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How Circuits Work

Series vs. Parallel Circuits: Current, Voltage, Combined Resistance, and Power with LEDs

Chapter 2: Series and Parallel ·

This page sums up what you play through in ElecLab Chapter 2, "Series and Parallel", with diagrams and worked numbers. When you connect two or more LEDs, putting them in a single line or side by side changes how the current and the voltage are shared. You see the difference in the LEDs' brightness, which is their current.

1. Series: one path for the current, shared voltage

Connecting LEDs in a single line is called series. There's only one path, so every LED carries the same current, and identical LEDs are equally bright. In exchange, the LEDs share the battery's voltage, so you need enough voltage for all of them. Two red LEDs need a battery above 2V + 2V = 4V.

B14.3VL115.0 mAL215.0 mA−++−+−
Two LEDs in a line on 4.3V: 15 mA through each

Wiring example from 1-5

The current around a single loop can be worked out for the whole loop at once.

Current = (battery voltage − sum of LED voltages) ÷ total resistance

For the circuit in the figure, (4.3 − 2.0 × 2) ÷ (10 × 2) = 0.3 ÷ 20 = 0.015 A, or 15 mA (10 Ω is the internal resistance of an ElecLab LED). If the battery voltage doesn't reach the sum of the LED voltages, no current flows at all. Two red LEDs in series on a 3V battery both stay dark, because 3V is short of 4V.

2. Parallel: the same voltage, currents add up

Connecting LEDs side by side is called parallel. Every branch (each LED's path) gets the same voltage as the battery. The current leaving the battery is the sum of the branch currents. In the figure each LED carries 20 mA, and 40 mA leaves the battery.

B12.2VL120.0 mAL220.0 mA−++−+−
Two LEDs side by side on 2.2V: 20 mA through each

Wiring example from 1-5

In parallel, the usual rule is one resistor per LED. If two LEDs share one resistor, they split the current that one resistor lets through, so each is dimmer. On top of that, every real LED has a slightly different forward voltage, so a shared resistor lets one LED hog the current. (ElecLab's LEDs of the same color are exactly identical, so they split the current exactly in half.)

3. LED colors and forward voltage

The voltage at which an LED starts to light, its forward voltage, depends on its color. In ElecLab, red is 2.0V, yellow is 2.1V, and green and blue are 3.2V, which is roughly what real LEDs need too.

B12.2VL1Red 20.0 mAL2Green−++−+−
2.2V: red lights, green stays dark
B13.4VL1Green 20.0 mAL2Blue 20.0 mA−++−+−
3.4V: green and blue light too

Wiring example from 2-4

With the same battery and the same resistor, a green or blue LED leaves less voltage across the resistor, so it gets less current and looks dimmer. To match the brightness (current) across colors, work out a resistor for each color with the formula from Chapter 1. Example: for about 15 mA from a 9V battery, red needs (9 − 2) ÷ 0.015 ≈ 467 Ω, so 470 Ω, and green needs (9 − 3.2) ÷ 0.015 ≈ 387 Ω, so 390 Ω. ElecLab calculates 14.6 mA for red and 14.5 mA for green, almost the same brightness.

4. Combined resistance: series adds up, parallel gets smaller

Resistors in series are like a longer path, so their combined resistance is the sum.

Series: R = R1 + R2 + …

Resistors in parallel give the current more paths, so the combined resistance is smaller than any single one.

Parallel: 1/R = 1/R1 + 1/R2 + … (for two, R = R1 × R2 ÷ (R1 + R2))

With equal resistors, two in parallel give half the value and three give a third. For example, three 1 kΩ resistors in parallel make about 333 Ω. You can combine resistors to make a value you don't have.

B15VR11kΩR21kΩ−+
Series: 1 kΩ + 1 kΩ = 2 kΩ, so 2.5 mA flows
B15VR11kΩR21kΩ−+
Parallel: 1 kΩ ∥ 1 kΩ = 500 Ω, so 10 mA flows

Wiring example from 2-5

5. Power = voltage × current

The energy a battery delivers each second is called power, measured in watts (W).

Power P = voltage V × current I

In the parallel circuit from section 2, the battery delivers 2.2V × 40 mA = 88 mW. To light the same LEDs at the same current, parallel adds up one LED's current per LED at the battery, while series needs only one LED's worth. With the same battery voltage, series uses less power and the battery lasts longer (but series needs a higher voltage, one LED's worth per LED).

The voltage across a resistor times its current (the resistor's power) turns into heat instead of light. Keep it under the resistor's rating, which is 1/4 W = 250 mW for the small resistors most often used.

Common misconceptions

In parallel, is the battery's voltage split among the LEDs?
No. Every parallel branch gets the battery's full voltage. The voltage is shared only in series.
In series, is the LED closer to the battery's + brighter?
They're equally bright. A series loop is a single path, so every LED carries the same current.
Does adding a resistor in parallel make it harder for current to flow?
The opposite. More paths make the combined resistance smaller, so more current flows.
Does a higher battery voltage always make the LED brighter?
Brightness depends on the LED's current. With a high voltage but a large resistor, the current stays small and the LED stays dim. Raising only the voltage while keeping the same brightness just turns more power into heat in the resistor.

Try it in the Chapter 2 stages

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