How this instrument works
The LM317 is a classic adjustable three-terminal linear voltage regulator, in continuous production since the 1970s and still one of the most widely used regulator ICs in hobbyist and professional electronics alike. Unlike fixed regulators (like the 7805, which always outputs 5V), the LM317 lets the designer set any output voltage in its range using just two external resistors, R1 and R2, arranged as a simple voltage divider that feeds back into the chip's adjust pin.
The regulator holds a fixed, precise 1.25V reference (Vref) between its output and adjust pins no matter what load is connected. That constant 1.25V across R1 sets a fixed current through the R1-R2 divider chain, and that same current flowing through R2 adds an extra voltage on top of the 1.25V already present — so the total output voltage becomes Vout = Vref x (1 + R2/R1). Choosing R2 = 0 (shorting the adjust pin directly to the output) gives the regulator's minimum possible output, exactly Vref = 1.25V.
In practice, R1 is almost always fixed around 220-240 ohms (per the datasheet's recommended value, chosen to guarantee enough minimum load current for the regulator to stay in regulation), and R2 is the value a designer varies to dial in the desired output voltage — everything from just above 1.25V up to the LM317's rated maximum (typically 37V, load and heat-dissipation permitting).
This instrument runs the datasheet formula directly: enter any R1/R2 pair and get the resulting Vout immediately, useful for both designing a new regulator circuit from a target voltage and for checking what an existing resistor pair on a board or in someone else's schematic will actually output.
- Enter R1 (Ω) — the fixed resistor between the output and adjust pins, commonly 240Ω per the datasheet's reference design.
- Enter R2 (Ω) — the resistor between the adjust pin and ground, the value you vary to set the output voltage.
- Read Output voltage Vout (V) for the resulting regulated output.
- Set R2 to 0 to confirm the regulator's minimum output equals the 1.25V reference voltage exactly.
- If you have a target output voltage in mind, try different R2 values with a fixed R1 until Vout lands close to your target — real designs then round to the nearest standard resistor value.
Worked example — R1 = 240Ω, R2 = 2.4kΩ, a standard 10:1 divider
Enter R1 = 240 and R2 = 2400. Output voltage Vout (V) reads 13.75. Working through the formula: 1.25 x (1 + 2400/240) = 1.25 x (1 + 10) = 1.25 x 11 = 13.75V.
This R1/R2 pair — a clean 10:1 ratio between the two resistors — is a commonly cited LM317 reference-design example precisely because the numbers work out evenly, making it easy to verify by hand and a natural starting point for anyone building their first adjustable regulator circuit, for instance to power a 13.8V nominal 'car electronics' rail from a higher unregulated DC supply.
Questions
What resistor values give the LM317's minimum output voltage?
Setting R2 to 0 ohms (effectively shorting the adjust pin to the output pin) gives the regulator's minimum output, exactly equal to its internal reference voltage of 1.25V, since Vout = 1.25 x (1 + 0/R1) = 1.25V regardless of R1's value. In a real circuit you'd typically just tie the adjust pin directly to the output rather than using an actual 0-ohm resistor to achieve this.
Why is R1 usually fixed around 220-240 ohms in LM317 designs?
The LM317 datasheet specifies a minimum load current (typically around 3.5-10mA depending on the specific part) needed to keep the regulator in proper regulation. Since the reference voltage across R1 is fixed at 1.25V, R1's value directly sets the minimum current flowing through the R1-R2 divider (I = 1.25V / R1) — a 240-ohm R1 gives about 5.2mA, comfortably above the datasheet minimum for most LM317 variants, which is why 220-240 ohms became the standard textbook and datasheet reference value.
How do I choose R2 for a specific target output voltage?
Rearrange the formula: R2 = R1 x (Vout/Vref - 1). With the standard R1 = 240 ohms and Vref = 1.25V, for a target of 12V you'd get R2 = 240 x (12/1.25 - 1) = 240 x 8.6 = 2064 ohms, and you'd then round to the nearest standard resistor value (commonly 2kΩ or 2.2kΩ) and check the resulting Vout with this instrument, since real resistors rarely land on the exact theoretical value.
What's the maximum output voltage the LM317 can produce?
The LM317's datasheet-rated maximum output depends on the specific part variant, but the classic LM317 is commonly rated for a maximum input-to-output differential around 40V and a maximum output around 37V, meaning R2 can theoretically be scaled up to reach that range — though in practice the resistor values needed for very high outputs get large, and real designs must also respect the chip's power dissipation limits, which depend on the current being drawn and the voltage dropped across the regulator itself.
Does the load current change the LM317's output voltage?
In an ideal analysis, no — the LM317's whole design goal is to hold Vout constant at Vref x (1 + R2/R1) regardless of load current, within its rated operating range. In practice, extremely low load current (below the datasheet minimum, which R1 is chosen to guarantee) can cause the regulator to drift out of tight regulation, and very high load current can cause thermal or dropout-voltage limitations, but under normal operating conditions the two-resistor formula this instrument computes holds accurately.