the center of the catalog, the ones at the beginning and end are harder to make reliably, if they weren't they would be nearer the center and different values would be at the beginning and end").

In rare cases all these techniques may fail and the values of certain resistors may have to be determined empirically by breadboarding the circuit with variable resistors substituted for the ones to be determined. The breadboard circuit is then operated and the variable resistors adjusted until the circuit operates as desired, then the circuit is powered down and the variable resistors removed and measured. Then pick the closest available resistors and reassemble the breadboard with them to verify correct operation with fixed value resistors. If it fails to operate correctly some or all of the variable resistors will have to be reinstalled and different settings found for correct operation, etc.

As an example of a simple circuit where the value of one resistor is impossible to determine without simultaneously determining others is a two resistor voltage divider. You must determine both resistor values, while also taking into account the thevenin equivalent circuit, individual and total power dissipation, whether standard tolerance resistor values are acceptable or if expensive high precision tolerances are needed, etc.

I have actually encountered circuits where a calculation of some component's value proved that no such value exists (e.g. R must satisfy both R > 5K ohms and R < 250 ohms at the same time), in which case the entire design had to be abandoned and a totally different circuit design approach taken.

Answer 2

You can either measure the resistor with an ohmeter, or read the color code printed on most small carbon resistors.

The four band color code is read with the first two colors as numbers and the third as a multiplier by 10^n (n being the number corresponding to the color) and the fourth band (which is separated from the first three) as a tolerance band or quality band.

The colors for resistor values are as follows:

Black 0 (x1)

Brown 1 (x10) 1%

Red 2 (x100) 2%

Orange 3 (x1,000)

Yellow 4 (x10,000)

Green 5 (x100,000) 0.5%

Blue 6 (x1,000,000) 0.25%

Violet 7 (x10,000,000) 0.1%

Grey 8 (x100,000,000) 0.05%

White 9 (x1,000,000,000)

Gold (x0.1) 5%

Silver (x0.01) 10%

No color 20%

So if you have a resistor that has Gold, a gap, green, brown, black, then you have it backward. Turn it the other way. It will always be read color, color, color, gap, color.

Now it is black, brown, green, gap, gold. That means 1, 0, (x100,000), 5%. Take the 1 and 0 (making 10) and multiply by 100.000. That's 1,000,000, or 1 mega-ohm. Take the tolerance band of 5% and add and subtract the number (50,000) from 1,000,000. This gives you the range of the resistor between 1.05 Mega-ohm and 0.95 Mega ohm.

So in a perfect world, this resistor would be exactly 1 MOhm, but the tolerance band tells us that it should be between 1.05 MOhm and 0.95MOhm.

Answer 3

There are various ways, according to the physical size of the resistor. For small resistors, the most common method is by a series of coloured rings applied towards one end of the resistor. Larger resistors used to use a colour code system called the 'body-tip-spot' method, but this is now obsolete. Large, wire-wound, resistors, may have their resistance values printed on them using an alpha-numeric code, such as 7K5 -meaning 7.5 kilohms.

Answer 4

measure the voltage drop divided by the resistance.