DC Circuit Simulator
Build a direct-current circuit on a grid, flip switches and watch the current move. Every change is solved instantly with modified nodal analysis, so ammeters, voltmeters, bulb brightness and the parts table always show the exact voltage, current and power.
Last updated:
Pick a part, then drag from one grid dot to another to place it. With Select, click a switch to flip it or drag a part to move it.
| Part | Type | Value | Voltage | Current | Power |
|---|
How to use the DC Circuit Simulator
- Start from an example such as bulbs in parallel or a voltage divider, or choose Blank board to draw your own.
- Pick a part in the toolbar and drag from one grid dot to another to place it. Draw a battery from its negative end to its positive end.
- Connect everything with wires. Wires join at their ends and wherever another part ends on them.
- Click a part with Select to change its resistance, voltage or bulb rating, flip it or delete it. Clicking a switch opens or closes it.
- Read the results from the meters on the board, the summary panel and the parts table, then download a PNG or save the circuit file.
The rules the simulator solves
Every DC circuit on the board obeys three laws, and the simulator writes them as one system of linear equations:
- Ohm's law for each resistor and bulb:
V = I × R, so the current through a part is the voltage across it divided by its resistance. - Kirchhoff's current law at each connection point: the currents flowing in equal the currents flowing out.
- Kirchhoff's voltage law via each battery: the voltage between its terminals is
E − I × r, whereris the internal resistance.
The negative terminal of the first battery is taken as 0 V (marked on the board), and all node voltages are measured from there. Power is P = V × I = I² × R; a positive power on a battery means it is delivering energy, a negative one means another battery is charging it.
Series vs. parallel at a glance
| Series | Parallel | |
|---|---|---|
| Current | Same through every part | Splits between branches; branch currents add up |
| Voltage | Divides between parts; drops add up to the source | Same across every branch |
| Total resistance | R = R1 + R2 + … | 1/R = 1/R1 + 1/R2 + … |
| One bulb removed or open | Everything goes out | Other branches stay lit |
| Try it | Example “Series resistors”: 9 V ÷ 650 Ω ≈ 13.8 mA | Example “Bulbs in parallel”: open the switch or delete a bulb |
Experiments to try
- Voltage divider: the output is
Vin × R2 ÷ (R1 + R2). With 12 V, 10 kΩ and 4.7 kΩ the voltmeter reads about 3.84 V. Add a 4.7 kΩ “load” across R2 and watch the output sag. - Balance the bridge: in the Wheatstone bridge example, change the 1.2 kΩ resistor to 1 kΩ. The ammeter drops to zero when
R1/R3 = R2/R4. - Batteries in parallel: a 9 V and a 6 V cell with 1 Ω internal resistance each feed a 10 Ω load. The 6 V battery ends up being charged, which is why mismatched cells should never be paralleled.
- Maximum power: give a battery an internal resistance and vary a load resistor; the load gets the most power when its resistance equals the internal resistance.
Frequently asked questions
How does the circuit simulator calculate the currents?
It uses modified nodal analysis, the same method used by SPICE-style simulators. Kirchhoff's current law is written for every connection point, Ohm's law for every resistor and bulb, and one extra equation for every battery. The resulting linear system is solved exactly each time you change something.
Which way do the moving dots go?
By default the dots show conventional current, flowing out of the battery's positive terminal and back into the negative terminal. Tick Electron flow to see the electrons, which move the opposite way. The dot speed grows with the size of the current on a logarithmic scale so both milliamps and amps are visible.
How are bulbs modelled?
Each bulb is a fixed resistance calculated from its rating, R = V² ÷ P, so a 6 V 3 W bulb is 12 Ω. Its brightness on the board follows the power it actually receives compared with its rated power. Real filaments have lower resistance when cold, so the simulator is a good approximation once the bulb is lit.
Are the ammeter and voltmeter ideal?
Yes. The ammeter behaves like a wire with practically zero resistance, and the voltmeter draws no current at all. Place an ammeter in series with the part you want to measure and a voltmeter across it, in parallel. The voltmeter shows the voltage at its + end minus the voltage at its other end.
What happens if I short-circuit a battery?
Wires are modelled with a tiny resistance, so a battery connected only through wire produces a huge current. The battery turns red and a warning appears. Set an internal resistance on the battery to see a realistic short-circuit current, which is the battery voltage divided by that internal resistance.
Can it simulate capacitors, diodes or AC?
No. This is a steady-state direct-current simulator for resistive circuits with batteries, bulbs, switches and meters, which covers series, parallel, voltage divider and bridge circuits used in school physics and basic electronics.
Is my circuit saved?
Your current board is stored only in this browser so it is still there when you return. Use Download circuit to keep a file you can open again later or share, and Download PNG to get a picture of the schematic.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.