Source: https://mokxi.com/learn/series-and-parallel-circuits Updated: 2026-09-27 Learn # Series and parallel: one current, or one voltage Written by the Mokxi team, updated September 27, 2026 Sign up free or see every lesson - 192 parts on the bench - 25 boards running now - 1.00x real time, on every board Click to open it in the editor Left: two LEDs in series. Middle: two in parallel on one resistor. Right: two in parallel with a resistor each. Every circuit, however tangled, is made of two ways of joining parts. In series, parts sit one after another in a single path, so the same current has to go through all of them. In parallel, parts sit side by side between the same two nodes, so they all have the same voltage across them. Almost every question about a circuit comes down to spotting which is which. The circuit above shows both with LEDs, because an LED tells you what it is getting by how brightly it glows. On the left, two red LEDs in series on one 100 ohm resistor. In the middle, a red and a blue LED in parallel sharing one 220 ohm resistor. On the right, the same red and blue LED in parallel with a 220 ohm resistor each. All three run off one 5 V bench supply. Press Run and look before you read on. Both LEDs on the left glow alike. In the middle only the red one lights. On the right both do. The rest of this page explains each of those three results with a number. Want the step-by-step version? The lesson "Branches in parallel" walks through this with checkpoints. Open the lesson ## Series: the same current through everything In a series loop there is nowhere else for charge to go, so whatever current leaves the supply goes through every part in turn and comes back. That is why both LEDs on the left are equally bright: they cannot be carrying different currents. The voltages add instead. A red LED drops about 1.73 V at this current, two of them drop about 3.46 V, and the resistor gets what is left: 5 - 3.46 = 1.54 V, and 1.54 V / 100 ohm = 15 mA. Resistances in series add too. A 100, a 220 and a 330 ohm resistor in a line are 650 ohms, and the current is the supply divided by that. Series has one weakness you will meet on a real bench: open the loop anywhere, a broken leg or a dead LED, and everything in it goes dark, which is the old string of string lights problem. It also runs out of voltage. Three blue LEDs in series want around 8 V before they conduct at all, and 5 V will not light them, however small the resistor. Series strings of LEDs are for supplies with voltage to spare, which is why 12 V LED strip is wired in groups of three with one resistor per group. ## Parallel: the same voltage across everything Parts in parallel all connect to the same two nodes, so they all have the same voltage across them, and the currents through them add. On the right each LED has its own resistor, so each branch is its own little series loop from the same 5 V. The red branch passes about 15 mA; the blue LED drops more voltage, leaving less for its resistor, so its branch passes about 10 mA. The supply provides the sum, about 25 mA. Resistors in parallel combine as 1 / R = 1 / R1 + 1 / R2, which for two resistors is R1 x R2 / (R1 + R2). Two equal resistors in parallel are half of one. A parallel combination is always smaller than the smallest resistor in it, because every branch you add is another path for current. 10 k in parallel with 1 k is 909 ohms, which is what happens to a voltage divider when you load it. ## Why two LEDs cannot share one resistor The middle circuit is the mistake almost everyone makes once: one resistor, a row of LEDs in parallel after it, and the expectation that they will share the current. They share the voltage, not the current. The red LED starts conducting at a lower voltage than the blue one, and the moment it does, it holds the shared node at about 1.72 V. The blue LED needs more like 2.6 V before it passes any real current, never gets it, and stays dark while the red one takes everything the resistor allows. With two LEDs of the same color it looks as if it works, and on a real bench it half does. No two LEDs have exactly the same forward voltage, so one takes more current than the other, gets warmer, and a warmer LED’s forward voltage falls, so it takes more still. Mokxi’s LEDs of one color are identical and have no temperature, so a pair of reds on one resistor will share evenly here in a way a bag of real ones will not. The fix is the right-hand circuit: one resistor per LED. ## Working out a mixed circuit Real circuits mix the two, and the way through is to collapse them one piece at a time. Take a 1 k resistor in series with two 2 k resistors in parallel, from 5 V. The parallel pair is 1 k, so the whole thing is 2 k, and the supply current is 5 V / 2 k = 2.5 mA. The 1 k drops 2.5 V, so the node between them is at 2.5 V, and each 2 k passes 2.5 V / 2 k = 1.25 mA, which adds back up to the 2.5 mA you started with. When the currents in and out of a node add up, you have not made a mistake. ## Common mistakes, and how to check yourself Measuring current with the meter across a part instead of in series with it is the one that costs a fuse. A meter on its amps range is a fraction of an ohm, so across anything it is a short circuit. Break the loop and put the meter in the gap. Mokxi’s meter has a fuse in its current range, and it will blow one if you get this wrong, which is a cheaper way to learn it than the bench. The other two: expecting parallel resistors to add up, when they always come out smaller, and running a series string of LEDs off a supply that cannot cover their drops. When a series string will not light, add up the forward voltages before you blame a part. ## What to try next In the editor, swap the blue LED in the middle for a second red one and watch the pair share. Add a third red LED to the series string on the left and see it go out: 5.4 V of drops on a 5 V supply. Then take the resistor sums further with the voltage divider page, which is two resistors in series doing something useful. ## Questions Is the current the same everywhere in a series circuit? Yes. A series loop has only one path, so the same current goes through every part in it. The voltage is what divides up between the parts, in proportion to how much each one drops. Do LEDs in parallel need their own resistors? Yes. Parallel LEDs share a voltage, not a current, and the one with the lowest forward voltage takes most of it. With different colors the other one may not light at all. Give each LED its own resistor. How do I work out resistors in parallel? For two resistors, multiply them and divide by their sum: 10 k and 1 k give 10 000 000 / 11 000 = 909 ohms. For more, add the reciprocals and take the reciprocal of the total. The answer is always less than the smallest resistor. What happens if one part in a series circuit fails? If it fails open, the whole loop stops: no current flows anywhere in it. If it fails short, the rest of the loop gets more voltage and more current than it was designed for. Related ## Keep going Ohm's Law, in a Circuit You Can Run Picking a Resistor for an LED Voltage Dividers and Why They Are Bad Power Supplies How an LED behaves in the simulator Measuring current with the multimeter ## Build this for real Open the editor, change a value and watch the number move with it. Nothing to install, and no account needed. Start building Open the editor