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I'm currently trying to write out Kirchoff's voltage law loops for a LM317 voltage regulator circuit in order to derive the equation Vout = Vref(1+R2/R1) + (I_Adj * R2):

enter image description here

However, I'm currently stuck on trying to understand how the current flows before writing out the loop equations. I keep hearing about the LM317 maintaining a constant 1.25 V between the output and adjust terminals, but I'm still trying to intuitively understand how the current flows.

Could someone explain how the current is flowing through the entire circuit?

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  • \$\begingroup\$ Don't believe hearsay. Have you read the component datasheet? It usually tells how a part works and how to calculate things like voltages and currents. \$\endgroup\$
    – Justme
    Dec 30, 2022 at 1:33
  • \$\begingroup\$ Adjust terminal current = 100 uA max \$\endgroup\$
    – Antonio51
    Dec 30, 2022 at 10:40

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There is a voltage drop of 1.25V across R1. Let's say you set R1 = 1kOhm. What would you say the current is through R1?

Now once you calculate that current, looking at the bottom of R1, there are two paths. What do you know about the currents that takes those two paths? (I.e. if you know what Iadj is, and the current through R1, can you calculate the current through R2?).

That answers your question about currents. The step that would be to calculate the output voltage based off of those currents.

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  • \$\begingroup\$ The datasheet for the LM317 says that the current in R1 should be 10mA or more. Then R1 should be 1.25V/10mA= 125 ohms or less, usually 120 ohms. The more expensive LM117 can use 240 ohms for R1. \$\endgroup\$
    – Audioguru
    Dec 30, 2022 at 2:26
  • \$\begingroup\$ First off, that is incorrect, that 10mA is the total minimum output current that is needed for regulation. That current doesn't necessarily have to pass through R1. Secondly, it bears no relevance to the actual explanation, and large value of R1 and R2 will lead in to discussing the effects of the adj terminal current. Lastly, the LM317 datasheet has system examples with 240 Ohms and higher \$\endgroup\$ Dec 30, 2022 at 3:08
  • \$\begingroup\$ Adjust terminal current = 100 uA max \$\endgroup\$
    – Antonio51
    Dec 30, 2022 at 10:40
  • \$\begingroup\$ Most circuits shown on the datasheet of the LM317 show the more expensive LM117 with a 240 ohm resistor for R1 and no load, including the circuit on the first page. The LM317 needs to have a total load (R1 when there is no load) of 10mA only when Vin - Vout is high at 40V which could easily happen if the load is disconnected or its current drops. \$\endgroup\$
    – Audioguru
    Dec 31, 2022 at 15:52
  • \$\begingroup\$ @Audioguru Ok, and? You just reiterated what I said, and what you previously said. And I still fail to see how any of this is relevant to the question or answer. \$\endgroup\$ Dec 31, 2022 at 21:36

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