7.3 Voltage Dividers

This is the final section of Module 7 in the Navy Basic Electricity and Electronics series, a module on parallel circuits. The first two sections were:

7.1 Solving Complex Circuits
7.2 Voltage Reference

1. A circuit can be divided up in a way that provides different voltages to different parts of the circuit. Such a circuit is called a "voltage divider."

The picture below is an example of the kind of circuit that divides off voltage in order to work lamps of various volts and amps. The first lamp needs 6 volts at 2 milliamps. The second needs 12 volts at 8ma.

How many ohms does each resistor need to be in order to supply the appropriate volts and amps?

2. So R₃ is called the bleeder resistor. You can generally estimate the bleeder current as one tenth of the total current elsewhere. Since 8 + 2 = 10 ma, we can estimate the bleeder current through R₃ as 1ma.

Since voltage is common in parallel and E = IR, then 6v = (1ma)R. R₃= 6 kilohms.

3. By this process one can go on to determine the value of R₂ and R₁ as well. The 1ma from R₃ combines with the 2ma from DS1 yielding 3ma in R₂. The 12V in DS2 minus the 6V over R₃ suggests there is 6 volts across R₂. E = IR, so 6V = (3ma)R₂. So R₂ = 2 kΩ.

4. We can follow the same process to solve for R₁, which turns out to be 11ma over 12V or 1.1 kΩ.