Voltage drop in LM337 based PS! - diyAudio
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Old 5th December 2013, 06:13 AM   #1
Roushon is offline Roushon  India
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Default Voltage drop in LM337 based PS!

Built a power supply based on LM317/LM337 using
fixed resistors. Giving +- 11.3v. But when I connect the PS to the circuit (OPAMP AD8620) the + supply remain 11.3v but the - supply (LM337) drops by 1 v, that is, it shows about -10.3v. What could be the reason? Is it a standard fact about LM337? I have couple of these PS circuits built to check and LM337 shows the same phenomena!

The resistor from output to control pin is 120Ohm in both LM317 and LM337.

Thanks for any comment/reply.

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Roushon.
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Last edited by Roushon; 5th December 2013 at 06:16 AM.
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Old 5th December 2013, 07:20 AM   #2
John8 is offline John8  United Kingdom
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What is the input voltage to the LM337 with the output loaded?
There should be at least 3V drop across the IC to maintain regulation. Also input and output capacitors are a good idea mounted close to the IC to avoid instability as per the data sheet.
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Old 5th December 2013, 07:36 AM   #3
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Does the negative regulator have a minimum current (check the data sheet) that must be drawn at all times ?
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Old 5th December 2013, 08:22 AM   #4
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LM317/LM337 need 10mA to go through the regualator - otherwise they may fall out of regulation
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Old 5th December 2013, 09:50 AM   #5
Roushon is offline Roushon  India
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Thanks for all the comments. The input voltage is +-20v, so there is enough difference in voltage. Will check about the minimum current requirement and reduce the output-to-control pin resistor value (and hence the resistor from control to ground also).

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Old 5th December 2013, 03:51 PM   #6
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Just add 1Kohm across the output from each reg to 0v to ensure that the minimum load requirement is met under all conditions.
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Old 6th December 2013, 03:09 AM   #7
Roushon is offline Roushon  India
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Quote:
Originally Posted by martin clark View Post
Just add 1Kohm across the output from each reg to 0v to ensure that the minimum load requirement is met under all conditions.
Great! Thanks a lot. Will do that.

Roushon.
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Old 6th December 2013, 03:11 AM   #8
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In my experience getting the 'wrong' output voltage has been related to oscillation. Do you have a scope to check for that? The minimum current requirement would in my estimation result in a more negative output voltage being registered, not less - without enough current the magnitude of the output rises.
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Last edited by abraxalito; 6th December 2013 at 03:14 AM.
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Old 6th December 2013, 03:25 AM   #9
mirlo is offline mirlo  United States
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Question minimum load

Quote:
Originally Posted by martin clark View Post
Just add 1Kohm across the output from each reg to 0v to ensure that the minimum load requirement is met under all conditions.
Doesn't the 120 Ohm resistor between Output and Adjust pins take care of the min load? A little more from a 1k to ground won't hurt but I doubt this is the problem with the circuit here.

Last edited by mirlo; 6th December 2013 at 03:26 AM. Reason: misspelling
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Old 6th December 2013, 12:21 PM   #10
Roushon is offline Roushon  India
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Quote:
Originally Posted by abraxalito View Post
In my experience getting the 'wrong' output voltage has been related to oscillation. Do you have a scope to check for that? The minimum current requirement would in my estimation result in a more negative output voltage being registered, not less - without enough current the magnitude of the output rises.
Unfortunately, I do not have a scope. I am a bit confused now. I tried the PS with different circuits (one is a bass boost and the other is a dc- servo, both based on AD8620) which have different layouts and PCB designs. Not sure if both the circuits are oscillating. The final amp output is silent and clean. Will do some more experiments and post the result here.

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