# Resistor limiting current – Ohms law

The primary thing that resistors do is limit current (moving charges). The current (I) in amps through a resistor is the voltage (V) across the resistor divided by the resistor’s resistance (R) in ohms. I = V/R

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Always keep in mind that while passing current, resistors get hot. The recommended maximum wattage rating of most resistors is 1/4W, but it is still recommended to stay below 1/8W as much as possible.  Calculating the wattage (P for power) in watts, of any component, involves taking the voltage (in volts) across it and multiplying that by the current (I) (in amps) flowing through it. P = VI

## Resistor measurements:

Usually schematic diagrams will show a safe resistor value to use.

• I = V/R (current equals voltage divided by resistance). Ohms law formula for current from a certain voltage and resistance.
• Voltage (V) across the resistor.
• Current (I) flows through the resistor.
• Current has a linear relationship (straight line on a graph) with resistance for a given voltage. Assuming that the voltage stays the same. Using half the resistance will double the current. Twice the resistance will result in half the current.

The relationship between voltage, resistance and current is known as Ohms law. Here are the 3 main Ohms law formulas.

### Common values diagram:

• The three uneven length lines on the right side of the resistor indicates 0V ground (negative side of a DC supply).
• The voltage on the left side of the resistor indicates the supply voltage (positive side of DC supply).
• The supply voltage (Vcc) and 0V ground is the voltage difference across the resistor.  So 5 volts across the resistor is considered to have +5V on one side and 0V (ground) on the other side.
• 1/4W (0.25W) maximum is a common value resistor. It is still recommended to keep them below 1/8W (0.125).
• Be aware that any other components in series with the resistor will take away some of the voltage from the resistor. Semiconductors tend to drop a certain amount of voltage while resistive components divide up the voltage based on their percentage of the total resistance.

#### Detailed video

Clicking the links to watch videos directly on YouTube helps a lot!

##### Diagram used in video

If you really like this video/diagram format, here are some links to the next topics of the series.

##### Related topics:

Other basic electronics topics that you should know before moving on to more advanced topics.

555 timer is an integrated circuit (IC). Being an IC, it has complex circuitry combined in a single package with external pins/terminals to connect to other circuitry. You can easily make all kinds of fun circuits with just a 555 timer and the components covered above, so I think it’s a good component to learn next.

Transistors will probably be the most challenging components to learn. Understanding them will help you understand all of electronics much better, and help you the most in being creative while designing your own circuits.

Other topics:

These pages are still being compiled.

Circuits covered more quickly series:

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