Resistors are a two terminal passive component that provide a specific amount of resistance to a circuit. They are the most common and versatile of all the components. Understanding what each resistor is doing in any given circuit is a requirement for understanding the circuit.
- 2 terminals: Wires or other connection points.
- Passive: The amount of current through the resistor is directly related to the voltage across it and it’s resistance (Ohms law).
An active component has adjustable conductivity. A transistor for example has the ability to adjust how much of the supply power is provided to a load which is controled by a signal to a different terminal.
Primary resistor uses:
Most of the time you will see resistors being used to….
- Limit current.
Current through a resistor is the voltage across it, divided by it’s resistance in ohms.
A current meter or multimeter that is set to measure current has virtually no resistance. It just measures the current without affecting it.
- Divide voltage.
When only resistors are connected in series, the voltage across them gets divided across them by each resistor’s percentage of the total resistance.
The voltages can be used as signals for circuitry that responds to that voltage. Signals provide as little current as possible.
Two equal value series resistors will each have half of the total supply voltage across them. Whereas three equal value series resistors will have one third of the total supply voltage across them.
- Pull up/down resistor: A mechanical switch has pretty much either zero resistance, or infinite resistance. While a switch is in series with a resistor whiles a voltage is across them, then when the switch is open, there will be no voltage difference across the resistor. The voltage on one end of the resistor will be the same as the voltage on the other end of the resistor. That can be used as an output voltage as long as the input circuitry that it is going to doesn’t take much if any current. Having to provide current drops/divides the voltage. Closing the switch provides a direct connection to the other supply voltage. That overrides any other voltage that was there while the switch was open.
- Diodes and some other components will drop a certain amount of voltage and then let current flow pretty much freely. The rest of the supply voltage (if it isn’t all been dropped) ends up across the resistance, which then sets the current.
the current through a resistance in series with different types of diodes is covered in more detail when you study those topics.
Resistors are sometimes used for:
As you study circuits, you will occasionally come across these uses for resistors.
- Biasing. – A voltage divider can set a baseline input voltage/current to an input before a signal is applied.
- Divide current. – Parallel resistors will all pass some of the total current.
If you need 24mA from 12V across a resistance, then a single 500Ω 1/4W (0.25W) resistor will get too hot. 12V/500Ω = 0.024A (24mA). Multiplying the voltage across the resistor by the current in amps through it gets us the wattage. 12V x 0.024A = 0.288W. Quite a bit hotter than the maximum of 0.25W it is rated for.
2 parallel 1,000Ω resistors with 12V across them however, will each pass 12mA of current. 12V/1000Ω = 0.012A (12mA). Multiplying the current across the resistor in amps by the voltage across it, we get 12V x 0.012A =0.144W. That is what needs to be dissipated by each resistor independently.
Those 2 currents then combine, where the resistors connect together, for a total of 24mA. The heat generated by each resistor will be 0.144W, which is slightly more than desired for a 0.25W resistor, but not too bad.
As mentioned before, a diode will drop some of the supply voltage from the other series components. You need to factor that in before getting an accurate calculation. That’s covered while studying diodes.
- Heating. – Usually heat from resistance is wasted energy and you avoid it as much as you can. You have to give components access to room temperature air, or add fans, heat sinks, etc. if need be, to dissipate their rated value. A component without ventilation or covered (such as by dust) will be more likely to overheat.
Electric heaters purposely use resistance to create desired heat.
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Through hole (wired) resistor components are usually close to cylindrical, but with a slightly narrowed middle. They also usually have colored bands that indicate their resistance value and tolerance.
Much higher wattage resistors are a lot larger than 1/4W resistors, and typically look different. They usually have their resistance value written on them and they often also have their wattage written on them as well.
On schematic diagrams, the resistor component is usually indicated by jagged lines or less commonly as a rectangle. A recommended resistance value is usually written next to it if it is jagged lines, or inside of it if a rectangle is used as a schematic symbol.
- Ω – Omega symbol used to indicate the resistance in Ohms.
- Ohms – Unit of resistance.
Most resistors are rated for 1/4W (0.25W). However, it is still recommended to stay below 0.125W (1/8W).
Unless a schematic diagram indicates another value, or you calculate that a higher wattage component is needed, it should be OK to use a 1/4W resistor.
Higher wattage resistors are larger in size, while lower wattage resistors are smaller in size. However, size may vary a bit, depending on what material it is made of and it’s shape.
How to read schematic diagrams 01 resistor component basics for DIY electronics quarter watt 1W 10W: Click to watch directly on YouTube.
Knowing the value of a resistor:
- Value typically listed somewhere on packaging:
- Multimeter/ohmmeter measurement:
Simply set the meter to measure resistance of a value more than you expect it to be, if the meter is not auto ranging. Make sure the resistor is not part of a circuit (is open on one or both ends), then connect the meter probes to both sides of the resistor and read the value displayed
- Reading it’s color code (will be added later).
Current through a resistor:
Electric current flows through power sources, components and connectors in a manner similar to how water flows through pumps, pipes and appliances. Water and electric don’t behave exactly the same of course, but while studying basic electronics, you will be presented with water analogies that help you imagine current flow through components and the entire circuit.
Ohms law : I = V/R
Ohms law states that the current (I) in amps, through a resistance, is equal to the voltage (V), in volts, across the resistance, divided by the resistance (R) in Ohms. I = V/R
In the diagram shown here, we have 9 volts across a 1,000 ohm resistor to get 9V/1,000 = 0.009A of current.
The current is calculated in amps while making the calculation. But, if the result is less than 1 amp of current, then the value is typically converted into milliamps. For example, 0.009A = 9mA. Which is pronounced as nine milliamps.
Without resistance, such as short circuiting a voltage source (battery), which should always be avoided, you will end up with the maximum current that the power source can provide. Many power sources can provide a dangerous amount of current.
Always avoid short circuits!
- For an ideal (impossibly perfect) voltage source: Any voltage divided by zero resistance equals an infinite amount of current. Ideal voltage sources are impossible to make, but manufacturers try to get as close as they can.
- Internal resistance (Battery): There is always some opposition to current within a battery. Alkaline batteries have more internal resistance than lithium batteries. Therefore it is less dangerous to short circuit an alkaline battery than a lithium battery that doesn’t have short circuit protection added to it.
In the real world, power supplies have internal resistance and their voltage drops when too much current is demanded of them. Any part of the circuit may be destroyed by high current, which is a dangerous situation. For added safety, many power supplies have short circuit protection circuitry that limits how much current they can provide.
A 9 volt alkaline battery will likely only provide 1 amp of current when short circuited. That means it has about 9V/1A = 9Ω of internal resistance.
Most of the power (wattage) in electronics is in the form of heat. Unfortunately, too much heat is what destroys electronic components. Waste heat is unavoidable in electronics, but should be limited as much as possible.
You can’t see heat unless you are using thermal imaging, something gets hot enough to glow, catch fire, or release the magic blue smoke of a plastic covered component melting and emitting smoke.
Therefore, to avoid damage and possible injury, it is necessary to be able to calculate component wattage, and to use components that are rated to exceed those wattage calculations.
Usually you want to use a component that is rated to handle at least twice as much wattage or current as is expected of it.
Calculating wattage/power: P = VI
Power in electronics is voltage times current. That gives us the formula P = VI . For resistors, or any other kind of individual component, simply take the voltage that is across it and multiply that voltage (in volts) by the current flowing through it (in amps). That will give you the power in watts that it needs to dissipate.
Topics to be added:
- Color code
- Variable resistor/Potentiometer/Trimmer potentiometer (trimpot).
- Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.
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