Potential divider design

Last time

Before we start: last time, in LDR circuits and parallel networks, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Choosing any voltage you like

You have a twelve volt battery, but your circuit needs exactly eight volts. Two or three ordinary resistors will do it. The trick is choosing the resistors so that the output voltage is the right fraction of the supply, and then checking that the load does not spoil it.

The output voltage

For two resistors in series across a supply, the output taken across one resistor is the supply voltage times that resistor's share of the total. For an output of eight volts from twelve, the fraction must be two thirds. So the resistor across the output must be twice the other. A sixty ohm output resistor with a thirty ohm resistor gives twelve times sixty over ninety: eight volts.

Combining resistors to make the values

You may only have certain values: one hundred and eighty, ninety and thirty ohms. A hundred and eighty and ninety in parallel give one hundred and eighty times ninety over two seventy: sixty ohms. Put that sixty ohm pair as the output, in series with the thirty ohm resistor. The output across the pair is two thirds of twelve volts: eight volts.