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I'm trying to drive a LM317T with a PWM signal coming from my Arduino UNO.
Here is my circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

The PWM frequency is about 500Hz and I'm using a TLC2272 op amp and a LM317T regulator.
If the V_ref of the regulator is 1.25V, shouldn't I get an output voltage 1.25v greater than the adjust voltage?
When I measure V_out - V_adj I get about 2V.

What's wrong with my circuit?

Thanks

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    \$\begingroup\$ Does it work when you replace the PWM signal with a fixed voltage? \$\endgroup\$ – CL. Feb 14 '17 at 12:59
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    \$\begingroup\$ Have you tried reading the datasheet how the 317 works? It needs to sink current into the ADJ terminal. Plus it needs a minimal load. \$\endgroup\$ – PlasmaHH Feb 14 '17 at 13:06
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    \$\begingroup\$ Your circuit is not going to work, the LM317 does not like what you're doing. Have a look here: picbasic.co.uk/forum/showthread.php?t=6466 it shows how it can be done. Ignore the first schematic, scroll down to the second one. \$\endgroup\$ – Bimpelrekkie Feb 14 '17 at 13:20
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    \$\begingroup\$ Actually he's using a TLC2272, a bit more modern :-) He could have typed that in the schematic though. OK, I've done it already. \$\endgroup\$ – Bimpelrekkie Feb 14 '17 at 13:26
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    \$\begingroup\$ @FakeMoustache ah, thanks, yeah, I was stupid. But I have exactly that notion: OP wants to do something "special" with his LM317, but forgot to mention. XY Problem! (especially if OP's whole problem is to generate a voltage that is 1.5 V higher than the voltage he generates with a low-passed PWM) \$\endgroup\$ – Marcus Müller Feb 14 '17 at 13:44
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shouldn't I get an output voltage 1.25v greater than the adjust voltage? When I measure V_out - V_adj I get about 2V.

Yes, assuming other conditions of the circuit is satisfiedd.

What's wrong with my circuit?

Read the data sheet thoroughly. The circuit requires some minimum level of current to flow through for it to work.

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    \$\begingroup\$ Thanks, i missed that part of the datasheet. Now it works as intended \$\endgroup\$ – jagjordi Feb 14 '17 at 14:06

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