Current through a resistor – Learning electronics lesson 0001

Whenever you see a resistor in a circuit, it is important to be able to estimate how much current is going through it to help make sure that none of the components overheat, including the resistor.

Current through a resistor illustrated and schematic with calculations diagram by electronzap
Current through a resistor illustrated and schematic with calculations diagram by electronzap

Video:

Review of calculating current through a resistor using ohms law for DIY learning electronics

Measuring current with a multimeter.

It is possible to measure the current with a multimeter even though usually you should calculate it with Ohms law instead. Ohms law is explained further down on this page.

Measuring current with a multimeter schematic diagrams by electronzap
Measuring current with a multimeter schematic diagrams by electronzap
  • Open (disconnect part of) the circuit if there’s not already an open point you can take advantage of, such as an open switch.
  • Set the meter to measure more current than you expect there to be. It is always bad to have more current flow through a meter than what is set to measure. You will likely blow a fuse in the meter and it will have to be replaced.
  • Connect the probes to each end of the open circuit to complete the circuit through the meter.

Don’t measure the voltage of a power source directly! There are some weak power source exceptions, such as low power solar cells. 

Quick how to measure current with a multimeter DIY learning electronics LED circuit tutorial

Ohms law

Ideally, you will want to be able to use Ohms law to calculate the (I) current through the resistor, based on the voltage (V) across it in volts, divided by it’s resistance (R) in ohms (Ω).

I = V/R

Ohms law voltage current resistance circle graph diagram by electronzap electronzapdotcom
Ohms law voltage current resistance circle graph diagram by electronzap electronzapdotcom
Power

Resistors get hot as current flows through them. Higher value resistors fight the passage of current more,  therefore it takes more voltage to push the current through them and they generate more heat than a lower value resistor passing the same current.

Keep a 1/4W (0.25W) resistor below 1/8W (0.125W). Covered in more detail below.

Quarter watt resistors heat dissipation at a quarter watt and an eighth of a watt of power seen in thermal by electronzap and electronzapdotcom
Quarter watt resistors heat dissipation at a quarter watt and an eighth of a watt of power seen in thermal

Ultimately the heat generated (P for power) for a resistance based component is calculated by taking the voltage (V) across the component and multiplying that voltage by the current (I) through the component.

P = VI

Most resistors are rated to dissipate a maximum wattage of 1/4 watt (0.25W), however they should still be kept below 1/8W (0.125W), and given direct access to room temperature air to make sure they don’t get too hot.

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