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I'm building variable voltage regulator, the regulator has levels of 9, 12, 15 and 18 volts and a potentiometer that allows for incremental adjustment between the levels, so at level 18v you can drop down close to 15V.

My circuit works fine when testing, but I need to connect it to a load resistor. I've been using a 5k potentiometer and varying the resistance to make sure the voltage stays steady, which it does.

However without a load resistor the voltage regulation does not work, which brings up a question.

Do I need to implement a load resistor in the actual design?

The device I'm building will be used as a power supply for guitar effect pedals, which usually have a current output of 10mA to 450mA, so the device should work fine when connected to a pedal.

But my device also has to pass a test, so how do I check the voltage output when a load isn't attached?

Here is the data sheet for the chip I'm using and below is my schematic in three pictures to make it easier to see.

All the pins starting with an "S" will go to a dp4t switch and the pins starting with a "P" will go to a potentiometer.

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  • \$\begingroup\$ If you use a specific LM317 make or model, please add it to the question. Some require more or less current drawn out than others, but the resistor between Vout and Vadj should provide it already, as long as it is low enough. \$\endgroup\$
    – Justme
    Apr 13, 2021 at 6:39

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If the component is LM317TG from OnSemi, it says typically at least of 3.5mA current draw is needed to stay within regulation, but it can be as much as 10mA.

Your resistor network does not guarantee this.

Basically, it makes very little sense to try using so large resistances between Vout and Vadj to try conserve idle power consumption, and then add an extra load to stay within regulation. Adding the extra load on output is also pretty useless, as you need to make sure at least 10mA is always drawn, so 10mA at lowest voltage means much higher current at higher voltages.

It would be best to follow the datasheet, and use a single fixed resistor between Vout and Vadj to make sure it always consumes 5mA, or maybe 10mA even.

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The datasheet for the LM317 (the TI one, which acquired National) states a recommended 10mA minimum load; probably it won't work with the stated tolerances if you pull less than that from it. The typical value states is 3.5mA.

Considering that this regulator is floating its whole supply need to come from the output and the feedback terminal, consider these 10mA as the Iq of the regulator.

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