MODE
IC
Please enter valid values. Program resistor must be greater than zero.
RESULTS
Vout = Vreg * (1 + R2 / R1)
IC
Please enter valid values. Resistor value must be greater than zero.
RESULTS
Iout = Vreg / R
Pr = Vreg * Iout
Input Parameters Specification
Program Resistor (R1 or R) This resistor is placed across output and adjustment pins, maintaining a constant feedback current loop.
Adjustable Resistor (R2) Connected between the adjust node and ground, allowing tuning of the reference offset to get custom voltage limits.
IC Reference Voltage (Vreg) Native constant voltage offset value embedded inside the selected active silicon regulator (such as 1.25V for LM317).
Regulator IC Model Dropdown selector that defines the reference voltage profile matching standard linear regulators.
Practical Operational Examples
Standard LM317 Voltage Split
Use Voltage Adj mode. Input R1 = 240 ohms, R2 = 2400 ohms, and select LM317. The computed output matches standard low-noise electronics rails.
Constant Current LED Driver
Use Current Adj mode. Input R = 10 ohms and select LM317. This sets a steady 125mA driving current, ideal for high-power series LED configurations.
7805 Variable Output Hack
Using a fixed 5V regulator as adjustable controller. Input R1 = 220 ohms and R2 = 330 ohms elevates the output to a higher regulated potential.
High Power current Limit
Configuring a 7812 with a 15-ohm shunt resistor. The calculator estimates output current and the power rating needed to prevent resistor burnout.
Diagrams & Theory
Linear adjustable regulators maintain a constant reference voltage drop between output and adjustment terminals. Placing a divider network creates a feedback loop that determines the final regulated output level while suppressing raw input line noise.
Formulas & Mathematical Logic
Output Voltage: Vout = Vreg * (1 + R2 / R1)
Output Current: Iout = Vreg / R
Resistor Power Dissipation: Pr = Vreg * Iout
The mathematical operations convert input dimensions into raw base units before executing equations, preserving reliability and avoiding parameter drift.
Step-by-Step Example
Example: Configuring an LM317 (Vreg = 1.25V) to deliver a stable 9.0V output rail.
Step 1: Select the program resistor R1. A value of 240 ohms is typically chosen to fulfill minimum load requirements.
Step 2: Rearrange the voltage formula to solve for adjustment resistor R2: R2 = R1 * ((Vout / Vreg) - 1).
Step 3: Substitute the parameters: R2 = 240 * ((9.0 / 1.25) - 1) = 240 * (7.2 - 1) = 1488 ohms (approx. 1.5 k-ohms).
Step 4: Verify that power dissipation parameters are within safe limits.
How to Use This Calculator
Select either Voltage Adj or Current Adj mode depending on your hardware schematic.
Enter your divider resistor parameters and choose the matching unit multiplier (ohms, k-ohms, or M-ohms).
Select your linear regulator IC model to determine the core reference voltage profile automatically.
Click Calculate to generate output voltage, current limits, and power ratings instantly.
About This Calculator
Determine voltage divider and constant current configurations for linear IC regulators.
This design tool estimates output voltage values for popular series regulators like the LM317, LM338, or LM350, as well as fixed three-terminal ICs in adjustable configurations.
Linear regulators offer a simple, low-noise approach to analog power distribution. Unlike switching converters, linear ICs function by dissipating excess voltage drop as heat. This eliminates high-frequency electromagnetic interference (EMI), making them highly suitable for sensitive preamplifiers, audio equipment, and wireless transmitter boards.
When selecting resistor values, engineers evaluate the trade-off between idle current waste and loop stability. Most adjustable regulators need a minimum load of approximately 5mA to maintain safe regulation thresholds. Using a 240-ohm or 120-ohm resistor for R1 satisfies this load condition naturally when utilizing a 1.25V reference IC.
The calculator also supports configuring three-terminal regulators as floating constant current sources. By placing a sense resistor across the output and adjustment terminals, the internal op-amp circuit maintains a fixed current flow, preventing overload risks in sensitive loads like laser diodes and series LED matrices.
Ideal ConfigurationsLow-noise workbench supplies, LED array drivers, and simple chargers.
Key ParametersOutput voltage levels, constant current thresholds, and resistor dissipation metrics.
User BaseCircuit board designers, electrical engineering students, and repair specialists.
Thermal PracticeProvide sufficient cooling or heatsinks to prevent the silicon core from entering thermal protection.
Tip: Always install high-quality decoupling capacitors near the regulator input and output pins to suppress high-frequency line oscillations.
Frequently Asked Questions
Why is a 240-ohm resistor commonly used for R1 with the LM317?
The LM317 requires a minimum load current of 5mA to regulate properly. Under a 1.25V reference drop, a 240-ohm resistor draws exactly 5.2mA, which fulfills this design constraint without extra loads.
What is the dropout voltage of an adjustable regulator?
Dropout voltage represents the minimum margin required between the raw input and target output voltage. Standard models typically require a headroom of 1.5V to 2V depending on load current.
Can I use this calculator for other adjustable ICs like the LM338 or LM350?
Yes. These models share the same internal bandgap reference voltage (1.25V) and feedback topology, meaning they deliver identical calculation results.
How does configuring a 7805 regulator with a divider network function?
A fixed regulator can be adjusted upward by raising the ground pin's potential. The 7805 maintains a stable 5V drop across R1, forcing current through R2 to shift the total output voltage upward.
Why is resistor power dissipation calculated in current mode?
In constant current mode, the setting resistor bears the full reference voltage and load current. Calculating power dissipation prevents component burnout by ensuring you select a resistor with an appropriate power rating.
Are output adjust pin currents factored into the voltage calculation?
No. The adjust pin current (typically 50uA to 100uA) is extremely small and has a negligible effect on the output voltage when standard low-value resistors (e.g., 240 ohms) are utilized.
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