Free SPICE Simulator Online
Real ngspice, in your browser
Velxio runs the open-source ngspice engine compiled to WebAssembly. Real device models, full Modified Nodal Analysis, live transient simulation — and you can wire it to real Arduino / ESP32 / RP2040 firmware. Free, no install, no account.
Real ngspice · Free & open-source · Runs 100% in your browser
Why Velxio for SPICE?
Browser SPICE has historically meant either no SPICE at all (linear approximators) or hosted-only tools that need an account. Velxio runs the real engine, locally, for free.
Real ngspice
Same engine as professional EDA workflows, compiled to WebAssembly via eecircuit-engine. No simplified solver — full MNA, full device library.
No server, no account
The solver runs in your browser tab. Your circuits never leave your machine. Works offline after the first load.
Co-simulates with firmware
Arduino / ESP32 / RP2040 / ATtiny85 GPIOs drive SPICE sources, ADCs read solved node voltages. The unique combo no other in-browser tool offers.
Device library
Every component on the canvas maps to a real ngspice device card — not a behavioural stand-in.
Passives
Resistors, capacitors (electrolytic / ceramic / polarity-aware), inductors, potentiometers, photoresistors (LDR), photodiodes, NTC thermistors.
Diodes
1N4148, 1N4007, 1N5817 / 1N5819 (Schottky), Zener 1N4733 — with proper forward drop, reverse breakdown, and recovery.
BJTs
2N2222, 2N3055 (high-current NPN), 2N3906, BC547, BC557 — with cutoff, active region, and saturation behaviour.
MOSFETs
2N7000 (small-signal N-channel), IRF540 (power N-channel), IRF9540 (power P-channel), FQP27P06 — Level 3 model.
Op-amps
LM358, LM741, TL072, LM324, ideal — with realistic saturation rails so output clipping looks correct.
Linear regulators
7805, 7812, 7905, LM317 — with dropout voltage. Build a complete power supply on the canvas.
Logic ICs
74HC-series DIP-14 packages, basic gates (AND/OR/NAND/NOR/XOR/XNOR/NOT), edge-triggered D/T/JK flip-flops, SR latches.
Power-stage parts
L293D dual H-bridge, optocouplers (4N25, PC817 with CTR), SPDT relays with coil inductance, hysteresis, and flyback diode.
Sources & instruments
9V / AA / coin batteries with realistic ESR, signal generator (sine / square / DC), oscilloscope, voltmeter, ammeter.
Frequently Asked Questions
- Is this real ngspice running in my browser?
- Yes. Velxio loads ngspice compiled to WebAssembly via the open-source eecircuit-engine. The same SPICE engine that powers professional EDA workflows runs in your browser tab — no server, no install.
- What kind of analysis does it run?
- Real-time transient analysis. The solver runs Modified Nodal Analysis on every tick (~60 Hz) so you see waveforms evolve as the circuit runs. Non-linear devices iterate to convergence at every step.
- Which SPICE device models are supported?
- Resistors, capacitors, inductors, ideal voltage and current sources, diodes (1N4148, 1N4007, 1N5817, 1N5819, Zener 1N4733), BJTs (2N2222, 2N3055, 2N3906, BC547, BC557), MOSFETs (2N7000, IRF540, IRF9540, FQP27P06 — Level 3 model), op-amps (LM358, LM741, TL072, LM324, ideal) with saturation rails, linear regulators (7805, 7812, 7905, LM317), optocouplers (4N25, PC817), and relay coils with inductance.
- Can I write raw SPICE netlists?
- Velxio builds the netlist for you from the schematic — wire components on the canvas and the simulator generates the SPICE cards. You don’t have to handwrite netlists, but the underlying engine is full ngspice and accepts standard device cards.
- Does it include instruments?
- Yes — multi-channel oscilloscope, voltmeter, ammeter, and signal generator (sine / square / DC, configurable frequency, amplitude, offset). All instruments probe live SPICE node voltages and branch currents.
- Can SPICE talk to a microcontroller?
- Yes. This is the unique feature. Arduino / ESP32 / RP2040 / ATtiny85 GPIOs drive SPICE voltage sources; ADC inputs read solved node voltages. Build a real firmware-driven analog signal chain end-to-end in one tool.
Run real SPICE — no install, no account
Open the editor and start solving real circuits in seconds.
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