+-30Vout with LM31+337

merlin el mago · 12th April 2011, 03:47 PM

I make this regulator with LM317 & LM337 to get +-30Vout but only gives +-23Vout how can do to get +-30Vout?

AndrewT · 12th April 2011, 04:04 PM

Hi.
the resistor values Ra & Rb are correct.
You must have an assembly error.

AP2 · 12th April 2011, 04:09 PM

Hi,
what is the voltage on the input pin of the LM317?
Is better to reduce the output capacitors 10-22uF max. (not good 220uF).
large caps at output, not help ripple and PSRR. (on active regulators)

Regards

luka · 12th April 2011, 04:27 PM

larger output capacitance is not a problem, none would be, since it could oscillate

Andrew Eckhardt · 12th April 2011, 04:47 PM

If the nodes between the + and - output filter caps are floating as shown in the drawing, and you measured output with reference to there, you could get such funny readings.

AndrewT · 13th April 2011, 08:37 AM

centre tap is connected to output Zero Volts.

wintermute · 10:21 AM

The interesting thing there is that the value of 23V is the approximate value you get before multiplying by the reference voltage of 1.25V (23.85V). Was the figure 23V rounded down, or exact? I'm wondering if the LM317 and LM337 are out of spec and the reference voltage is around 1V or a bit lower, though it seems unlikely that they would both be out of spec by the same amount... could you take a voltage measurement across Ra and Rd (it should be ~1.25V)

In order for your output voltage to be 23V (if all is working properly) you would need either a 315 ohm resistor in place of the 240 ohm, or a 4k176 resistor in place of the 5k49 resistor (assuming the other one is correct). Both of those values don't look like they could be easily mistaken.

If the resistors were marked as the correct values but out of tolerance enough (higher and lower in the right way) they would need to be more than 10% out assuming the LM's ref voltage is 100% accurate. However the standard LM3x7 has a tolerance of +-4% on the ref voltage. In the worst case scenario with 10% resistors, with ra +10% rb -10% and Vref -4% we would get ~22.5V

I'm guessing however that this is not the case, but hopefully the above is an aid for troubleshooting

Also as AP2 said, what is the input voltage!! ?? Though I also agree with luka about the large caps not being bad (however I'm only going off my interpretation of the datasheet, not yet real world experience, which I will hopefully have soon).

Tony.

merlin el mago · 09:48 AM

Quote:

Originally Posted by AndrewT

centre tap is connected to output Zero Volts.

My tx isn't center tapped, have I to connect to mains earth?

Quote:

Originally Posted by wintermute

The interesting thing there is that the value of 23V is the approximate value you get before multiplying by the reference voltage of 1.25V (23.85V). Was the figure 23V rounded down, or exact? I'm wondering if the LM317 and LM337 are out of spec and the reference voltage is around 1V or a bit lower, though it seems unlikely that they would both be out of spec by the same amount... could you take a voltage measurement across Ra and Rd (it should be ~1.25V)

In order for your output voltage to be 23V (if all is working properly) you would need either a 315 ohm resistor in place of the 240 ohm, or a 4k176 resistor in place of the 5k49 resistor (assuming the other one is correct). Both of those values don't look like they could be easily mistaken.

If the resistors were marked as the correct values but out of tolerance enough (higher and lower in the right way) they would need to be more than 10% out assuming the LM's ref voltage is 100% accurate. However the standard LM3x7 has a tolerance of +-4% on the ref voltage. In the worst case scenario with 10% resistors, with ra +10% rb -10% and Vref -4% we would get ~22.5V

I'm guessing however that this is not the case, but hopefully the above is an aid for troubleshooting

Also as AP2 said, what is the input voltage!! ?? Though I also agree with luka about the large caps not being bad (however I'm only going off my interpretation of the datasheet, not yet real world experience, which I will hopefully have soon).

Tony.

