Approximate values for voltage level with no load condition

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rajaram04 said:
okk so how to protect that ?
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Protect what?
It operates in class-A so the active device (the LM317) and its DC-load (the 10 ohm resistor) both waste a lot of power by making a lot of heat.
Most audio amplifiers use two complementary transistors operating in class-AB so their idle current is very low. They drive the speaker PUSH-PULL.
 




my god amazing info you have a terrific idea about basic . . really

okk i ll follow that

Now here coming to our original ciruit with 120 ohms i connected R2 as 470 ohms for 6 volts but on scale its showing 6.25 volts . . is that ok or need to make value close to 6 volts . . please comment
 

ICs are not exactly the same. Some are better than others even when they have the same part number and are made by one manufacturer.
The LM317 has a range of "minimum load current" from a typical amount of 3.5mA to a maximum of 10mA. The maximum for the more expensive LM117 is only 5mA. The datasheet says, "If there is insufficient load on the output, the output will rise."

The two voltage-setting resistors are the load when there is no other load. 1.25V/120 ohms is a little more current than 10mA so 120 ohms is fine for all LM317 ICs that have a very low current to an external load like a Cmos logic circuit. Some LM317 ICs like yours might work if the 120 ohm resistor is 470 ohms but the voltage might change when the temperature changes.
 





hmm & so now its showing voltage of 6.40 volts (starting previously from 6.24 volts) . .
but i connected a blue LED with a 100 ohm resistor at output but still getting voltage of 6.40 v
now what to connect or what to do ?
 

The datasheet for the LM317 shows that the voltage across the 120 ohm resistor can be from 1.20V to 1.30V. The 120 ohm resistor might have 5% tolerance so it can be from 114 ohms to 126 ohms. The 470 ohm resistor might also have 5% tolerance so it can be from 447 ohms to 494 ohms. The adjustment pin current might be max at 100uA.

Then the 120 ohm resistor at 114 ohms with 1.3V plus 100uA has a total current of 11.5mA. The 470 ohm resistor at 494 ohms will have a voltage of 5.7V.
Then the total output voltage is 1.3V + 5.7V= 7.0V. If the tolerances were the opposites then the output voltage will be lower than you want.

Your output voltage drifted maybe because the resistors are cheap and their value changed as they warm up.
 





hmm i see a lot of idea i still have to learn from you

well , should i use presets in place of resistors ??

Too a problem is occurying that output is varying like say if its showing 9.17 at starting up then after half a minute the value oscillates between 9.17 to 9.18 in a gap of 3 or 4 seconds continously , so i connected a 100 uf cap. but no change . . .
 

If you use cheap resistors or a cheap copy of an LM317 then the voltage might wander a little like yours does. Maybe your voltmeter is not stable.
I use name brand carbon film or metal film resistors and National Semi (Texas instruments today) LM317 ICs and their voltage is very stable.
 




no multimeter is ok but may be devices i am using are not from such manufacturers
well i ll have to manage with these only . .
 

so please tell , should i apply presets here in place of resistors ?
 

i have this type of LM 317 from ST devices

 

Use trimpots if you want to adjust the output voltage.
National Semi (Texas Instruments now) invented the LM317 and their manufacturing is excellent. I have never used anything made by ST Micro so I do not know if theirs is any good. Maybe you have a fake one.
 





till the time come i have to implant this one only . .

Actually as i told earlier my 12 volt supply is varying & due to that the output is wandering here

so should i connect a zener at the output end of dc adapter ?

- - - Updated - - -

how thats possible to manage it a lilte bit ?????
 

The datasheet of the LM317 shows that its output voltage is extremely stable.
When the input voltage changes from 3V to 40V then the output voltage typically changes only 4mV. When the load current changes from 10mA to 1.5A then the output voltage typically changes only 0.1%. When the chip temperature changes from 0 degrees C to 125 degrees C the output voltage typically changes only 0.1%. The LM117 and the LM317A are better.

If you connect a zener diode to the output of an AC-DC adapter then the zener diode and the adapter will overheat since there will be nothing limiting the current.
An LM317 is much better than a zener diode anyway.

The datasheet shows that poor wiring (which is common when a breadboard is used) can degrade voltage regulation of an LM317 a lot.
 







hmm so is it possible to limit current in AC DC adapter ? any simple technique ?
 

Do you want to limit the current from an AC-DC adapter so it can feed a zener diode?

A zener diode has poor voltage regulation when it is compared to an LM317.
The current in a zener diode is usually limited with a series resistor. The zener diode is a SHUNT regulator so it gets the hottest when there is no load. The LM317 is a SERIES regulator so it gets the coolest when there is no load.
 




hmm i see , But i don't want to do current limiting by zener actually . . i just want to make a balance so as to stop that wandering value of voltage i explained earlier , so planing diffrent ways . . well we may leave that its nothing serious i ll change that adapter later . . Now its time to move for circuit building as i am on the final stage wish me luck pray for me & i need al l your bless , rest on almighty
 

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