Voltage

Voltage is the electrical potential difference between two points. It tells you how much energy is transferred per unit of electric charge as charge moves between those points. Voltage is measured in volts (V), and it is central to understanding batteries, power supplies, sensors, microcontrollers, and almost every electronic circuit. This lesson explains voltage from first principles, shows how to measure and calculate it, distinguishes voltage from current, and introduces practical limits that matter when building circuits. The examples use low-voltage DC electronics; mains electricity is hazardous and should not be used for beginner experiments.

1. What voltage means

A voltage of one volt means one joule of energy is transferred per coulomb of charge. In symbols, voltage is energy per charge: V = W/Q, where W is energy in joules and Q is charge in coulombs. If two points differ by 5 V, a coulomb of charge moving through the relevant energy-transfer process corresponds to 5 joules. This definition is a useful physical foundation, even though you rarely calculate voltage from individual electrons in everyday circuit design.

Voltage is always a comparison between two points. Saying “this wire is 3.3 V” usually means it is approximately 3.3 V relative to the circuit’s ground reference. Ground in a battery-powered project is commonly the chosen zero-volt reference; it does not automatically mean earth ground or a protective-earth connection. A point can be at 3.3 V relative to one reference and a different voltage relative to another.

The word potential can sound abstract, but the practical idea is simple: a voltage difference can provide the conditions for current to flow through a connected path. Whether current actually flows, and how much, depends on the circuit and its components. An open switch can have voltage across it while carrying essentially no current.

2. Voltage, current, and resistance are different

Voltage, current, and resistance are related, but they are not interchangeable. Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω). Voltage describes a potential difference; current describes the rate of charge flow; resistance describes a component’s opposition to current under a particular set of conditions.

For an ideal fixed resistor, Ohm’s law states V = IR. If 1 kΩ is connected across 5 V, the expected current is 5 mA. The voltage is the condition across the resistor, while the current is the resulting flow through it. If you change the resistance or supply voltage, the current changes. This relationship is why a voltage source does not force the same current through every load.

A common misconception is that a 5 V supply always pushes 5 V “into” every connected part. In a series circuit, voltage is divided across components according to their behaviour. In a parallel circuit, each branch is connected across the same pair of nodes and ideally has the same voltage. Real wires, connectors, batteries, and supplies have non-zero resistance or internal impedance, so actual voltages may differ from ideal calculations under load.

3. DC voltage and AC voltage

Direct current (DC) circuits have a polarity that stays the same in normal operation. A battery is a familiar DC source. Its voltage may decline as it discharges, and it may sag when a load draws substantial current. Regulated DC power supplies are designed to maintain a target output over a specified range of loads and input conditions.

Alternating current (AC) changes with time and periodically reverses polarity in common sinusoidal systems. Household electricity is AC, but it is not appropriate for beginner breadboard experiments. AC voltage may be described using instantaneous, peak, peak-to-peak, or RMS values. For a pure sine wave, RMS voltage equals peak voltage divided by the square root of two. That relationship does not automatically apply to every waveform.

A multimeter’s AC mode has a specified frequency range and waveform accuracy. Some meters are true-RMS instruments, while others are calibrated for sine waves and can be inaccurate with distorted waveforms. Never assume that a reading is meaningful outside the meter’s ratings. For learning, use an isolated, low-voltage source designed for experiments, and keep AC mains equipment enclosed and untouched.

4. Voltage sources and ratings

Common voltage sources include alkaline batteries, rechargeable cells, USB supplies, bench power supplies, voltage regulators, and the regulated outputs of development boards. A source’s nominal voltage is not necessarily its exact voltage at every moment. Battery chemistry, state of charge, temperature, load current, and internal resistance can all affect the terminal voltage.

A power supply label usually specifies a target output and a maximum current capability. A 5 V, 2 A supply does not force 2 A into every circuit; a load draws the current determined by its electrical characteristics, up to the source’s limits. The 2 A figure is the supply’s rated capacity, not the current automatically delivered. A load that tries to draw more than the supply can provide may cause voltage sag, current limiting, shutdown, or overheating depending on the design.

