Electronics: understand, build and measure 5 V circuits

This course uses small 5 V direct-current circuits, powered by a commercially enclosed, isolated, regulated supply with an adjustable current limit. For the proposed indicator, set 5.0 V and a 20 mA limit according to the supply manual, keeping its output off before connecting the build. Do not open the supply. A computer USB port is not a substitute for this bench current limit.

A supply labelled “5 V, 2 A” does not necessarily push 2 A through an LED: the load determines current while the supply maintains voltage. However, it can deliver enough current to heat a shorted wire. The resistor limits branch current; the supply limit helps when wiring is wrong. Neither replaces the other, and protection response is not instantaneous in every supply.

Before applying power, identify positive, return and polarised components. Turn the output off before moving wires. This project does not use exposed mains, high-current batteries, motors or opened appliances. If you do not have a suitable supply yet, you can study the circuit in simulation and calculate its results.

Voltage is a potential difference between two points, measured in volts (V). Current is charge flow per unit time through a branch, measured in amperes (A). In this example, conventional current goes from supply positive through the resistor and LED to the negative return. The LED converts some energy into light and heat; it does not “use up the current” and leave the return wire with none.

An interrupted path can have voltage across its ends with practically no current. A short creates a very low-resistance path bypassing the intended load. “The LED went dark” fits different situations: a loose wire, reversed LED or a path around it. Appearance alone cannot identify the cause.

We call the return GND, the 0 V reference. This does not necessarily mean protective earth at a wall socket. This isolated circuit uses one reference; do not connect other equipment or supplies merely because they also have a terminal called GND. First understand the connections those devices already impose.