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Old 20th April 2012, 08:58 AM   #1
linxiaoxin is offline linxiaoxin  China
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Default 45v lm317?

hello,i need 45v power,i get i picture on the internet ,but i do not know it is right or not ,who can help me ?Click the image to open in full size.
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Old 20th April 2012, 09:18 AM   #2
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by linxiaoxin View Post
hello,i need 45v power,i get i picture on the internet ,but i do not know it is right or not ,who can help me ?Click the image to open in full size.
Easy. The voltage across R1 is ALWAYS 1.25V. So if you want to know the output voltage, you select the total of Vr1 + R2 to add voltage to the 1.25V.
An example: for output of 11.25V, you know that there will be 10V across R2+Rv1. (11.25 - 1.25). 10V is 8 times 1.25V on R1 so R2 + Vr1 = 8 times R1 = 8 times 220 = 1720 ohms.

For your 45V you know on R2 + Vr1 is 45-1.25 = 43.75V, so R2 + Vr1 = 43.75/1.25 * 220 = 7.7k....

For the -45 you can use an LM337 but the calculation is the same.

BTW This is also in the datasheet, very simple, you could easily find that yourself without worrying what is true or not. Datasheets are your friend! ;-)

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Last edited by jan.didden; 20th April 2012 at 09:20 AM.
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Old 20th April 2012, 09:22 AM   #3
woodturner-fran is offline woodturner-fran  Ireland
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Plus there are loads of online calculators to make it even easier. There are also LM317ht that can do 57v
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Old 20th April 2012, 09:28 AM   #4
linxiaoxin is offline linxiaoxin  China
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Originally Posted by janneman View Post
Easy. The voltage across R1 is ALWAYS 1.25V. So if you want to know the output voltage, you select the total of Vr1 + R2 to add voltage to the 1.25V.
An example: for output of 11.25V, you know that there will be 10V across R2+Rv1. (11.25 - 1.25). 10V is 8 times 1.25V on R1 so R2 + Vr1 = 8 times R1 = 8 times 220 = 1720 ohms.

For your 45V you know on R2 + Vr1 is 45-1.25 = 43.75V, so R2 + Vr1 = 43.75/1.25 * 220 = 7.7k....

For the -45 you can use an LM337 but the calculation is the same.

BTW This is also in the datasheet, very simple, you could easily find that yourself without worrying what is true or not. Datasheets are your friend! ;-)

jan didden


thany you very much ,i see.
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Old 20th April 2012, 09:38 AM   #5
abraxalito is offline abraxalito  United Kingdom
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Default Beware the input-output voltage limit

For any input voltage above 40V, there's no short circuit protection with this device - I've seen at least one explode, so be very wary of shorting the output. On power-up transients too its possible to exceed the abs max.
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Old 22nd April 2012, 09:47 AM   #6
Nisbeth is offline Nisbeth  Denmark
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LM783?

/U.
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Old 22nd April 2012, 10:08 AM   #7
KatieandDad is offline KatieandDad  United Kingdom
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You'd be better off building a regulator using discrete components at that voltage.

Have a look at http://www.kk-pcb.com for some ideas.
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Old 22nd April 2012, 10:29 AM   #8
KatieandDad is offline KatieandDad  United Kingdom
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Post #6 shows another example of a higher voltage regulator.
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Old 22nd April 2012, 03:00 PM   #9
Gavin Jaing is offline Gavin Jaing
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agree with KatieandDad
You'd be better off building a regulator using discrete components at that voltage.
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Old 22nd April 2012, 06:29 PM   #10
MarianB is online now MarianB  Romania
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LM317 can only handle about 1,5A so be carefull on that part, if you need higher current than you will need to add a power tranzistor on it, but as the other have sayd it would be better with zenner diodes and a few transistors to supply the needed current, it would be much safer as you can add the protection you want/need.
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