What is a Resistor? Ohms law
Resistor components limit current, divide voltage, and create heat in electronics circuits based on their resistance. The electrical properties of resistive components, such as resistors, are explained by some simple formulas called Ohms law.
For example, each volt across a 1000 ohm resistor causes 1 milliamp (1/1,000th of an amp) of current to flow through the resistor. 1V across a resistor and 0.001A (1mA) of current through it generates 0.001W (1mW) of heat that the resistor must be able to dissipate. Most resistors you are likely to come across are rated to handle a maximum of 1/4W (0.25W).
Topics covered below
- Color code to read value
- Ohms law elements
- Calculating current
- Calculating power
- Series resistors/voltage dividers.
Color code to read value
New resistors are usually in well labeled packaging. In addition, it is really easy to measure resistance with a multimeter, especially when the resistor is not part of a circuit. But, it’s still a good idea to pay attention to the color code of resistors that you use use.
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- Hold resistor so that the tolerance band is to the right. usually gold for a 4 band beige resistor, and usually brown (1%) for a blue resistor. Tip: Unfortunately both ends may be brown, hopefully the tolerance band is spaces further apart from the other bands.
- First (left) band is the first digit, in accordance with the color code below.
- Second band is second digit in the number. Tip: If the first band is brown (1), and the tolerance is brown (1%) then the second band probably won’t be red orange or yellow, which are more common as multipliers with resistors that start with 1 (brown).
- Third band: For Blue resistors it is normally the third digit. – For beige resistors, it is usually the multiplier. If it is a color in the 0-9 list below, which is usually is, then it is simply the number of zeroes that follow the digits. Some sources show multipliers as what you multiply the digits by, for example, black = x1, brown equals x10, red – x100 … etc.
- Forth Band: Usually the beige resistor’s tolerance. Tolerance is the percentage that the actual value of resistance can be expected to be higher or lower due to manufacturing imperfections. – Blue resistors usually have their multiplier indicated by the fourth band.
- Fifth band: Usually the tolerance of the Blue resistors. Beige resistors usually don’t have a 5th band.
- An extra band may indicate the temperature coefficient (not covered here). It will then be the last band, following the tolerance. Most people won’t deal with them.
Ohms law elements (plus power)
- Voltage – Unit: volt – symbol: V
- Resistance – Unit: Ohm – Symbols: R and Ω (the Greek letter Omega)
- Current – Unit: Amp – Symbols: I and A
- Power – Unit: Watt – Symbols W and P
- Main formula variations: I=V/R – R=V/I – V/IR – P=VI
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Calculating current (electric flow)
Below is a couple examples of Ohms law for current (I). Note that the current in low power electronic circuits is usually less than an Amp. Therefore the value in Amps is typically converted to milliamps (thousandths of an amp) after doing the Ohms law calculation. A milliamp (mA) is 1/1000 of an amp, and inversely, 1,000mA is the same as 1A
- 1V/100Ω = 0.01A (10mA)
- 1V/1000Ω = 0.001A (1mA)
- 5V/1,000Ω = 0.005A (5mA)
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Calculating power (heat generation)
After calculating the current through a resistive component, it is easy to calculate how much heat (power) it will generate. Of course, the component needs to be rated for that much power dissipation at a minimum, but it is recommended to limit power dissipation to half of a components rated value. Most resistors are 1/4W (0.250)
- 0.01A x 1V = 0.01W (10mW)
- 0.001A x 1V = 0.001W (1mW)
- 0.005A x 5V = 0.025W (25mW)
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Series Resistors/voltage dividers
Series Resistors (connected end to end) simply add up their resistance.
- 1KΩ resistor in series with another 1KΩ resistor, will provide 2KΩ of resistance.
- 1KΩ resistor in series with a 100Ω resistor will provide 1,100Ω of resistance.
When you put components in series, and then apply a voltage across the ends of those series components, then the voltage will be split up among the series components. Some components block a certain amount of voltage, but resistors divide up voltage based on their percent of resistance in the circuit.
- 2 equal value series resistors will have 1/2 of the supply voltage across each of them.
- 3 equal value series resistors will have 1/3 of the supply voltage across each of them.
- a 2KΩ resistor will have 2/3 of the supply voltage across it if it is in series with a 1KΩ resistor, which will have 1/3 of the supply voltage across it.
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