Online circuit simulator for DC circuits

An online circuit simulator lets you build an electric circuit on screen and see the currents and voltages in it without touching a wire. Our online circuit simulator, the Circuit Builder, works with direct current (DC). Place cells, resistors, lamps, LEDs, switches, sensors and meters on a grid and it solves the circuit as you build, giving the voltage, current and power in every part and warning you about short circuits, open loops and parts pushed past their limits. Five guided circuits go further, from resistor networks to capacitors charging.

Try it

Circuit Builder

Build any DC circuit with meters, lamps, LEDs and sensors, and see live readings

Open the simulatorGo to its page

What the circuit simulator does

It has two modes, chosen at the top of the controls:

• Build a circuit: a free grid. The parts are cells (with an optional internal resistance), resistors, variable resistors, filament lamps, LEDs, switches, thermistors, LDRs, ammeters and voltmeters. Meters read live, lamps and LEDs glow, and selecting a part shows its voltage, current and power, with a graph of its I-V characteristic and its working point marked.
• Guided circuits: five set-ups with inputs, worked formulas and graphs. Resistor network (series branches in parallel), LED driver (it picks a standard resistor), Voltage divider (with a thermistor, LDR or a load), EMF and internal resistance, and RC timing (a capacitor charging or discharging).

Every mode has practice questions with worked solutions, 22 kinds in all, and you can share a circuit as a link.

How to use the online circuit simulator

1. Open the Circuit Builder. It starts in Build a circuit with a cell, switch, ammeter, lamp and voltmeter already wired.
2. Pick a part from the parts row and click a gap between two dots to place it. Choose Wire and drag between dots to connect parts, and Erase to remove them.
3. Choose Select and click a part to set its values and see its readings. Click a switch to open or close it.
4. Read the table of every part, the meters on the grid and any warnings below it.
5. To try a set-up, open Load an example, or switch to Guided circuits and pick one. For questions, press Random problem.

The breadboard simulator guide goes through building an LED circuit and a light sensor step by step, and how to copy a circuit from the grid onto a real breadboard.

How the simulator solves a circuit

It uses a method called nodal analysis. Every junction where wires meet is a node. Kirchhoff's current law says the current flowing into each node equals the current flowing out, and Ohm's law, I = V ÷ R, gives the current through each part from the voltages at its ends. That makes one equation per node, and the simulator solves them all at once. Lamps and LEDs don't have a fixed resistance: a lamp's rises as it heats, and an LED only conducts one way. So it solves, adjusts them, and solves again until the answers settle.

Worked examples you can check in the simulator

Example 1: lamps in series and in parallel

Load Lamps in series: a 6 V cell and two lamps rated 3 V, 1.5 W. Each lamp gets 3 V and 0.5 A: in series the lamps share the voltage and carry the same current. Now load Lamps in parallel: a 3 V cell and the same two lamps. Each gets the full 3 V and 0.5 A, and the ammeter by the cell reads 1 A, the sum of the two branch currents.

Example 2: a resistor network

Switch to Guided circuits and choose Resistor network. It opens on a 5 V supply with a 220 Ω and a 330 Ω resistor in series in one branch, and 1,000 Ω in another. The branch in series adds to 550 Ω, then the two branches combine in parallel:

The 220 Ω resistor gets 2 V and the 330 Ω resistor 3 V, both carrying 9.09 mA, while the 1,000 Ω branch takes the full 5 V and 5 mA. Raise the supply and resistors past their power rating are flagged Over rating.

Example 3: EMF and internal resistance

In EMF and internal resistance, set a 1.5 V cell with an internal resistance of 0.5 Ω and a 2 Ω load. The current is 1.5 ÷ (2 + 0.5) = 0.6 A. The cell loses 0.6 × 0.5 = 0.3 V inside itself, the lost volts, so the terminal pd across the load is 1.5 − 0.3 = 1.2 V. The graph of terminal pd against current is a falling straight line whose slope is −r.

Example 4: a capacitor charging

In RC timing, a 5 V supply charges a 100 µF capacitor through 10 kΩ. The time constant is RC = 10,000 × 0.0001 = 1 s. Set the elapsed time to 1 s: the capacitor is at 3.16 V, 63.2% of the supply. At 5 s, five time constants, it is at 4.97 V, almost full.

Here x is the time in seconds and y is the voltage across the capacitor. It rises quickly at first, then more and more slowly as it approaches 5 V.

Example 5: loading a voltage divider

In Voltage divider, 10 kΩ over 20 kΩ on 5 V gives an output of 5 × 20 ÷ 30 = 3.33 V. Tick the load box and add 10 kΩ across the output. The bottom half becomes 20 kΩ in parallel with 10 kΩ, which is 6.67 kΩ, and the output drops to 2.00 V. That is why a divider can scale a signal but can't power anything that draws much current.

Example 6: a lamp's I-V characteristic

Load Lamp I-V experiment: a 12 V supply, a variable resistor, an ammeter, and a lamp rated 12 V, 6 W, with a voltmeter across it. The lamp reads 4.72 V and 0.364 A, so its resistance is 4.72 ÷ 0.364 = 12.9 Ω. Select the lamp to see its I-V curve. Drag the variable resistor's slider down and the lamp's current rises, but its resistance climbs towards 24 Ω at full brightness, because the hot filament resists more. The curve bends over: a filament lamp isn't ohmic.

Read a resistance from an I-V graph, then press Mark to check your answer in the Electricity course.

What the simulator doesn't do

• AC circuits. Everything is direct current: no alternating supplies, transformers or frequency.
• Transistors, diodes other than LEDs, chips and logic gates.
• Capacitors and inductors in your own circuits. Capacitors appear only in the RC timing guided circuit.
• Wire resistance. Wires and meters are ideal, and resistor tolerance and temperature effects aren't modelled, except in the lamp, thermistor and LDR.

Common mistakes

• An ammeter in parallel. It has almost no resistance, so it shorts out what it is across. Put it in series.
• A voltmeter in series. Its huge resistance stops the current. Put it across the part.
• An open loop. Current only flows round a complete path from the + terminal back to the −. The simulator tells you when there isn't one.
• A wire straight across a cell. That is a short circuit: the simulator refuses to give a current and says why.
• Adding parallel resistors like series ones. Parallel resistance is always less than the smallest branch.

Read next

Physics · Electricity and magnetismBreadboard simulator: test a circuit before you build itTest a breadboard circuit online before you build it: place LEDs, resistors, switches and sensors, read live voltage and current, and spot mistakes first.