Electronics Basics: Notice Divider Loading

Check whether a connected load changes the voltage divider.

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An unloaded divider has R_top and R_bottom in series: V_out = V_in R_bottom/(R_top + R_bottom). A load across the output is parallel with R_bottom. Recalculate that parallel combination before applying the divider rule. Use ideal resistors and a 6 V ideal source. Use diagrams and calculations only. Never use wall outlets, short batteries, open powered equipment or handle charged capacitors.

Worked example

With R_top = R_bottom = 1000 ohms, unloaded V_out = 3 V. A 1000-ohm load makes the lower equivalent resistance 500 ohms. Loaded V_out = 6 x 500/1500 = 2 V.

A six-volt source feeds a 1000-ohm top resistor. A 1000-ohm bottom resistor and 1000-ohm load connect in parallel from the output node to the reference node. The loaded output is two volts rather than the unloaded three volts.

Would you believe it?

Adding a resistor in parallel provides another path and lowers the equivalent resistance. The way parts connect matters, not just their individual values.

OpenStax: series and parallel circuits

Question 1 What is the unloaded output voltage? 6 V 2 V 3 V

Question 2 What is 1000 ohms in parallel with 1000 ohms? 1000 ohms 500 ohms 2000 ohms

Question 3 What is the loaded output voltage in the example? 2 V 3 V 4 V

Question 4 What current flows through the 1000-ohm load at 2 V? 4 mA 6 mA 2 mA

Question 5 Why did output voltage fall? The input became 2 V The load changed the lower equivalent resistance Voltage was eaten by the wire label

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