Calculator Inputs
Formula Used
The calculator uses the standard adjustable regulator equation:
Vout = Vref × (1 + R2 / R1) + Iadj × R2
To solve for R2, it rearranges the formula:
R2 = (Vout - Vref) / ((Vref / R1) + Iadj)
To solve for R1, it uses:
R1 = (Vref × R2) / (Vout - Vref - (Iadj × R2))
Thermal loss is estimated with:
Power Loss = (Vin - Vout) × Total Current
Junction Temperature = Ambient + (Power Loss × Thermal Resistance)
How To Use This Calculator
- Select the calculation mode.
- Enter the reference voltage and adjust pin current from the regulator data sheet.
- Enter R1 and R2, or enter the resistor used with the target voltage.
- Add input voltage, load current, dropout voltage, and thermal data.
- Select a preferred resistor series when solving resistor values.
- Press Calculate to show the result above the form.
- Use CSV or PDF download options to save the calculation.
Example Data Table
| Regulator Type | Vref | R1 | R2 | Vin | Load | Approx Output |
|---|---|---|---|---|---|---|
| LM317 style | 1.25 V | 240 Ω | 720 Ω | 12 V | 100 mA | 5.04 V |
| Low current design | 1.25 V | 330 Ω | 1000 Ω | 9 V | 50 mA | 5.08 V |
| Higher output rail | 1.25 V | 240 Ω | 2000 Ω | 18 V | 150 mA | 11.77 V |
Adjustable Voltage Regulator Design Guide
An adjustable voltage regulator lets one circuit serve many output needs. A common three terminal device uses a reference voltage, a set resistor, and an adjust resistor. The calculator helps choose these values before parts are fitted. It also checks practical limits that often cause field trouble.
Why The Resistor Pair Matters
The lower resistor sets current through the divider. The upper adjust resistor raises the output above the internal reference. Small changes in either resistor shift the final voltage. For this reason, tolerance should be checked with the nominal result. A standard value can be close, yet still miss the design target after tolerance drift.
Input Headroom And Dropout
A regulator needs enough input voltage to control the output. The difference between input voltage and output voltage must be greater than the dropout value. Extra margin is useful. It protects the supply during ripple, cable loss, battery sag, and load steps. When the margin is negative, the regulator may pass a lower voltage instead of regulating.
Load Current And Heat
Linear adjustable regulators burn extra voltage as heat. Power loss equals voltage drop multiplied by load current. High current and high input voltage can create a hot package. The thermal estimate uses ambient temperature and thermal resistance. It gives an estimated junction temperature. Keep this below the device limit and leave safety margin.
Using Preferred Resistors
Real designs use available resistor series. The calculator can compare the calculated resistor with E12, E24, or E96 values. This makes the result easier to build. It also shows the output expected from the rounded value. Precision parts may be needed when the output rail feeds sensors, references, or analog stages.
Good Design Practice
Use the result as a design check, not the only proof. Read the regulator data sheet. Confirm minimum load current, capacitor values, stability rules, and package limits. Measure the finished circuit under full load. Test at low input voltage and high ambient temperature. A small bench test can prevent damaged boards.
For battery products, repeat the check at fresh, nominal, and nearly empty voltage. For mains adapters, include ripple and tolerance. These simple comparisons reveal weak operating points before detailed layout work begins safely.
FAQs
What is an adjustable voltage regulator?
It is a regulator that uses external resistors to set output voltage. The same device can create many output rails within its allowed input, output, current, and thermal limits.
What does R1 do?
R1 sets the divider current through the feedback network. A common value is 240 ohms for many LM317 style circuits, but the data sheet should always guide the final choice.
What does R2 do?
R2 adjusts the output voltage above the reference voltage. Larger R2 values usually produce higher output voltage, assuming R1 and the reference voltage stay fixed.
Why is adjust pin current included?
Adjust pin current flows through R2 and adds a small voltage term. It may look minor, but it can affect accuracy when resistor values are large.
What is dropout margin?
Dropout margin is the remaining voltage after subtracting output voltage and required dropout voltage from input voltage. Positive margin helps the regulator stay in control.
Why does the calculator estimate heat?
Linear regulators turn unused voltage into heat. The heat estimate helps check whether the package may exceed its safe junction temperature under load.
Should I use preferred resistor rounding?
Yes, when building a real circuit. Preferred series values show practical parts you can buy and the output voltage expected after rounding.
Can this replace a data sheet?
No. Use this tool for quick design checks. Always confirm capacitor needs, minimum load, safe current, package rating, and layout guidance from the regulator data sheet.