Circuit Lab — Schematic Capture & Simulation
Circuit Lab at /circuitlab is UtilBloom’s browser Multisim-style workspace for drawing analog schematics—or wiring a virtual breadboard—and running DC, transient, AC phasor, Bode, and parameter-sweep analysis without installing SPICE or uploading netlists. Place resistors, capacitors, inductors, diodes, NPN BJTs, NMOS FETs, ideal op-amps, DC and AC sources, ground, and voltage probes on a grid, or drop jumpers, LEDs, and batteries on a half-size breadboard that compiles into the same MNA engine. Solve locally—including complex MNA for frequency response and Newton–Raphson for nonlinear DC/transient. Use it for homework and lab prep: RC steps, diode drops, inverting amplifiers, transistor stages, breadboard LED builds, and RC low-pass Bode curves, with an on-page scope (dual cursors) plus mag/phase and sweep plots. Catalog helpers such as Ohm’s law and RC calculators stay available for quick arithmetic; open this lab when you need a full schematic, breadboard practice, or analysis plot instead of a single formula.
What you can do
- Schematic capture on a grid
Drop R, L, C, diode, NPN, NMOS, op-amp, VDC, VAC, ground, and probe symbols, rotate parts, drag to reposition, and draw orthogonal wires that merge at shared grid points so Multisim-like netlists form automatically from the drawing.
- DC operating point
Run a modified nodal analysis DC bias: capacitors open, inductors short, AC sources at zero for bias, diodes and transistors via Newton–Raphson, ideal op-amps as high-gain VCVS, with probe voltages and source branch currents in the results panel.
- Transient analysis
Step through time with backward-Euler companion models for capacitors and inductors, re-linearizing nonlinear devices each step. Set tStop and dt, then plot every voltage probe as a waveform.
- AC and Bode
Complex MNA at a single frequency (magnitude and phase at each probe) or a log-spaced Bode sweep of |H| in dB and phase versus frequency—driven by a VAC stimulus for filters and transfer-function labs.
- Parameter sweep
Vary one component value over a linear or log range and re-run DC or AC at each point, plotting probe voltage or |V| against the swept parameter for what-if design checks.
- Oscilloscope chrome
Toggle channels, drag cursors A and B, and read Δt, implied frequency, and ΔV on the primary channel—enough Multisim-like instrument feedback for lab write-ups without a separate scope tool.
- Virtual breadboard
Switch to the Breadboard tab to place jumpers, resistors, LEDs, capacitors, buttons, and batteries on a half-size board. Strips and rails follow real solderless connectivity; Run DC compiles into the same MNA engine and lights LEDs from forward current.
- Example circuits
Load RC step, diode drop, inverting op-amp, NPN CE, NMOS inverter, RC low-pass Bode, or breadboard LED demos in one click, then edit values in the inspector. Schematics and boards persist in local storage on this device.
Worked examples
Series RC step toward 5 V
V1=5 V, R1=1 kΩ, C1=1 µF, probe on C
Open /circuitlab, leave the demo topology, choose Transient with tStop around 5 ms, and Run. The probe waveform should rise with τ = RC = 1 ms and settle near the DC source voltage.
Breadboard LED + resistor
5 V rail → 330 Ω → LED → GND
Open /circuitlab, choose Breadboard, load LED + resistor, and Run DC. The LED glow and probe voltage reflect the diode bias through Circuit Lab’s MNA.
RC low-pass Bode
Vac — R — probe/C — GND, fc ≈ 159 Hz
Load the RC low-pass sample (Analysis already set to Bode), Run, and confirm magnitude near −3 dB around 159 Hz with roll-off toward high frequency.
How it compares
Desktop NI Multisim, LTspice, and PSpice remain the standard for large component libraries, Monte Carlo, and PCB handoff. Circuit Lab is the UtilBloom answer when you need a free Multisim-like schematic plus virtual breadboard with DC, transient, AC, Bode, and sweeps in the browser without an install or account wall. It does not claim SPICE model-card parity, MCU co-sim, AC linearization of nonlinear devices, or Ultiboard PCB layout. For single-formula checks keep /engineering/ohms-law-calculator, /physics/rc-circuit-calculator, and /physics/rl-circuit-calculator; graduate here when the problem needs a drawn circuit, breadboard practice, or a plot. Pair with /scientific for numeric scratch work and /graph when you want to replot exported sample tables.
Questions
Is Circuit Lab the same as NI Multisim?
No. It is inspired by Multisim’s schematic-plus-instruments workflow but is an independent browser studio. Current coverage includes R, L, C, diode, NPN, NMOS, ideal op-amp, sources, ground, probes, a virtual breadboard mode, DC, transient, AC, Bode, parameter sweep, and scope cursors.
Which analyses and devices are available now?
DC operating point and transient (backward Euler) with Newton–Raphson for diodes and transistors; ideal op-amps as high-gain VCVS; AC phasor and Bode via complex MNA; DC/AC parameter sweeps; and a breadboard tab for jumpers, LEDs, and discretes that compile into the same MNA. Monte Carlo, MCU co-sim, and PCB layout are still planned later.
How does the virtual breadboard work?
Open the Breadboard tab, place parts on holes, and add jumpers. Rows a–e and f–j follow real solderless strips; power rails run the length of the board. Run DC compiles the layout into Circuit Lab’s netlist and lights LEDs from diode current. For Arduino sketches and MCU firmware use /arduinolab instead.
Why does my matrix say it is singular?
Every circuit needs a ground reference, a voltage source, and no floating islands disconnected from ground. Shorted ideal voltage sources or missing wires also produce singular stamps—check connectivity on the grid.
How are capacitors treated in DC versus AC?
In DC bias capacitors are open circuits and inductors are near-shorts. In transient, companion models integrate from rest. In AC/Bode, RLC use complex admittances at the chosen frequency (nonlinear devices are left open in the AC stamp).
Does my schematic leave the device?
Ordinary simulation stays in the browser tab and can persist to local storage on your machine. UtilBloom does not upload the netlist for Circuit Lab analysis runs.
Can I use this instead of LTspice for lab reports?
It is suitable for small teaching circuits and quick what-if edits. For manufacturer SPICE models, hierarchical designs, or formal lab toolchains, keep LTspice or Multisim as the course standard and use Circuit Lab for private sketching.