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Lm317

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The LM317 does not have a ground connection.
If you limit the voltage between its input and output to 40V then its input could be 50V and its output could be 10V to 47V.
But then it gets fried if the output is shorted to ground.

I was looking at this:
**broken link removed**
and
LM317 - Wikipedia, the free encyclopedia

And it says the max input voltage is 40v. Are they wrong or is the datasheet being interpreted wrong?

The way the datasheet looks as long as the difference is 40 it should work. As in I can input a max of 80V and output 40V(80-40=40)max. But they are stating the input is max of 40.
 
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You've interpreted it wrong.

It says this on the first page of the datasheet.
LM317datasheet said:
Since the regulator is
“floating” and sees only the input-to-output differential voltage,
supplies of several hundred volts can be regulated as
long as the maximum input to output differential is not exceeded,
i.e., avoid short-circuiting the output.
https://www.electro-tech-online.com/custompdfs/2009/07/LM117-2.pdf
 
You've interpreted it wrong.

It says this on the first page of the datasheet.

https://www.electro-tech-online.com/custompdfs/2009/07/LM117-3.pdf

So you could put 200V in and 170 out but since it's less than 40 it's ok?

I assume this is the way it is but the way they word it is really misleading. Quote from the wikipedia page:
LM317 is a positive voltage regulator supporting input voltage of 3V to 40V and output voltage between 1.25V and 37V

That would suggest a MAX of 37V out. Would it not?
 
It has a dropout voltage of 3V meaning the output must always be 3V below the input in order for it to regulate properly: 40V - 3V = 37V.
 
I have used the LM317 as a 160V supply, from NS "linear brief" 47.
Worked fine. AND the LM337 as a (-) 160V supply
 
Sorry to make this post alive from back again . . . .But I want to ask a question here
How can I get to know that if I have like say 50V at input and about 20V at output means; Vin - Vout = 30V & lets say am requiring 1A output . . .So how can I get to know that for these values I wont need a heatsink or would I?
 
If you have some of the newer one, they can handle a little more voltage, but if you have 50 volt in, regulate to 20 volts out, your dropping 30 volts on the regulator and if you passing 1 amp, that's 30 watts. mount it on a heat sink and immerse in oil or huge heat sink with fan. Really though, figure out how much voltage the 317 is dropping and how much current it is passing and that gives you wattage.
Kinarfi
 
Ok Sir. and one thing more. I have the datasheet of LM317 , so like where exactly I have to look to find out how much watt can my LM317 bear ?
 
You should have been taught about datasheets in University.

The datasheet shows that the absolute maximum temperature of the chip in an LM317 is 125 degrees C.
It shows that if you have a PERFECT heatsink and some kind of PERFECT cooling then its chip heats at 4 degrees C for each Watt of power it dissipates. Then if the ambient temperature is 40 degrees C then the chip can heat 125 - 40= 85 degrees C and it can dissipate 85/4= 21.25W.

But a normal heatsink without a fan gets hot. Its thermal resistance is listed in its spec's and might be 2 degrees C per Watt for a pretty big heatsink. If an insulator is used then it might have a thermal resistance of 1 degree C per Watt. Then the total thermal resistance from the chip to the ambient is 4 + 2 + 1= 7 degrees C per Watt.
If the ambient is 40 degrees C then the chip will be at its absolute max allowed temperature of 125 degrees C when it dissipates only 85/7= 12.1W.

Without a heatsink, the TO-220 package has a thermal resistance of 50 degrees C. Then with a 40 degrees C ambient it can dissipate
85/40= 2.1W when its chip will be at its max allowed temperature of 125 degrees C.
 
Your phone line is 50VDC when it HAS NO CURRENT!
When it has a phone that is off-hook (or has your charger circuit connected) and is drawing about 20mA then the voltage drops to about 10V (measure the voltage yourself).
 
The LM317 and LM117 are IDENTICAL die and are made from the same mask sets in the same fab. The 117 is tested to tighter limits, and parts that fail those tests are retested and can become LM317 devices. So, they are the same part but tested to different spec limits.
 
You should have been taught about datasheets in University.

