Circuits and Ohm's law

Every circuit on a car comes down to three numbers. Know what each one means and you can make sense of any electrical fault, from a dim lamp to a fuse that keeps blowing.

• What voltage, current and resistance are, and the units they're measured in
• How Ohm's law ties them together
• How series, parallel and mixed circuits share voltage and current
• Why one bad earth can dim everything on it

Voltage is the push

A battery pushes current round a circuit the way a pump pushes water round a pipe. A car battery gives about 12 volts (V). Push on its own does nothing, though. The circuit has to be complete: a full loop from positive, through the load and back to negative.

Current is the flow

Current is how much electricity flows, measured in amps (A). A dash warning light takes a trickle, a headlamp takes a few amps and a starter motor takes hundreds. That's why the wires on a car come in such different sizes.

Resistance holds it back

Resistance is anything that holds current back, measured in ohms (Ω). A lamp's filament has a few ohms on purpose: that's where the work gets done. Good wires and connectors should be close to zero.

The three numbers are tied together by one rule: voltage equals current times resistance. V = I × R. The I stands for current. Know any two of the numbers and you can work out the third.

The triangle makes it easy to remember. Cover the one you want, and what's left tells you the sum. Side by side means multiply. One over the other means divide.

Change one, change the others

Our lamp circuit has 12 V and a 2 Ω lamp, so 12 ÷ 2 = 6 A. Double the resistance to 4 Ω and the current halves to 3 A. The lamp dims.

Now go the other way. If the resistance drops to almost nothing, the current shoots up. That's a short circuit, and it's what the fuse is for. The fuse is a thin link that melts and breaks the circuit before the wire can overheat.

In a series circuit the parts sit end to end on one single path. Current leaves the battery, goes through the first lamp, then the second, then back to the battery. There's nowhere else for it to go.

So break the path anywhere and everything stops. If one filament snaps, the other lamp goes out too, even though there's nothing wrong with it.

Same current, resistances add

With only one path, the current is the same at every point round the loop. To find it, add up every resistance first. Two 2 Ω lamps make 4 Ω, so 12 V ÷ 4 Ω = 3 A. That's half what one lamp drew on its own, which is why both glow dim.

The voltage is shared

The 12 V is shared between the parts. Two equal lamps get 6 V each. Swap lamp 2 for a 4 Ω lamp and the split becomes 4 V and 8 V: the bigger resistance takes the bigger share.

A single lamp circuit on a car is a series circuit: fuse, switch, lamp and earth, all on one path. A corroded earth joins that queue and takes a slice of the 12 V for itself.

When one brake lamp bulb blows, the other stays lit. If they were in series, both would go out. They're wired in parallel: side by side, each lamp on its own branch between a shared feed and a shared earth.

Full voltage on every branch

Each branch connects straight across the battery, so each lamp gets the full 12 V. A 2 Ω lamp draws 12 ÷ 2 = 6 A, exactly as if it were the only lamp in the circuit. That's why cars use parallel: full voltage, full brightness, and each part works on its own.

The feed carries the lot

The branch currents add up in the shared feed wire. Two lamps at 6 A each put 12 A in the feed. Three lamps put 18 A. More branches give the current more paths, so the total resistance goes down: 12 V ÷ 12 A = 1 Ω, less than either lamp on its own. The shared feed and its fuse have to be big enough for every branch at once.

Most car circuits are a mix. Take two tail lamps in parallel, with one shared earth wire taking both currents back to the battery. The two lamps are in parallel with each other. The earth wire is in series with the pair, because all the current has to pass through it.

Work from the inside out

There are two steps. First, replace each parallel group with the single resistance it adds up to. Then add up the series parts, the same as any series circuit.

Two rules for parallel

When the branches are equal, divide one branch by how many there are: two 2 Ω lamps make 2 ÷ 2 = 1 Ω. When there are two different branches, use product over sum: multiply them, then divide by their total. For 3 Ω and 6 Ω, that's 3 × 6 = 18, 3 + 6 = 9, and 18 ÷ 9 = 2 Ω. Check it: the answer is always less than the smallest branch.

In a healthy circuit, the pair of lamps adds up to 1 Ω and the earth is close to 0 Ω, so the total is about 1 Ω. That gives 12 A in the shared earth wire and 6 A through each lamp. Both are bright.

Now corrode the shared earth to 1 Ω, the same crusty joint from the volt drop video. The total becomes 1 + 1 = 2 Ω, so the current falls to 12 ÷ 2 = 6 A. The earth uses up 6 A × 1 Ω = 6 V of the push. That leaves the lamps only 6 V, so each draws 3 A and both dim together.

The clue

When every branch shows the same symptom, look at the part they share. When only one branch is affected, the fault you're looking for is in that branch.

Here are five symptoms from the workshop. Every circuit is the same: two tail lamps in parallel, a 10 A fuse on the shared feed and a shared earth. Pick the most likely cause, then see the circuit explain it.

Ten questions covering the whole module. You need 8 out of 10 (75%) to pass. Each answer comes with a short explanation.