Formula Used
How to Use This Calculator
- Choose Forward to compute Vout from known resistors.
- Enable Include Load when your circuit loads the divider.
- Set Tolerance to estimate worst‑case Vout range.
- Use Design to solve for R1/R2 from a target Vout.
- Download CSV or PDF for documentation.
Example Data Table
| Vin (V) | R1 (Ω) | R2 (Ω) | RL (Ω) | Ideal Vout (V) | Loaded Vout (V) |
|---|---|---|---|---|---|
| 5 | 10,000 | 10,000 | — | 2.5 | 2.5 |
| 12 | 18,000 | 10,000 | 100,000 | 4.2857 | 4.1509 |
| 24 | 47,000 | 10,000 | 10,000 | 4.2105 | 3.4286 |
| 3.3 | 6,800 | 10,000 | 1,000,000 | 1.9643 | 1.9550 |
Voltage Divider Performance Snapshot
A divider ratio of 0.50 delivers 2.50 V from 5.00 V. Using 10 kΩ and 10 kΩ draws 0.25 mA. At 12.0 V with 18 kΩ and 10 kΩ, ideal Vout is 4.286 V. Adding a 100 kΩ load reduces it to about 4.151 V. These shifts matter for sensor scaling and reference generation.
Load Interaction and Effective Resistance
When a load is attached, R2 becomes R2 ∥ RL. If RL equals R2, the bottom leg halves. For 47 kΩ over 10 kΩ at 24 V, ideal Vout is 4.211 V. With RL at 10 kΩ, Vout falls near 3.429 V. Keep RL at least ten times R2 for small error.
Current Draw and Source Impedance
Divider current is Vin divided by the total resistance. Higher totals save energy, but increase output impedance. The Thevenin resistance is R1·R2/(R1+R2). With 10 kΩ and 10 kΩ, Rth is 5 kΩ. If the next stage has 10 nF input capacitance, τ is about 50 µs. Five time constants need roughly 250 µs settling. Higher impedance also raises susceptibility to interference and bias currents, so buffering can improve stability.
Power Dissipation Checks
Each resistor dissipates P = I²R. With Vin 12 V and total 28 kΩ, current is about 0.429 mA. Power in R1 is roughly 3.3 mW and in R2 about 1.8 mW. Small resistors are safe. At 48 V with the same total, power rises by sixteen times. Always check resistor wattage and temperature rise. Higher Vin or lower resistance can quickly raise power.
Tolerance Range and Worst-Case Planning
A 1% tolerance can move the ratio enough to break thresholds. This tool checks worst-case corners. For a mid-scale divider, Vout shifts both directions when R1 and R2 drift oppositely. For example, a 2.50 V target may swing by tens of millivolts. Comparator trip points and ADC codes can change. Use tighter tolerance for references, and recalibrate for precision sensors.
Design Mode for Target Scaling
Design mode solves resistors from Vin and a target Vout. If Vin is 12 V and target is 5 V, the ratio is 0.4167. Selecting a 20 kΩ total yields R2 about 8.33 kΩ and R1 about 11.67 kΩ. Choose nearby standard values and verify loaded behavior.
FAQs
1) Why does Vout drop when I connect a load?
The load sits in parallel with R2, reducing the effective bottom resistance. That lowers the divider ratio and pulls Vout downward, especially when RL is close to R2.
2) What is a good rule for choosing RL relative to R2?
Aim for RL at least 10× R2 for small loading error. For higher accuracy, use 100× or buffer the divider with an op-amp or ADC input buffer.
3) How do I pick total resistance for low power?
Increase R1+R2 to reduce current draw. Then confirm your circuit accepts the higher source impedance and that noise, leakage, and ADC sampling effects remain within limits.
4) What does Thevenin resistance tell me here?
Rth is the divider’s output resistance seen at Vout. It affects settling time with capacitive loads and interacts with ADC sample-and-hold circuits, shaping accuracy and speed.
5) Are tolerance results statistical?
No. The tolerance range is worst-case corner analysis using ±tolerance on each resistor. Real parts usually cluster closer, but worst-case planning avoids failures in thresholds.
6) Can I use this for resistor ladder or multi-tap networks?
This calculator focuses on a two-resistor divider with an optional load. For ladders, analyze each segment’s Thevenin equivalent or simulate the full network for best results.