Voltage, current, resistance

This is probably the first page of electronics you have read. It does not assume anything. Nothing in it needs a board, a meter or a soldering iron, and there is one equation at the end of it.

The three words

One circuit, drawn twice — as wires, and as water, because the water is the version your intuition already has. Tap each word and watch which part of the picture it is.

Voltage (V, volts) is the push. It is always a difference between two points, never a property of one wire on its own. "3.3 V" is shorthand for 3.3 volts higher than something else, and that something else is nearly always the circuit's shared bottom rail, called ground. This is why touching one meter probe to one wire tells you nothing at all: you have measured a difference between a wire and mid-air.

Current (I, amps) is charge actually moving — the water going past. It is what lights an LED, what warms a resistor, and what kills a chip. An amp is a lot. Almost everything you build lives in milliamps, thousandths of an amp: 20 mA lights an LED comfortably, a Wi-Fi chip with its radio on pulls about 240 mA, and the same chip asleep pulls about 10 µA — ten millionths of an amp.

Resistance (R, ohms) is how hard the path fights back. Copper wire has almost none, which is the point of copper wire. A resistor has a lot, on purpose: its entire job is to be deliberately in the way, so that the current comes out where you chose rather than where physics would have put it.

One equation, and that is the lot

It is called Ohm's law, after a schoolmaster who found it with a thermocouple and a compass needle, and was called a fraud for publishing it.

The arithmetic is not the point. The point is that you only ever set two of the three. The supply fixes the voltage. You choose the resistor. The current is then decided for you, and it does not negotiate — if the sum says 180 mA through a chip's pin that is rated for 20 mA, the pin loses.

Move both sliders. Watch the number you did not touch.

What this buys you today

• A resistor is not optional on an LED. With nothing in the path, R is a couple of ohms of wire, and I is as much as the supply can find.
• "5 V" and "3.3 V" are pressures, not sizes. Both push through the same wire; the higher one just pushes harder, which is why the same circuit runs hotter on 5 V.
• Current is shared, voltage is spent. Follow one loop and the current is the same everywhere in it. The voltage gets used up across each part in turn.

The mistake almost everyone makes first

Reaching for a bigger power supply because something seems weak.

A 2 A supply does not push 2 A into your circuit. Amps are not delivered, they are drawn. The circuit takes whatever V ÷ R allows, and the supply's rating is only a promise about how much it could give before it gives up.

The opposite mistake is the one that costs parts. A supply is perfectly happy to deliver far more current than a component can survive, and nothing in the box will warn you first. That is the whole reason you do the sum.