Voltage compatibility is only one part of choosing a source. Check polarity, connector type, ripple, transient behaviour, current capacity, isolation where relevant, and the load’s permitted input range. Some boards accept 5 V at a designated input but use 3.3 V logic internally. That does not mean every pin can tolerate 5 V. Follow the exact board documentation rather than assuming that supply input ratings apply to GPIO pins.

5. Voltage in series and parallel circuits

In a series circuit, components share one current path. The sum of the voltage drops around a closed loop equals the source voltage for an ideal steady-state circuit, consistent with Kirchhoff’s Voltage Law. For example, if a 9 V source feeds two series resistors and one drops 3 V, the other must drop 6 V in the ideal loop. The split depends on the component values and any non-linear behaviour.

In a parallel circuit, each branch is connected to the same two nodes. Each ideal branch therefore has the same voltage as the source across those nodes. The branch currents can differ because the loads differ. A 1 kΩ resistor and a 2 kΩ resistor placed in parallel across 5 V each have 5 V across them; their ideal currents are 5 mA and 2.5 mA respectively. The source must provide the sum of the branch currents, plus any other circuit demand.

Voltage is also lost across non-ideal wires, connectors, switches, and protection devices when current flows. If a circuit works on a bench but fails at the far end of a long cable, measure the voltage at the load while it is operating. The supply may read correctly at its terminals while wiring resistance causes a lower voltage at the device.

6. How to measure voltage with a multimeter

For voltage measurement, connect the meter probes in parallel with the two points whose potential difference you want to know. Put the black lead in COM and the red lead in the voltage/ohms socket, then select the appropriate DC or AC voltage mode. Use a range that covers the expected value and check the meter’s maximum input rating. With an auto-ranging meter, still verify that the selected function is correct.

For a simple DC circuit, place the black probe on the chosen reference point and the red probe on the point being measured. A positive reading means the red probe is at a higher potential than the black probe; a negative reading means the opposite. Reversing the probes changes the sign, not the physical voltage difference. A circuit’s ground reference is a measurement convention and should not be confused with a safe connection to earth.

A voltage meter has high input resistance so it draws relatively little current from the circuit, but the reading can still be affected by poor probe contact, noisy signals, floating nodes, or the meter’s limitations. An unconnected wire can show a changing or misleading voltage due to capacitive coupling or the high input impedance of the meter. Confirm the circuit reference and measurement conditions before concluding that the node is driven strongly.

Do not move the red lead to a current socket and then probe a supply as if measuring voltage. Before measuring, inspect the lead sockets and meter mode. Do not probe exposed mains terminals; use qualified professionals and properly rated instruments for mains work.

7. Voltage in Arduino and ESP32 projects

Microcontroller projects commonly use a regulated logic supply, such as 3.3 V or 5 V, but the exact permitted voltage depends on the board and pin. Arduino Uno-class boards commonly use 5 V logic, while many ESP32-family GPIO pins use 3.3 V logic and are not generally 5 V tolerant. Board revisions and individual chip datasheets matter, so verify the exact model before wiring.

Suppose an ESP32 GPIO output is at approximately 3.3 V when high. That is a logic-level output, not a general-purpose power supply for motors, relay coils, or other high-current loads. A GPIO pin has current and voltage limits. Use an appropriate transistor or driver for loads that exceed the pin’s capability, provide the required flyback protection for inductive loads, and ensure the power supply and grounding arrangement match the design.

Sensor modules also need voltage checks. Some modules accept a broad supply range but output signals at their supply voltage; others include level shifting or regulators, and some do not. Before connecting a sensor’s output to a microcontroller, check its datasheet or board schematic. Never assume that a module is safe for a 3.3 V GPIO merely because it can be powered from 5 V.

If readings are unstable, measure the voltage directly at the board and at the sensor while the circuit is active. A supply that appears correct without a load may dip during Wi-Fi transmission, motor startup, or relay switching. Local decoupling, correct wiring, and a supply sized for peak current can help, but diagnose the cause instead of adding capacitors blindly.

