The units of measurement in electronics are the way to quantify voltage, current, resistance, and power in a circuit. The four you will constantly encounter are the volt, the ampere, the ohm, and the watt.
In the electricity course, we covered the physics behind these quantities. Here, we will focus on how they are used when designing a circuit: which resistor prevents an LED from burning out, how much current a power supply must deliver, or what power rating a component needs to handle.
If you need to review the basics, you can go back to the Electricity Course.
Volts: The Operating Voltage
The volt (V) is the unit used to measure the voltage or potential difference between two points. In electronics, voltage determines both the power supply for a circuit and the levels used for signal exchange.
It is common to encounter several values:
- 5 V: Appears on many Arduino boards and USB devices.
- 3.3 V: Common in modern microcontrollers and sensors, such as the ESP32.
- 12 V or 24 V: Used in motors, lighting, and industrial automation.
Just because two circuits work doesn’t mean they can be connected directly. Applying a 5 V signal to an input that only supports 3.3 V can damage it.
Voltage is always measured between two points. Saying a node is at 5 V only makes sense if we know the reference point, usually GND.
Amperes: Current and Consumption
The ampere (A) measures the current intensity, i.e., the charge passing through a point in the circuit per unit of time.
In electronic circuits, we often work with small quantities, so we frequently use the milliampere:
A power supply does not force the entire current indicated on its label to flow. That value indicates the maximum current it can supply within its operating conditions; the load draws the current resulting from its own behavior and the applied voltage.
A 5 V, 3 A power supply can power a circuit that consumes 200 mA. What matters is that it delivers the correct voltage and has sufficient capacity.
This also doesn’t mean we can connect an LED directly to a powerful supply. An LED needs a component to limit its current because its behavior is not that of a fixed resistive load.
Ohms: Opposition to Current Flow
The ohm (Ω) is the unit of electrical resistance. Resistors allow us to limit current, divide voltages, set logic levels, and establish operating points.
Since standard values cover a huge range, we use prefixes:
1 kΩequals1000 Ω.1 MΩequals1,000,000 Ω.
The relationship between voltage, current, and resistance is given by Ohm’s law:
We can solve for any of the three quantities:
For example, if we apply 5 V to a 1 kΩ resistor, 5 mA will flow:
Watts: Power and Heat
The watt (W) is the unit of power, i.e., the rate at which a component delivers or transforms energy.
In a DC circuit, we can calculate it as:
Combining this expression with Ohm’s law gives us two other useful forms:
Power is not a theoretical detail. Part of it is usually converted into heat, so it determines the size and dissipation capability of the component.
If a resistor dissipates 0.5 W and we use a 0.25 W model, we will exceed its power rating. It will overheat and can be damaged.
It is advisable to leave a margin between the calculated power and the rated power. How much margin we need depends on the ambient temperature, ventilation, package type, and manufacturer recommendations.
Example: Resistor for an LED
Let’s use all four quantities in a simple circuit. We have a regulated 12 V power supply, a red LED with an approximate forward voltage drop of 2 V, and we want a current of 20 mA.
The resistor must drop the remaining voltage:
We calculate its value using Ohm’s law:
Since 500 Ω is not a common value in many standard series, we can choose 510 Ω. The actual current will be slightly below 20 mA.
Now we calculate the power the resistor will dissipate:
A 1/4 W resistor would be too close to its limit. A 1/2 W resistor offers a more reasonable margin and will operate at a lower temperature.
This calculation uses nominal values. In a real design, we would also verify the power supply tolerance, variation in the LED’s forward voltage, and temperature, because physical components rarely read our notes.
