The standard voltage divider rule (Vout = VS × R2 / (R1 + R2)) is a powerful tool. However, it relies on a hidden assumption: that nothing is drawing current from Vout.
12V
1 kΩ
1 kΩ
1 kΩ
Tap (Vout)
Load (RL)
Equation Output
12V × 1k / (1k + 1k) = 6V
Actual Output Voltage
6.00 V
What happened?
The voltage sagged! When you connected the load, it formed a parallel circuit with R2. Two 1kΩ resistors in parallel have an equivalent resistance of just 0.5kΩ.
The bottom half of the divider lost resistance, so it lost its share of the voltage. The standard rule only works perfectly when unloaded.