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Voltage Divider

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Recipe #2 — the voltage divider

Two resistors can produce a smaller voltage from a larger one. This is one of the most common little circuits in electronics.

Example

R1 = 10 kΩ, R2 = 10 kΩ, VIN = 12 V.

VOUT = VIN × R2 / (R1 + R2)

VOUT = 12 × 10 / 20 = 6 V

Where it appears: transistor bias circuits, sensor inputs, reference voltages, volume controls, battery monitors, and measurement circuits.

Another way to think of it

The supply voltage is shared by the two resistors in proportion to their resistance. Equal resistances share it equally.

The catch: loading

Once another circuit is connected to VOUT, that circuit may draw current. If it draws enough current, it effectively changes the lower part of the divider and the output voltage shifts.

This is one reason buffers — tube cathode followers, transistor emitter followers, and op-amp voltage followers — are so useful.

Try these yourself

With two equal resistors across 12 V and no significant load, what is the midpoint voltage? 0 V 6 V 12 V 24 V

Two equal resistors divide the supply equally, giving about half the supply at the midpoint.

What happens if a heavy load is connected to the divider output? The output can shift because the load becomes part of the divider Nothing can ever change The resistors disappear The source becomes AC

A voltage divider works as calculated only when the load is light enough that it does not significantly alter the resistor ratio.

Where might a divider be used? Biasing, sensing, or creating a reference voltage Only speaker crossovers Only vacuum-tube heaters Only transformers

Voltage dividers commonly provide bias voltages, references, and scaled-down measurements.

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