Circuit Inspector
File Inspector, but for circuits. Paste a SPICE-style netlist and Circuit Inspector runs real nodal analysis (DC operating point + AC frequency sweep), tells you what kind of circuit it is, solves every node voltage and branch current, computes the quick wins (cutoff, gain, divider ratio, resonance, LED current), draws the schematic and a Bode plot, flags faults — no ground, floating nodes, shorted sources, unprotected LEDs, op-amps with no feedback — and routes you to the right Utilities Bunker electronics tool.
How to Use
- Paste a netlist — one component per line: <code><Name> <node1> <node2> <value></code>. Node <code>0</code> (or <code>GND</code>) is ground. The first letter of the name is the type: <strong>R</strong>esistor, <strong>C</strong>apacitor, <strong>L</strong>inductor, <strong>V</strong>oltage source, <strong>I</strong>current source, <strong>D</strong>iode/LED, <strong>Q</strong> BJT (<code>Q1 c b e npn</code>), <strong>M</strong> MOSFET (<code>M1 d g s nmos</code>), <strong>X</strong>/OP op-amp (<code>Xop in+ in- out</code>).
- Values take SI suffixes: <code>1k</code> = 1 kΩ, <code>100n</code> = 100 nF, <code>4.7u</code> = 4.7 µF, <code>10m</code> = 10 mH, <code>2M</code> = 2 MΩ. Units after the prefix (<code>Ω F H V A</code>) are optional.
- Circuit Inspector instantly runs Modified Nodal Analysis: it names the <strong>circuit type</strong>, solves the <strong>node-voltage table</strong> and <strong>branch current / power table</strong>, computes <strong>quick wins</strong> (cutoff, gain, resonance, divider ratio, LED current), and draws the <strong>schematic</strong>. Add a capacitor or inductor and it sweeps the frequency response into a <strong>Bode plot</strong>.
- Read the <strong>Bunker Warnings</strong> — it catches a missing ground, floating/dangling nodes, shorted or conflicting sources, an LED with no current-limiting resistor, and an op-amp wired with no feedback.
- Click a <strong>Recommended Tool</strong> to open the matching dedicated calculator (often prefilled), or <strong>Copy report</strong> / <strong>Export JSON</strong> to keep the autopsy.
Paste a SPICE-style netlist. Circuit Inspector runs real nodal analysis — DC operating point and an AC frequency sweep — names the circuit, solves every node voltage and branch current, draws the schematic and Bode plot, flags faults, and routes you to the right dedicated tool.
One component per line: Name node1 node2 value, node 0 = ground. R/C/L/V/I/D and op-amps (Xop in+ in- out). It's a heuristic front-door analyzer — for a final design number, open the linked dedicated calculator.
What it inspects
The engine — Modified Nodal Analysis
Circuit Inspector is a real solver, not a lookup table. It builds the same system of equations SPICE does: Kirchhoff's current law at every node gives G·V = I, where G is the conductance matrix, V the unknown node voltages and I the source currents. Voltage sources, inductors and op-amps each add a branch-current unknown (that's the "modified" part). Gaussian elimination solves it exactly for the DC operating point; for frequency response the matrix is rebuilt with complex admittances — jωC for capacitors, 1/jωL for inductors — and swept across frequency to plot the Bode magnitude and locate the cutoff.
The Inspectors
Circuit Inspector is the third flagship in the Utilities Bunker "Inspector" family — the same DNA each time: take a whole, messy input, classify it, do the quick wins, warn on what's broken, and route to the right deep tool.
About the Circuit Inspector
Circuit Inspector is a quick, free tool for electronics and circuit design. It works in your browser and keeps everything on your device. File Inspector, but for circuits. Paste a SPICE-style netlist and Circuit Inspector runs real nodal analysis (DC operating point + AC frequency sweep), tells you what kind of circuit it is, solves every node voltage and branch current, computes the quick wins (cutoff, gain, divider ratio, resonance, LED current), draws the schematic and a Bode plot, flags faults — no ground, floating nodes, shorted sources, unprotected LEDs, op-amps with no feedback — and routes you to the right Utilities Bunker electronics tool.