I verified all connections & changed both regulators LM317 & LM337, measurements:

Rd -0.392V without load (measured with dvm 237R instead 240R)
Ra +0.859V without load (measured with dvm 239R instead 240R)
Vout -24.1V measured without load
Vout +21.9V measured without load

Rc 5K48 instead 5k49
Rb 5k48 instead 5k49

Vout = 1.25 V (1+Rc or Rb / R1) V.
Vout = 0.392V + (1+5480/237) = -24.514V
Vout = 0.859V + (1+5480/239) = +24.787V

240R 1% 50ppm
5K49 0.1% 15ppm

I think if I use a 6K8 for Rc & Rb could get +-30Vout, right?

AndrewT · 14th April 2011, 10:35 AM

Then you have the wrong rectifier.

A centre tapped + 4diode bridge rectifier is capable of dual polarity supplies.

A single secondary must use half wave rectifier for each polarity.
Not nearly as good for ripple.

merlin el mago · 14th April 2011, 10:46 AM

I have two secondaries x 30V in parallel so I can do in series to get a center tapped, right?

12th April 2011, 04:27 PM	#4
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	larger output capacitance is not a problem, none would be, since it could oscillate __________________ home page @ http://www.classdaudio.tk/ @ 24/7 all year long I FEEL SLOVENIA

13th April 2011, 10:18 AM	#7
wintermute just another diyAudio Moderator Join Date: Aug 2003 Location: Sydney Blog Entries: 22	The interesting thing there is that the value of 23V is the approximate value you get before multiplying by the reference voltage of 1.25V (23.85V). Was the figure 23V rounded down, or exact? I'm wondering if the LM317 and LM337 are out of spec and the reference voltage is around 1V or a bit lower, though it seems unlikely that they would both be out of spec by the same amount... could you take a voltage measurement across Ra and Rd (it should be ~1.25V) In order for your output voltage to be 23V (if all is working properly) you would need either a 315 ohm resistor in place of the 240 ohm, or a 4k176 resistor in place of the 5k49 resistor (assuming the other one is correct). Both of those values don't look like they could be easily mistaken. If the resistors were marked as the correct values but out of tolerance enough (higher and lower in the right way) they would need to be more than 10% out assuming the LM's ref voltage is 100% accurate. However the standard LM3x7 has a tolerance of +-4% on the ref voltage. In the worst case scenario with 10% resistors, with ra +10% rb -10% and Vref -4% we would get ~22.5V I'm guessing however that this is not the case, but hopefully the above is an aid for troubleshooting Also as AP2 said, what is the input voltage!! ?? Though I also agree with luka about the large caps not being bad (however I'm only going off my interpretation of the datasheet, not yet real world experience, which I will hopefully have soon). Tony. __________________ Any intelligence I may appear to have is purely artificial! Some of my photos Last edited by wintermute; 13th April 2011 at 10:21 AM.

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12th April 2011, 04:04 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi. the resistor values Ra & Rb are correct. You must have an assembly error.

12th April 2011, 04:09 PM	#3
AP2 diyAudio Member Join Date: Dec 2009 Location: Italy	Hi, what is the voltage on the input pin of the LM317? Is better to reduce the output capacitors 10-22uF max. (not good 220uF). large caps at output, not help ripple and PSRR. (on active regulators) Regards

12th April 2011, 04:47 PM	#5
Andrew Eckhardt diyAudio Member Join Date: Nov 2005 Location: PA	If the nodes between the + and - output filter caps are floating as shown in the drawing, and you measured output with reference to there, you could get such funny readings.

13th April 2011, 08:37 AM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	centre tap is connected to output Zero Volts.

14th April 2011, 10:35 AM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Then you have the wrong rectifier. A centre tapped + 4diode bridge rectifier is capable of dual polarity supplies. A single secondary must use half wave rectifier for each polarity. Not nearly as good for ripple.

14th April 2011, 10:46 AM	#10
merlin el mago diyAudio Member Join Date: Sep 2009 Location: Catalonia new Europe state	I have two secondaries x 30V in parallel so I can do in series to get a center tapped, right?