Electrical – the input – output voltage relationship of an LM317 voltage regulator circuit

lm317voltage

I'm looking to create the simple adjustable voltage regulator below

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However, I am confused by the input-output voltage relationship. Looking at the LM317 data sheet (https://www.onsemi.com/pub/Collateral/LM317-D.PDF, pages 1,2 and 7 in particular), the Vref (the voltage across resistor R1) is specified as being a constant 1.25V.

Page 2 under characteristics of Vref states
"Reference Voltage, 3.0 V ≤ VI−VO ≤ 40 V … "

  • What is the input-output relationship in the LM317?
  • Can Vin be any voltage as long as Vout is between 1.25 – 37V but the
    difference between Vin and Vout (Vin – Vout) cannot be greater than
    40V or less than 3V?
  • is Vin – 3 the maximum allowed Vout?

Best Answer

The LM317 will raise or lower the output voltage until the voltage it sees on its Adj pin is the reference voltage (of 1.25 V).

It does that by dynamically adjusting the effective resistance between in- and output. Simply imagine an adjustable resistor between in- and output, and someone tuning that resistor so that the output is always at the same voltage.

It does that within it's physical limits: it can't drop less than 3 V (being a very, very, VERY old regulator), and the more it drops between in and output, the more power it converts to heat (in that resistance mentioned above).

At some point, the things simply breaks down and can't act as resistor anymore. 40 V (depending on the actual manufacturer of the LM317, that thing is so old, there are multiple masks out there) sounds realistic.