In many circuit applications there is a requirement to convert 12 Volt to 5 Volt.
For example, a circuit may not be able to tolerate input voltages greater than 5V. However, the output device feeding this circuit can only provide 12V.
In this case it becomes necessary to divide +12V down to +5V.
One way to do this is with a voltage divider network that consists of two resistors in series as shown in the picture below.
This tool calculates the resistor values for a maximum +5V drop across resistor R2.
Enter either R1 or R2 in milliOhm / Ohm / kiloOhm / megaOhm. The tool will calculate the other resistor value.
Formula
Ohm’s law is used to calculate the voltage drop across each resistor. The current through each series resistor is the same as the total current through the circuit.
Therefore the total current Itotal is given by
Itotal = Vin/(R1+R2) = V1/R1
V1=Vin*R1/(R1+R2)
Itotal = Vin/(R1+R2) = V2/R2
V2=Vin*R2/(R1+R2)
In this case, V2 = +5V and Vin = +12V.
How to Pick Resistor Values
In general, there are many combinations of resistor values that will give the same result. For instance,
- R1 = 10 kΩ and R2 = 7.14 kΩ
- R1 = 10 MΩ and R2 = 7.14 MΩ
Both combinations provide +5V at the output.
Why use a Voltage Divider?
A voltage divider configuration is used to scale a voltage so that it can be analyzed without damaging the device that’s sensing the voltage level. All analog-to-digital converters (ADC) for instance have a max voltage rating on their analog input pins.
Let’s say for instance, there is a requirement to monitor a battery voltage. You may want to detect if it falls below a certain threshold (75% of it’s rated level).
The voltage for a new battery may be 12V. If you are using an Arduino to detect this level, let’s first take a look at the data sheet for the ADC built into the micro-controller.
According to the table below the max VIN = 5.5V. Providing some headroom, the input voltage should not be allowed to exceed +5V. To enable this reduction, a voltage divider should be implemented between the battery and the micro.
Check the impedance requirements for the circuit that requires +5V. This will determine which resistor combination should be used.
In the same table we can see that the Analog Input resistance, RAIN varies from 55 MΩ to 100 MΩ.
In this situation, how well will the resistor divider values of R1 = 10 MΩ and R2 = 7.14 MΩ work?
To answer this question, let’s use the parallel resistor calculator.
7.14 in parallel with 55 gives an effective resistance value of 6.3 MΩ. This will impact the voltage levels into the ADC as there is effectively an 11% discrepancy in R2.
Instead let’s take a look at using a value of R2 = 550 kΩ (1% of the lower value of ADC input impedance). In this case, the effective resistance of the parallel combination is dominated by R2. The effective resistance is now 545 kΩ. This represents less than a 1% error. Using this value of R2 in the calculator gives R1 = 770 kΩ.
Notes
[1] Check the tolerance of the resistor. A higher % means that the value will deviate more from the stated value. For instance + 10% on a 3 kΩ resistor means its value can be as high as 3.3 kΩ and as low as 2.7 kΩ. This discrepancy can impact the value of the divided voltage significantly.