12v to 1.5v

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qwikv6

Member - Posts: 4

Joined: August 23, 2006

Location: Texas, United States

Posted: March 04, 2007 at 4:10 PM / IP Logged

I've got an LM317T-Positive adjustable voltage regulator that i'm planning on using to take 12v down to 1.5v to power an mp3 player. But I'm not sure what size resistors i need to use between the out/adj and the adj/gnd. Or any sites that have charts with this kind of info? Thanks in advance.

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master5

Silver - Posts: 1,123

Joined: October 10, 2006

Location: United States

Posted: March 04, 2007 at 8:50 PM / IP Logged

I am curious why you would need any resistors if the voltage regulator is adjustable. Am I missing something?

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master5

Silver - Posts: 1,123

Joined: October 10, 2006

Location: United States

Posted: March 04, 2007 at 9:11 PM / IP Logged

oops..sorry I goofed..you are using this component to build one yourself.

I found this formula for that particular regulator...V out = V ref (1 + r1/r2) + I adj r2

V ref = 1.25 volts nominal. r2 goes to gnd

Personally...this is a bit much algebra for me..been too long...I would get one of those cheesy cheap cigarette lighter plugs with the adjustable voltage output dial on it and hardwire that into 12v source. All the works done...lol

Best of luck either way.

If you find that formula is incomplete for that regulator just google LM317T

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KPierson

Platinum - Posts: 3,527

This member consistently provides reliable information

Joined: April 14, 2005

Location: Ohio, United States

Posted: March 05, 2007 at 4:29 PM / IP Logged

You can usually ignore the Iadj * R2 at the end, as Iadj is limited to 100uA max (very small) per the data sheet.

Vout = Vref (1+R2/R1)

Vout = 1.50 (for MP3 player)

Vref = 1.25 (per data sheet)

R1 = 10,000 ohms (starting point)

1.5 = 1.25 (1+r2/10000)

R2 = 2000 ohms

Checking:

Vout = Vref (1+R2/R1)

Vout = 1.25(1+2000/10000)

Vout = 1.25(1+0.2)

Vout = 1.25(1.2)

Vout = 1.5vdc

One alternative to fine tune this is to go with a 1K resistor for R2 and then use a 10K pot for R1. You should be able to dial in 1.5vdc exactly.

Kevin Pierson

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12v to 1.5v

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