Switch Pulls Low - Electronic Circuits II & III: Decision Networks and Interfaces Step by Step

A switch-pulls-low scan treats the closed switch as a path to ground, pulling V_out = 0 V and setting the pull-up current I = V / R row by row. The arithmetic is exact only for the stated output voltages, source voltages, resistances, currents, switch states, and rows in the ideal pull-up and ideal switch model. Real pull-up circuits add resistor tolerance, switch contact resistance, leakage, input impedance, supply tolerance, probe/calibration limits, finite precision, and measurement uncertainty.

Closing the switch turns the output node into an ideal low node and sets the pull-up current. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Closing the switch gives a path to ground

The closed switch connects the output node to the return node, so the ideal output voltage is 0 volts.

V_{\text{out}} = 0\ \text{V}

The pull-up current is exact

The source is 12 volts and the pull-up is 6 ohm, so the current is 2 amperes.

I = \frac{V}{R} = \frac{12\ \text{V}}{6\ \text{ohm}} = 2\ \text{A}