What the Three-Resistor Divider Gives You
A three-resistor voltage divider creates two intermediate voltage nodes instead of just one. This is useful when a circuit needs more than one reference level from the same input supply.
Voltage Drops vs Node Voltages
A common mistake is mixing up voltage drop and node voltage. The voltage drop across a resistor is the voltage lost across that resistor. A node voltage is measured from that node to the reference point, usually ground.
| Quantity | Meaning |
|---|---|
| VR1 | Voltage drop across resistor R1. |
| VR2 | Voltage drop across resistor R2. |
| VR3 | Voltage drop across resistor R3. |
| Node after R1 | Voltage from the node between R1 and R2 down to ground. |
| Node after R1 + R2 | Voltage from the node between R2 and R3 down to ground. |
Formula Used
First, calculate total series resistance:
Divider current is:
Each resistor voltage drop is:
Example
Vin = 12 V R1 = 2 kΩ R2 = 3 kΩ R3 = 5 kΩ
Total resistance is 10 kΩ. Current is 12 V / 10 kΩ = 1.2 mA. Voltage drops are 2.4 V, 3.6 V, and 6.0 V.
When This Simple Divider Is Not Enough
A real voltage divider can shift when another circuit is connected to one of its output nodes. That connected circuit acts like a load resistance and changes the equivalent resistance.
Practical Design Tips
- Use resistor values low enough to avoid excessive loading error.
- Use resistor values high enough to avoid unnecessary power waste.
- Check resistor power dissipation, especially at higher Vin.
- Account for resistor tolerance if the voltage needs to be precise.
- For ADC inputs, check the microcontroller input impedance and sampling requirements.
Frequently Asked Questions
Can I use this for AC voltage?
For pure resistors, the same ratio applies to instantaneous or RMS AC voltage. For capacitors, inductors, or frequency-dependent circuits, use impedance instead of simple resistance.
Why does the sum of voltage drops equal Vin?
In a series loop, Kirchhoff’s Voltage Law says the voltage rises and drops around the loop must sum to zero. So the resistor drops add up to the input voltage.
What happens if I connect a load to a node?
The load becomes part of the circuit and changes the divider ratio. The unloaded voltage from this calculator may no longer be accurate.
Can I use more than three resistors?
Yes. The same principle applies: add all series resistances, then calculate each voltage drop as Vin multiplied by each resistor divided by total resistance.