A voltage divider is a broadly used series resistor circuit. By arranging two carefully chosen resistors in series, we can manipulate the voltage across the second resistor. To find the correct resistor values we simply use the voltage divider equation and the ten percent rule.
The circuit below is a simple voltage divider. We see +12 Volt at Vin. This voltage is reduced to 7.5 Volt across the second resistor. Let’s assume that there is no current drawn from the divider.
So if you want to know the voltage out from the voltage divider we simply use the voltage divider equation.
After filling in the blanks we get:
Pretty easy, but we assumed that there was no significant load on the divider.
There are two problems with this approach. First, we don’t do anything useful with the divider. Secondly, we do not take the power dissipation into account.
Using the 10 percent rule
The example above showed a voltage divider with no purpose. Normally spoken we would connect a parallel resistive load to the voltage divider. In the circuit below we added a 10k resistor as a load. Now current is flowing through the load as well.
The voltage from the divider has dropped from 7.5V to 6.3V by adding a 10k load. So if we need 7.5V instead we need to adjust our calculation. Here’s where the ten percent rule comes in.
First, we need to know how much current our resistive load needs to operate. That’s simply applying Ohm’s Law.
Next, we pick the value of the second resistor in the voltage divider. Its value needs to be set so it draws 10 percent of the desired load current. In this case that would be 75 micro Ampere. Using Ohm’s law again to calculate the value of the second resistor.
Next, we need to select the first resistor, so that the output is still at 3V. To do this, just calculate the total current through the resistor…
… and use Ohm’s Law again.
Our voltage divider in full swing.
Now, 5.45 is not a standard resistor value, but a 5.6 or 5.1 kOhm will do the trick. Just pick a standard value as close as possible to the calculated value.