Capacitor values in 317/337 based PSU - diyAudio
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Old 11th March 2005, 08:08 AM   #1
klitgt is offline klitgt  Denmark
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Default Capacitor values in 317/337 based PSU

This thread came almost with the answer but still not quite

Regulator choices...

I plan to make a +24V/0V/-24V PSU for an audio buffer after a low pass active filter (crossover freq. 100Hz). The buffer has extremely low consumption (25mA).
I have calculated R3 and R5 to 200Ohms and R4 and R6 to 3,6K. This will regulate the input voltage to 24V.
The resistors R1 and R2 should be 1 Ohm and the caps C5, C6, C7 and C8 must be small, 100nF.
But I am not sure which values to use for C1, C2, C3, C4 and C9 and C10.
Can they all be approx. 470uF or is it advisable to use bigger values before the regulators than after them?
Any suggestions are welcome!
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Old 11th March 2005, 09:03 AM   #2
Zombie is offline Zombie  Sweden
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Bigger values before...have a look at the spec sheet
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Old 11th March 2005, 10:32 AM   #3
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470uF is at the bottom end of what I would use. You might also like to check the data sheets for the LM317 and LM337 as your resistor values are not optimal due to the current requirements of the adj terminals.

I have designed a very nice PCB for the sort of circuit you propose. It even uses the CRC input filter like you have. I have additional bypassing on the adj terminals. It also has the option to use an AC wall wart or split secondary transformer.

I can send you a copy of the layout if you want and you can make your own board from it. I am confident that the board gets the most from these regulators and a lot of effort has gone in to the design. I have thought about a group buy if enough people are interested. If people want any more info just ask here or email me.
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Old 11th March 2005, 10:39 AM   #4
Werner is offline Werner  Europe
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http://www.tnt-audio.com/clinica/reg..._noise2_e.html

http://www.tnt-audio.com/clinica/regulators2_impedance1_e.html
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Old 11th March 2005, 12:16 PM   #5
klitgt is offline klitgt  Denmark
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Quote:
Originally posted by richie00boy
...as your resistor values are not optimal due to the current requirements of the adj terminals.

I do not quite understand what you mean, "current requirements of the adj terminals"??

What values are required for 24V output? it can be obtained with R3=240 Ohm and R4=4,3K or R3=549 Ohm and R4=10K etc.

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Old 11th March 2005, 01:26 PM   #6
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If you look at the datasheets for both parts you will notice that they give a recommended fixed value of one of the resistors and that these resistors are different for the 317 and 337. This is because the current drawn through the adj terminal is different for the 317 to the 337 and hence the voltage drop slightly different. Also, these values are recommended because they are optimised.

You are not far off with the value for the 317, but it makes good practice IMO to optimise things.
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Old 11th March 2005, 02:18 PM   #7
klitgt is offline klitgt  Denmark
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Richie00boy, I understand, but look at the schematic from the National Semiconductor datasheet. Same values for voltage regulating resistors, 120 Ohms both at 317 and 337! AND they are both specified to 1% tolerance.

Your statement ..."they give a recommended fixed value of one of the resistors and that these resistors are different for the 317 and 337"... would make me believe that there was 120 Ohms at the 317 and e.g. 124 Ohms at the 337 in some datasheets, but I had not yet found a manufacturer that recommends this in any datasheet. Do you know of any?

Are the variable resistors 2K the ones that make sure that the resistor values are different at the 317/337 regulators?

Should I put a variable resistor at one of the 4 resistor positions and with a DVM tune in to the same output voltage at + and -?
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Old 11th March 2005, 04:24 PM   #8
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Check the individual datasheets for the 317T and 337T. The L suffix versions are a little different so don't use those. I believe I have the latest ones and they are May 2003 and November 2001 respectively.

I see 240 ohms for the 317 and 120 ohms for the 337. If you use these values then you simply apply the formula in the datasheets (both are very slightly different) to work out what value of other resistor you need.
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Old 11th March 2005, 04:27 PM   #9
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I have had a few emails regarding my regulator. To save sending info out many times I will update my website with some preliminary information on this project over the next few days. I will post the link here when done
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Old 11th March 2005, 07:06 PM   #10
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Very brief info now available on my website. Click the www button
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