How it works
Enter what you have and read the result as it updates live. It all runs on your own device, so it is quick and private, with nothing to install.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What is a netlist and what format do you accept?
A netlist is a text list of components and how they connect. Each line is <code>Name node1 node2 value</code> — for example <code>R1 in out 1k</code> is a 1 kΩ resistor between nodes <em>in</em> and <em>out</em>. Node <code>0</code> or <code>GND</code> is ground. It's a practical subset of SPICE: R, C, L, V, I, diodes/LEDs, BJTs (<code>Q</code>), MOSFETs (<code>M</code>) and ideal op-amps, with <code>k / M / m / u / n / p</code> engineering suffixes. Lines starting with <code>*</code> are comments; <code>.out node</code> picks the Bode output node.
How does it actually solve the circuit?
By <strong>Modified Nodal Analysis (MNA)</strong> — the same method SPICE uses. It builds the conductance matrix from your components, adds a current unknown for every voltage source and inductor, and solves the linear system by Gaussian elimination to get every node voltage and branch current. For frequency response it rebuilds the matrix with complex admittances (jωC for capacitors, 1/jωL for inductors) and sweeps ω to plot the Bode magnitude and find the −3 dB cutoff.
How are op-amps and diodes handled?
Ideal op-amps are modelled as <strong>nullors</strong> — a virtual short between the inputs plus an output that supplies whatever current the feedback demands. That's exact for any op-amp circuit with negative feedback (inverting, non-inverting, buffer, summing…). If there's no feedback path it's flagged as an open-loop comparator instead of being force-solved. Diodes and LEDs are <strong>recognised, not nonlinearly simulated</strong>: the tool detects the topology, computes the real LED/diode current analytically as (V<sub>supply</sub> − V<sub>f</sub>)/R, and routes you to the dedicated calculator. It never pretends to run a full nonlinear transient. <strong>Transistors work the same way</strong>: a BJT or MOSFET makes the circuit nonlinear, so Circuit Inspector identifies the stage (common-emitter, emitter follower, Darlington, current mirror, MOSFET switch…), estimates the bias with the standard hand-analysis approximations (V<sub>BE</sub> ≈ 0.7 V, base divider), and routes you to the exact transistor calculator — rather than force-solving a linear matrix that would be meaningless.
What faults can it catch?
Missing ground reference, floating / dangling nodes (a node touching only one component), a voltage source shorted to itself or two sources fighting in parallel, an LED or diode straight across a supply with no current-limiting resistor, an op-amp with no feedback, and a resistor dissipating more than a power rating you give it (e.g. <code>R1 a b 100 P=0.25</code>). A singular (unsolvable) circuit matrix is reported with its likely cause.
Is this the same as the individual electronics calculators?
It's the <strong>front door</strong> to them. Utilities Bunker has ~300 individual electronics calculators (voltage divider, RC filter, op-amp gain, LED resistor, Wheatstone bridge, LC resonance and so on). Circuit Inspector takes a <em>whole circuit</em>, works out which of those it is, solves it, and hands you off to the exact deep tool — prefilled where it can. Think of it as File Inspector and Math Inspector, but for electronics.
Does anything run on a server?
No. The netlist parser, the MNA solver, the complex AC sweep, the topology classifier, the fault detectors and the schematic/Bode renderers all run in your browser. Nothing you paste is uploaded, and it works offline.
How do I use the Circuit Inspector?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Check a design fast
Paste a netlist and get every node voltage, branch current and power in one shot.
Learn circuit analysis
See the DC operating point, the Bode plot and the working, not just a single number.
Catch wiring mistakes
Missing ground, floating nodes, shorted sources, unprotected LEDs, no-feedback op-amps.
Identify a circuit
Not sure what you built? It names the topology and its key parameters.
Find the right tool
Routes a whole circuit to the exact dedicated calculator, prefilled.
Frequency response
Drop in a cap or inductor and get the cutoff, filter type and Bode curve.
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