# I need help with determining voltage wave form of LM317

Now I calculated Vout as being 5V, so is the Vout wave form the same as Vin?

• What do you already know about how an LM317 behaves? Commented Jul 1, 2020 at 15:41
• This looks suspiciously like homework or an exam. There's no sane reason to build this, the reason for asking about what would happen would be to see if the student understands the behavior of such a regulator. Additionally if this were for some odd real purpose, you'd have to consider that it will behave differently with an external load than without. Also the question is defective as it neglects to give a time scale. If this is an assignment consider a multi-part answer introducing and discussing issues one at a time with an emphasis on the bounds of what is knowable. Commented Jul 1, 2020 at 15:59
• The datasheet shows that the 240 ohm resistor is used with the more expensive LM117. The LM317 uses 120 ohms and the other resistor value also must be half. Commented Jul 1, 2020 at 16:30

The LM317 is a linear voltage regulator. It expects a DC input voltage at least 2 volts above the desired output voltage for proper operation.

If you give it a sine wave input as shown, it will produce a pulsed, varying voltage, with a maximum voltage of 5 volts.

No,

LM317 is a regulator and will try to hold the output voltage at the predefined value determined by your setpoint resistors.

There is a minimum input voltage that the LM317 needs to be able to regulate the 5V level on the output. Anything below that level, the output level will fall. Anything above that minimum input voltage level, the output should remain relatively close to 5V.

There is not really enough information on the data sheet to allow you to answer this question accurately.

I would expect it to look a bit like the 7805 input-output characteristics:

Figure 5 in the datasheet shows you where the output voltage starts to decline linearly, but I see no information on what happens at the output as the input voltage approaches zero. At some point it will likely drop to near zero, similar to the LM340 aka 7805.

So the output waveform would stay at zero for a spell, zoom up to some voltage, increase like a segment of a sine wave until the output hits 5.0V (at which point the input will be somewhat higher) and flat-line until the input falls again to the drop-out voltage at the output current (in your case, just the resistors form the load).