Ohm’s Law & DC Circuits

The Ohm’s Law & DC Circuits Simulator is a free, browser-based interactive tool for building and analysing direct-current circuits. Drag components from the palette — batteries, resistors, lamps, LEDs, fans, switches, ammeters, voltmeters — onto the canvas, wire them up by clicking ports or tapping anywhere along an existing wire, click a component to open a floating edit popover, and watch animated current flow in real time. It covers V = IR, Kirchhoff’s Laws (KCL and KVL), series and parallel resistance, voltage dividers, and power dissipation.

The simulator uses a full circuit solver (Modified Nodal Analysis — the same algorithm behind professional SPICE tools) so any topology you can draw will be solved, not just presets. Each node’s voltage is computed, each branch’s current is computed, and the results drive animated current dots and live voltage/current readouts on every component. Before energising, a fault-detection pass catches common wiring mistakes — shorts, conflicting batteries, ammeters across loads, duplicate wires, open loops — and flashes the offending parts red with an explanatory banner. Designed for electrical engineering students, physics learners, vocational trainees, and instructors.

Drag a component from the palette on the left onto the canvas. Start with a Battery, add one or more Resistors (or real-life loads like lamps and fans), and then click the round ports on each component to create wires between them. Click Run Circuit to energise — current dots flow along the wires, live readouts appear, and any wiring fault is highlighted in flashing red.

• Click a blank canvas point while wiring to drop a waypoint for cleaner routing.
• Tap anywhere along an existing wire to attach a new wire — a junction node is created automatically at the tap point.
• Use an explicit Junction (3-way or 4-way) component when you want a visible node.
• Load a Pre-Built Circuit pill to start from a ready example (single resistor, series, parallel, mixed, LED, Wheatstone bridge, two-loop KCL, etc.).
• Click any component to open a floating popover with a value input, unit selector, slider, and rotate / delete buttons — no side panel needed.

Simulate mode is the free-form workspace where you build any DC circuit. Available components:

• Battery: Adjustable 1–48 V DC source with clear + and – terminals. Multiple batteries supported — the solver handles series-aiding, series-opposing, and identical-parallel configurations.
• Resistor / Rheostat: Adjustable resistance with Ω / kΩ / MΩ unit selector.
• Lamp / Bulb: Incandescent load; brightness scales with P = V×I.
• LED: Diode with ~2 V forward drop; glows when forward-biased.
• Fan / Motor: Animated rotation proportional to current.
• Buzzer / Heater: Visual indication of power dissipation.
• Switch / Push Button: Click during simulation to toggle (open / closed).
• Ammeter: Reads branch current in series — wiring it in parallel is now caught as a fault before Run.
• Voltmeter: Reads node-to-node voltage in parallel — wiring it as the only return path is caught as an open-loop fault.
• Junction: 3- or 4-way wire splitter/combiner.
• Ground: Reference node (0 V).

The readouts show supply voltage, total current, equivalent resistance, total power, component count, and status. Verify KVL (voltage drops around a loop sum to 0) and KCL (currents at a junction sum to 0) by reading the on-canvas labels.

Click any component on the canvas to open a floating edit popover anchored right next to it. It lets you change the component without ever leaving the canvas:

• Value field — type an exact value (e.g. 470, 9, 2.2).
• Unit selector — pick Ω / kΩ / MΩ for resistors, V / kV / MV for batteries. The stored SI value updates automatically.
• Slider — drag for fast adjustment while watching readings update live.
• Rotate — cycle 0° → 90° → 180° → 270°.
• Delete — remove the component and all its wires.

The popover hides while you drag the component and re-appears at the new position on drop. Click elsewhere or press Esc to dismiss.

Hit Run Circuit on a broken wiring and the faulty components plus their wires flash red, and a red warning banner explains the issue in plain language. The engine runs two passes:

Pre-flight topology check — catches problems before the solver even runs:

• No battery — nothing to energise.
• Hard short across a battery — – terminal wired directly to + without any load.
• Disconnected battery terminal — an unwired + or – pole.
• Conflicting parallel batteries (KVL violation) — two batteries of different voltage wired directly in parallel.
• Ammeter in parallel with a load — a near-zero-Ω ammeter across a resistor shorts it out and in real life would blow the meter fuse.
• Duplicate wires between the same two ports — distorts readings.

Post-solve numerical check — catches problems only visible after solving:

• Open loop / no current — battery is live but no complete conductive path back (e.g. an open switch in the only loop).
• Voltmeter as the only return path — a voltmeter’s near-infinite resistance cannot carry load current; the hint tells you exactly this.
• Singular / ill-conditioned configurations — the solver aborts cleanly instead of returning nonsense.

The identical-parallel-batteries case is handled silently by the solver (treated as one source), not flagged as an error.

Annotation Toolbar: Move (select, drag, resize), Sketch (freehand with 6 colours & 3 widths; the pencil cursor stays active for continuous drawing), and Shapes (rectangle, circle, ellipse, line, arrow, double arrow, text label). Picking a shape from the dropdown activates it for a single placement. Click any annotation to select it — a dashed box with corner handles appears; drag inside to move, corners to resize (including text labels), use the action icons above to rotate / duplicate / delete. Double-click text to edit.

Zoom & Pan: Toolbar buttons or Ctrl = / Ctrl - / Ctrl 0 / Ctrl 1. Hold H or toggle the ✋ pan button for pan mode. Ctrl+Scroll zooms; pinch-to-zoom on touch.

Export & Toggle: The 📷 button exports the canvas as a PNG with watermark. The 👁 eye toggles annotation visibility. The 🧹 button clears annotations with a category chooser (all, sketches only, shapes & text only).

Fullscreen: Click the yellow fullscreen button (top-right) to expand the simulator to the full viewport — it changes to a red × while active so you can close it at a glance, or press Esc.

Explore provides structured content across Fundamentals (Ohm’s Law, current, voltage, resistance), Laws (KCL, KVL, power, series), and Components (parallel, voltage divider, current divider, conductance) — each with formula, description, and worked example.

Practice offers 12 randomised problem types on V=IR, series/parallel totals, dividers, and power, with step-by-step solutions.

Quiz gives 5 randomly selected questions from a pool of 15 MCQ + numeric questions with instant grading.

Circuit: Space = Run/Stop, Ctrl+Z = Undo, Ctrl+Shift+Z = Redo, R = Rotate, D = Duplicate, Delete/Backspace = Delete, Esc = Cancel / dismiss popover / exit fullscreen.

Zoom/Pan: Ctrl+= In, Ctrl+- Out, Ctrl+0 Reset, Ctrl+1 Fit, H Pan toggle, Esc Exit pan.

• Every circuit needs a closed loop from + to – of the battery — an open loop triggers the post-solve fault banner.
• Always put a series resistor with an LED to limit current (typical 150–470 Ω).
• Wire an Ammeter in series, a Voltmeter in parallel. The wrong orientation is now flagged before Run.
• Use the Rheostat or the popover slider to sweep resistance live while the simulation runs and watch the V / I response.
• Verify KVL: voltage drops across series resistors should sum to the supply voltage.
• Verify KCL: at any junction, incoming currents = outgoing currents.
• Match battery voltages exactly if you want to parallel them — conflicting voltages are caught as a KVL violation.