Circuit calculators – dividers, filters, impedance and logic

Some questions come before the schematic: which two resistors give 3.3 V, which capacitor sets a 1 kHz corner, how small can this logic function get? Ohmforge's circuit calculators answer them and then place the result on the canvas as one undoable step.

Open them with Instruments → Circuit calculators…. The dialog has five tabs: Voltage divider, Filter designer, Impedance, Resistor networks and Truth table. Use the arrow keys to move between tabs. Results update while you type; values accept engineering notation such as 4.7k, 4k7, 100n or 10m.

Voltage divider

Open the Voltage divider template so there is already a circuit on the canvas, then open the calculators. The divider tab starts with Solve for set to Output voltage Vout, 12 V, R1 = 10k and R2 = 4.7k. The status line reads Vout = 3.837 V. Below it you see the current from Vin (816.3 µA), the power in each resistor and the output resistance R1 ∥ R2 (3.197 kΩ).

Change Solve for to Bottom resistor R2, enter a Vout of 3.3 and type a Load resistance (optional) such as 100k. The bottom leg is now R2 in parallel with the load, and the tab shows how much the load pulls the output down.

The table Closest E24 pairs lists standard pairs with their error; pick another Standard resistor series for E6 up to E192. Use copies a pair into R1 and R2 and switches back to solving Vout, so you see the exact output of the pair.

Choose Add this divider to the schematic. The divider appears below the template's circuit, selected, with its own source and ground. Because the template already has a net called OUT, the new output label is renamed OUT2 and the message says so; the two circuits never connect by accident. Press Run to see both outputs, and Ctrl+Z to remove the new divider again.

Filter designer

Switch to Filter designer. With RC low-pass (1st order), R = 1k and C = 100n the status reads The −3 dB cutoff is at 1.592 kHz. – the familiar 1/(2πRC). The two charts show magnitude and phase from the exact transfer function.

Set Task to Design for a target frequency: enter the frequency, the capacitor you want to use and the resistor series. The tab computes the exact resistor, suggests a standard one and shows the frequency error with that part. For the series RLC filters you also set Quality factor Q; 0.7071 gives the flattest low- or high-pass, a higher Q a narrower band-pass or notch. A Load resistance (optional) moves the −3 dB points, and the tab shows where they really are.

Add this filter to the schematic places the source, the filter and an OUT label. In an empty project it also sets up the AC analysis and the Bode plotter for it, so pressing Run draws the curve straight away.

Impedance and resistor networks

The Impedance tab takes a network such as R100 + (L10m || C1u): + is series, || is parallel and binds tighter. At 1 kHz it shows Z = 100 Ω + j103.8 Ω, the admittance, the reactance of each part and a parallel resonance at 1.592 kHz.

Resistor networks has two tasks. Find standard resistors for a target looks for up to three parts: for 1234 Ω from E12 the best is (68 ∥ 68) + 1.2k, exact, while a single 1.2k is 2.76 % low. Evaluate a network expression gives the total resistance of 100 + (220 || 330) (232 Ω) and the voltage, current and power of every resistor.

Truth table

The Truth table tab accepts a Boolean expression or a minterm list with don't cares. The example A'B'C + A'BC + AB'C + ABC' gives Minimal SOP: A·B·C' + A'·C + B'·C and the matching product of sums, the Karnaugh map, the prime implicants and gate counts.

Add the gate circuit to the schematic builds it from Ohmforge's 2-input gates: one switch with a 10 kΩ pull-down per input (closed = 1), NOT gates for inverted inputs and a logic probe on the output. Close SW1 and SW2 and run the operating point: the output reads 5 V, as row 6 of the truth table says. Close SW3 as well and it drops to 0 V.

The same calculations as tools

Each tab has a standalone tool with a form, JSON API and MCP access: Voltage Divider Calculator, Passive Filter Designer, Impedance Calculator, Resistor Network Calculator and Truth Table & Logic Minimiser. They use the same engine as the dialog and add simulator cross-checks and project downloads; see Automate with the API and MCP.