Circuit lab
Open, change and compare real circuit files
Four experiments and ten comparison cases with editable circuit files. Trace the connections, change a value, and compare the simulator with an independent hand calculation.
How to reproduce these results
Choose a condition in a table to open its populated editor, press Sim and select the wire marked PROBE to inspect voltage. For the parallel-resistor experiment, inspect VCC. You can also load the RAW JSON through More → Import RAW. Open an example in a new tab to compare it while keeping another board open.
The tables are saved solver output. Each board was run through the deployed source’s DC engine, compared against a separately written circuit equation, then saved and restored through the editor URL to check the result again. Download the exact files and the raw report to inspect the conditions.
A circuit you can reproduce
How much does an LED current change with its resistor?
Compare 330 Ω, 1 kΩ, 100 Ω and a reversed LED on the same layout.
How can you distinguish a resistor problem from reversed polarity when an LED is dim or off?
- Open the 330 Ω case and press Sim. R1 is in series between VCC and the LED.
- Select the wire marked PROBE to read the LED anode voltage. The table reports total supply current.
- Compare the 1 kΩ, 100 Ω and reversed cases. The 100 Ω case is a virtual diagnostic demonstration, not a build recommendation.
Calculate it by hand first
The red LED model uses a 1.9 V threshold and 12 Ω dynamic resistance. Including the 0.2 Ω supply resistance gives I = (5 − 1.9) / (R + 12 + 0.2): about 9.06 mA at 330 Ω, 3.06 mA at 1 kΩ and 27.63 mA at 100 Ω. This explains the difference from a calculator that omits dynamic resistance.
| Condition / open board | Node voltage | Supply current | File |
|---|---|---|---|
| Red LED · 330 Ω | 2.009 V | 9.059 mA | RAW JSON |
| Red LED · 1 kΩ | 1.941 V | 3.059 mA | RAW JSON |
| Red LED · 100 Ω (overcurrent) | 2.232 V | 27.629 mA | RAW JSON |
| Red LED · 330 Ω · reversed | 5.000 V | 0.000 mA | RAW JSON |
What the comparison tells you
A larger resistor reduces current. In the reversed case, current is near zero while PROBE is near 5 V. That voltage alone does not mean the LED works: inspect polarity and current together. The 100 Ω case checks that exceeding the model’s 20 mA limit raises COMPONENT_STRESS.
Open this circuit in the editor
Read next: getting-started-led-circuit·led-current-limiting·reading-diagnostics
A circuit you can reproduce
Why does a 2.5 V divider fall to 1.67 V under load?
Connect a 10 kΩ load to the midpoint of two 10 kΩ resistors.
Why can a correct divider calculation fail once you connect a load?
- Open the unloaded case, start Sim and read the PROBE voltage.
- Open the loaded case. RL shares both nets with R2, so it is parallel with R2.
- Compare supply current as well as output voltage. Adding the load reduces total resistance and increases supply current.
Calculate it by hand first
Without a load, 5 × 10 / (10 + 10) = 2.5 V. A 10 kΩ load makes the lower leg 10 kΩ ∥ 10 kΩ = 5 kΩ, giving 5 × 5 / (10 + 5) ≈ 1.667 V. Supply current rises from about 0.250 mA to 0.333 mA. The simulator includes supply resistance, which slightly changes the final digits.
| Condition / open board | Node voltage | Supply current | File |
|---|---|---|---|
| 10 kΩ / 10 kΩ · unloaded | 2.500 V | 0.250 mA | RAW JSON |
| 10 kΩ / 10 kΩ · 10 kΩ load | 1.667 V | 0.333 mA | RAW JSON |
What the comparison tells you
A divider is not an ideal voltage source. Its unloaded output resistance is R1 ∥ R2 = 5 kΩ, so a 10 kΩ load cannot be ignored. Distinguish a small input load from a circuit drawing sustained power.
Open this circuit in the editor
Read next: voltage-divider-guide·level-shifting-3v3-5v·resistor-color-code
A circuit you can reproduce
Why a released button still has a defined 0 V input
Compare pressed and released states of a four-pin button with a 10 kΩ pull-down.
Which pins of a four-pin switch should you use, and where does the pull-down go?
- In the released case, check that 10 kΩ connects PROBE to GND.
- Compare the pressed case. The upper two switch pins form one pair and the lower two another; pressing joins the pairs.
- Toggle the switch in the editor, then read PROBE and supply current again.
Calculate it by hand first
Released, the VCC path is open while the resistor ties the input to GND, giving about 0 V. Pressed, the input joins VCC and the resistor carries about 5 / 10000 = 0.0005 A, or 0.5 mA.
| Condition / open board | Node voltage | Supply current | File |
|---|---|---|---|
| 10 kΩ pull-down · released | 0.000 V | 0.000 mA | RAW JSON |
| 10 kΩ pull-down · pressed | 5.000 V | 0.500 mA | RAW JSON |
What the comparison tells you
The pull-down defines the idle input voltage. Connecting across an internally joined pair conducts even without pressing. Confirm internal pairs with continuity rather than assuming their arrangement from the number of pins.
Open this circuit in the editor
Read next: pullup-pulldown-tact-switch·beginner-mistakes-top10·troubleshooting-checklist
A circuit you can reproduce
Why adding a parallel resistor increases current
Compare current and power for one 1 kΩ resistor and two in parallel.
How do you calculate current and power when two resistors share both nets?
- Open the single-resistor case and inspect the supply and resistor terminals. At an ideal 5 V, current is 5 mA.
- In the parallel case, both left terminals join VCC and both right terminals join GND. Each resistor has the same voltage.
- Compare total supply current with each resistor’s power. Do not confuse total current with a single branch current.
Calculate it by hand first
Equivalent resistance is 1 / (1/1000 + 1/1000) = 500 Ω. An ideal supply gives 10 mA total and 5 mA per resistor. Each dissipates 5²/1000 = 0.025 W, or 0.050 W in total. The model’s 0.2 Ω supply resistance lowers the voltage slightly, to about 4.998 V.
| Condition / open board | Node voltage | Supply current | File |
|---|---|---|---|
| One 1 kΩ resistor | 4.999 V | 4.999 mA | RAW JSON |
| Two 1 kΩ resistors in parallel | 4.998 V | 9.996 mA | RAW JSON |
What the comparison tells you
Adding a parallel branch reduces total resistance. Calculate dissipation separately for each physical resistor rather than comparing only total network power with a single resistor rating.
Open this circuit in the editor
Read next: resistor-color-code·power-rails-and-grounding·getting-started-led-circuit
Verification scope and rerunning
The source is fixed at 5 V open-circuit voltage, 0.2 Ω internal resistance and a 1 A current limit. A 1 A limit does not mean the experiment consumes 1 A: each table shows actual supply current. Verification covers converged DC solutions of simplified resistor, LED and switch models, not temperature, contact resistance, part tolerance or wire inductance.
With the source checkout, run npm run content:verify to repeat all ten cases. If a component model or wiring changes, regenerate both the report and screenshots. Send the example name and reproduction steps through Contact if you find a discrepancy.