The datasheet shows that the absolute maximum temperature of the chip in an LM317 is 125 degrees C.
In truth, that is the limit for the part to meet the specs guaranteed "over full temp range" but any 317 would be "functional" to at least 150C since the mil version is guarateed to work to 150C and they are identical die. They don't like to admit that, but it should work as a regulator up to 150C.
 
Thankyou Thankyou Sir. Thankyou very very much for your so much detailed explanation. With your so much detailed explanation I am able to get things clearly just like this one.
You should have been taught about datasheets in University.

The datasheet shows that the absolute maximum temperature of the chip in an LM317 is 125 degrees C.
It shows that if you have a PERFECT heatsink and some kind of PERFECT cooling then its chip heats at 4 degrees C for each Watt of power it dissipates. Then if the ambient temperature is 40 degrees C then the chip can heat 125 - 40= 85 degrees C and it can dissipate 85/4= 21.25W.

But a normal heatsink without a fan gets hot. Its thermal resistance is listed in its spec's and might be 2 degrees C per Watt for a pretty big heatsink. If an insulator is used then it might have a thermal resistance of 1 degree C per Watt. Then the total thermal resistance from the chip to the ambient is 4 + 2 + 1= 7 degrees C per Watt.
If the ambient is 40 degrees C then the chip will be at its absolute max allowed temperature of 125 degrees C when it dissipates only 85/7= 12.1W.

Without a heatsink, the TO-220 package has a thermal resistance of 50 degrees C. Then with a 40 degrees C ambient it can dissipate
85/40= 2.1W when its chip will be at its max allowed temperature of 125 degrees C.

& Sir one thing more. as you have said " Without a heatsink, the TO-220 package has a thermal resistance of 50 degrees C. Then with a 40 degrees C ambient it can dissipate
85/40= 2.1W when its chip will be at its max allowed temperature of 125 degrees C. "
Does this mean that without heat sink and cooling, my LM317 would be just able to bear 2.1Watt ? Thats all ?
 
Sir one thing more. as you have said " Without a heatsink, the TO-220 package has a thermal resistance of 50 degrees C. Then with a 40 degrees C ambient it can dissipate 85/40= 2.1W when its chip will be at its max allowed temperature of 125 degrees C. "
Does this mean that without heat sink and cooling, my LM317 would be just able to bear 2.1Watt ? Thats all ?
Correct. Then it will be at its maximum allowed temperature. It must not be confined inside a box.

It might fail early due to thermal fatigue if it is turned on and off many times because it will heat and cool many times.
 
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Sorry for reposting here again but I have came up with another question now . . . . You have taught me this everything by assuming the ambient's (IC) temperature to be 40 degree C . . . I wana ask you that how am I going to calculate my ambient's temperature or which limit of ambient should I take for making a circuit accordingly? Is the ambient's temperature same as my surrondings(while keeping the input voltage and other factors under the limit)?
 
The ambient temperature is the temperature of the air around your LM317, not around you. If the LM317 is in a case then the temperature of the air in the case might be higher than the air outside the case.
The temperature of air in an air conditioned room is about 25 degrees C in North America.
 
um mean to say the ambient temperature(the temperature of the air around my lm317) is the same as temperature of the air which can also be checked by a normal thermometer placed anywhere in this room right?
 
um mean to say the ambient temperature(the temperature of the air around my lm317) is the same as temperature of the air which can also be checked by a normal thermometer placed anywhere in this room right?
I don't know if your LM317 is in open air or if it is inside a box. If it is in open air then its ambient temperature is the temperature of the air around it.
Yes, a thermometer in its room will show its ambient temperature.
 
Thankyou thankyou :)
now its clear to me much more:)

one thing more i want to ask here , its that as you have said "
Then if the ambient temperature is 40 degrees C then the chip can heat 125 - 40= 85 degrees C and it can dissipate 85/4= 21.25W."

means by using a heatsink and proper cooling system, my lm317T would dissipate about 21.25W right? so like even if I use my lm317t with proper cooling,heatsink and while dissipating 21.25W for no matter how many hours would it still keep on working properly? without any harm or danger?
 
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