This thread came almost with the answer but still not quite
http://www.diyaudio.com/forums/showthread.php?s=&threadid=49292&perpage=10&pagenumber=3
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!
http://www.diyaudio.com/forums/showthread.php?s=&threadid=49292&perpage=10&pagenumber=3
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!
Attachments
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.
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.
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.
You are not far off with the value for the 317, but it makes good practice IMO to optimise things.
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 -?
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 -?
Attachments
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.
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.
If you are a bit lazy you can use my Excel calculation for LM317.
http://www.sjostromaudio.com/hifi_files/misc/lm317_calc.zip
http://www.sjostromaudio.com/hifi_files/misc/lm317_calc.zip
In that schematic the voltage is variable, so precisely accounting for adjust pin current is a waste of time when it can just as easily be done by turning the pot a little.klitgt said:
As for capacitor values: you need to size C1/2/3/4 so that the input voltage never falls too low (i.e. the lowest part of the ripple must always exceed the output voltage + minimum dropout of the regulator), also taking into account the voltage drop across the resistor (do you really need a CRC filter and a regulator?). For C9/10, personally I wouldn't bother going any larger than 10uF, with probably another 10-100uF more directly on the PCB of whatever circuit is being supplied by the regulator. There is no particular benefit to ripple from increasing the capacitance beyond that (although the noise measurements linked to by Werner are interesting).
I am following the recommended values for those resistors that is why they are 240 and 120 ohms. The 317 and 337 have different adj pin current demands so by using these values of resistor you create the same offset voltage across them.
MrEvil, the adj pin draws a set current from the potential divider network. You are right that the current flow in this network will change with different resistor/pot settings, but the adj pin current stays constant.
The regulator will work with other resistor values, I'm just trying to optimise things as much as possible
That is why I have a CRC filter and a regulator as you seem to find unneeded Doing this enables me to get the best noise and regulation performance.
The reason I have 47uF on the output is because if you add large capacitance here it improves the stability of the regulator. If you only have small capacitance here then large capacitance on the op-amp PCB then the stability and o/p impedance of the regulator is not improved as effectively because of the inductance of the wires/traces to the bigger caps.
MrEvil, the adj pin draws a set current from the potential divider network. You are right that the current flow in this network will change with different resistor/pot settings, but the adj pin current stays constant.
The regulator will work with other resistor values, I'm just trying to optimise things as much as possible
That is why I have a CRC filter and a regulator as you seem to find unneeded Doing this enables me to get the best noise and regulation performance.The reason I have 47uF on the output is because if you add large capacitance here it improves the stability of the regulator. If you only have small capacitance here then large capacitance on the op-amp PCB then the stability and o/p impedance of the regulator is not improved as effectively because of the inductance of the wires/traces to the bigger caps.
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