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# Voltage Regulator LM317 to 120ohm or to 240ohm

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Hello,

I'm in little dilemma with LM317, I searched trough almost all posts but the final answer wasn't there.
**If value is more than 120 ohms then the output voltage will rise, ok if that is correct.

So  I'm going to put a straight question: if I put 120ohm as R1 where is the difference towards 240ohm (except R2 value, except **), if there is no difference why put any other value?

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Hi Jhanus,
The datasheet shows 240 ohms as R1 with a more expensive LM117 on its first page.
The spec's in the datasheet show that an LM117 costs more than an LM317 because its operating current has a max amount of only 5mA but an LM317 has a max amount of operating current of 10mA. The operating current flows through R1 when there is no load.

Your LM317 might have an operating current of only 5mA then a voltage of 5mA x 240 ohms= 1.20V which is less than the 1.25V used for regulation, so it will work fine.
But if your LM317 has an operating current of more than 5mA then the output voltage will rise without a load. 10mA x 240 ohms= 2.4V (it must be less than 1.25V) so the voltage regulation will not occur!

They are all different. You cannot order an LM317 to have a low operating current like an LM117. Its operating current might be high enough to cause the ouput voltage to rise without a load if R1 is 240 ohms.

The operating current is spec'd with 40V across the IC and is a little less at lower voltages. It doesn't cost more to use 120 ohms for R1 so why not always use 120 ohms with an LM317?

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It doesn't cost more to use 120 ohms for R1 so why not always use 120 ohms with an LM317?

Exactly thats the answer I was looking for, so if there is no loss in voltage or current really why not always use 120ohm, if that puts us in secure area.

I'm using LM317 and LM337 for (-25)-0-(+25)V DC, could I drop 1.25V to 0 with diodes at output?
Can I put on the same coil with already LM337 another LM337 (parallel) to create -1.25V to eliminate 1.25V from positive regulator LM317,and vice-versa?!

Thanks again!

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The output voltage will not go to 0V with diodes in series with the input.

If two diodes are connected in series with the output then their output will go to 0V sometimes, but the voltage regulation will be ruined and the output voltage will change with output current and temperature as the diodes warm up.

The datasheet for the LM317 shows an LM113 1.2V zener diode used for the pot to get a negative 1.2V so the output will go down to 0V. an LM337 can also make a negative 1.25V instead.
Then the negative regulator could have a positive 1.25V for its pot.

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Thanks,

But now I found that I have a real problem, my second output is 28.28V AC, and that means 28.28*sqrt(2)=39.99V DC, but LM338K only allows 35V cross it, I don't need 35-40V DC, but I need 1.25-35V or 1.25-30V.

What to do now?!

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The extra votage will make a lot of heat if there is any load current. You have the wrong transformer.

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Oh no!, but thanks!

This setup all in all will cost me very much, ahh... :'(

I know I must be already very boring, sorry for that, but what if I would use redone circuit (http://www.electronics-lab.com/projects/power/003/index.html) for 5A on weaker transformer, let's say 80% of refered power, but wouldn't push it to the end?
And if output isn't 30VAC, but 28VAC?

Thank you.

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The transformer in the 30V/3A project is too small. With it the project has a max output voltage of 25VDC at 2.1A when the transformer is at its max power rating.
Its supply voltage for its opamps is higher than their max voltage allowed.
Some parts get too hot and fail.

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Well what to say,this is bad, I will continue to search for some solution, thanks.

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• 1 month later...

:)

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Hi Steven,
Altronics got it wrong like many people get it wrong.
National Semi invented the expensive LM117 and the cheaper LM317. They show 240 ohms with the LM117 and describe what happens if its value is higher than the spec's for the device.
The LM317 is supposed to have 120 ohms.

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