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Old 16th January 2012, 10:01 PM   #1
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Default LM317

Hi There,

Is it feasable to use an LM317 regulator on each side of a split transformer? The idea is to use two LM317s to get more amps available to an audio IC. Will this work?

(I'm normally into mechanical engineering type stuff, and relativly ignorant of electronics)

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Old 16th January 2012, 10:34 PM   #2
SY is offline SY  United States
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In a word, yes, assuming you have one for each channel. An easier way is to use an external pass transistor (see National semi app notes).
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Old 16th January 2012, 10:55 PM   #3
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How much current do you actually need? the official limit of an LM317 is 1.5A (depending on package), but as SY said see the app notes (or datasheet) to see common circuits for increasing the current capability, as I think this would be a much simpler (and probably less problematic) way to do it

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Old 16th January 2012, 11:11 PM   #4
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Default Thanks guys

As to current, 1.5 amps is pushing it. I hope to get 56 watts out of an audio amplifier that is powered by this setup, and I'm planning on about 30 t0 35 volts, so I'd like to make sure that I have enough power. And that means, to me, that I need two LM317s. Also, I try to stay away from complicated thinmgs like transistors. The amplifier is based on a lm3876 chip, with caps and resistors in various places. I'm mainly shooting in the dark; my first priority is to avoid burning down the house. :-)

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The Happy Hippy

PS. Does anyone have a schematic that shows exactly what resistors to use for getting 30 volts or 35 volts out of this Lm317? A pot or variable resistor does me no good in this situatiuon.
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Old 16th January 2012, 11:27 PM   #5
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The application is a bit unclear to me. The LM317 will not provide more amps - the only way to do that is to increase the voltage or lower the resistance. I have doubts that the LM3876 requires a regulated supply, so a typical 3-pin regulator circuit would seem somewhat counterproductive.
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Old 16th January 2012, 11:29 PM   #6
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Default Eureka!

OK! I just answered one of ny own questions. When all else fails, read the instructions. On the LM317 datasheet, it clearly states that you can use a LM137-337 series regulator to put on the negative side of the power supply. Is't that wonderful! However, I still don't know what resistors and cap to use for 35 volts. Any thoughts will be appreciated.

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Old 16th January 2012, 11:30 PM   #7
Minion is offline Minion  Canada
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When useing a Pass transistor there isn"t anything complicated about it , it is just a component like any other and probably less complicated to work with and take up less space than useing 2 regulators .......

If you look in the Datasheet there is a circuit in there useing a pass transistor that isn"t complicated and if you use the values shown you shouldn"t have any issues .....
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Old 16th January 2012, 11:53 PM   #8
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The formula for output voltage is given on the datasheet- 1.25(1 + R2/R1). So if you choose R1 as 120R (this causes the regulator to put out ~10mA minimum, since output-to-sense is 1.25V), then you can solve for R2. Output voltage is 35V, so 35 = 1.25 (1 + R2/120), and then R2 = 120(35 - 1.25) = 4050 ohms. Nearest 1% values are 4020 and 4070, good enough.
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Old 16th January 2012, 11:58 PM   #9
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Default Idea?

My idea was that, if I need more that 1.5 amps, then I can put one regulator, each good for 1.5 amps, on each side of a split power supply, and get the necessary amperage. I have been told, by my genius friends, that if I have 50 volts AC coming out of a transformer, it will knock down to 35 volts DC. (50 times the sine of 45 degrees, .7071) Also, voltage regulators are supposed to smooth out the flow of the electricity, I think. and getting 56 watts out of the amplifier, which is the purpose of all this, seems to mean that I'll need a little over 1.5 amps. And there maybe a voltage drop someplace, down to around 32v or even 30v. So, that's the basic idea. Will that work?

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Old 17th January 2012, 12:05 AM   #10
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Yes, that's more or less the idea. If you have higher voltage from the transformer than the chip amp can handle, some of it has to be dropped somehow. If you drop 15 volts across the regulator at 2 amps or more, it will need a substantial heatsink.

edit: As far as the voltage for the chip, National says, "Operation is guaranteed up to 84V, however, distortion may be introduced from SPiKe Protection Circuitry when operating above 70V if proper thermal considerations are not taken into account. Refer to the Thermal Considerations section for more information."
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Last edited by sofaspud; 17th January 2012 at 12:10 AM.
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