8. Voltage drop and power

Voltage drop is the potential difference across a component or conductor when the circuit operates. In a resistor, the drop relates to current through Ohm’s law. In a cable, even a small resistance can matter at high current: the lost voltage is I × R, and the heat generated is I²R. This is why power wiring for motors or LED strips needs appropriate conductor size, connectors, and current capacity.

Power is the rate of energy transfer. For DC, power is commonly calculated as P = VI when voltage and current refer to the same component and the sign convention is understood. For a resistor, P = I²R and P = V²/R are useful rearrangements. If a resistor has 5 V across it and carries 10 mA, it dissipates 0.05 W. Select a suitable power rating with margin and consider temperature and enclosure conditions.

Voltage drop is not always a defect. A resistor is intentionally used to create a drop, and a regulator intentionally maintains an output within its operating range. The key is whether the drop is expected and within specifications. When a device resets or behaves erratically, measure at its supply pins during the fault; a reading taken elsewhere or with no load may miss the problem.

9. Common mistakes and troubleshooting

Confusing supply voltage with logic voltage: a board powered from 5 V may still have 3.3 V GPIO. Forgetting the reference point: a voltage reading requires two points. Ignoring polarity: electrolytic capacitors, diodes, and many modules must be connected with correct polarity. Measuring only at the source: wiring losses can reduce voltage at the load. Treating a supply current rating as forced current: the load determines its demand within the source’s capability. Using the wrong meter mode: this can produce incorrect readings or create a short circuit.

If a reading is unexpected, first verify the meter mode and lead sockets. Then check the reference node, probe contact, supply polarity, and whether the circuit is powered as intended. Measure both at the source and at the load under operating conditions. Compare the value with the device’s documented input range, not just with a generic “5 V” or “3.3 V” label.

For intermittent resets, monitor the voltage during the event if the meter has a min/max function, or use an oscilloscope when fast dips matter. A standard multimeter may average or miss brief transients. Inspect ground connections, cable resistance, regulator thermal limits, and peak current demand. Make one change at a time and record what changes so that troubleshooting remains evidence-based.

10. Practice questions

1. A battery measures 8.7 V between its terminals. What does that tell you? It tells you the measured potential difference under the measurement conditions; it does not by itself reveal the battery’s remaining capacity or ability to supply a high current.

2. A 5 V source is connected across a 1 kΩ resistor. What current is expected? Using Ohm’s law, I = 5/1,000 = 0.005 A, or 5 mA.

3. A sensor needs 3.3 V, but the available supply is 5 V. Can you connect it directly? Only if the sensor’s documented supply range and signal interfaces permit it. Otherwise use a suitable regulator or interface; do not infer compatibility from connector fit.

4. Why might a microcontroller reset even if the supply reads 5 V when idle? The voltage may dip briefly or at the board under peak load, or there may be a wiring, grounding, regulator, or current-capacity problem.

5. What does a negative multimeter voltage reading mean? With the chosen probe orientation, the red probe is at a lower potential than the black probe. It does not necessarily indicate a fault.

11. Key takeaways and next steps

Voltage is a potential difference measured between two points. It can be DC or AC, and its meaning depends on the reference point, waveform, and measurement method. Voltage influences current through a circuit’s components, but it does not determine current on its own. Check supply and signal limits separately, measure at the load when diagnosing problems, and use component datasheets to confirm safe operating ranges.

Continue with Current, Resistance, Ohm’s Law, and Power to connect the basic quantities. Then study Series Circuit and Parallel Circuit to understand how voltage is distributed. Using a Multimeter is the practical companion to this lesson. When moving into Arduino or ESP32 builds, check the exact board’s pin voltage limits and treat GPIO as a signal interface rather than a power source for heavy loads.

Safety note: this lesson is educational and focuses on low-voltage circuits. Household mains voltage can cause fatal electric shock and fire. Do not open, probe, or modify mains-powered equipment; seek a qualified electrician for mains installations and repairs.