Unloaded LM317/337 behaviour - diyAudio
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Old 19th June 2007, 06:33 AM   #1
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Default Unloaded LM317/337 behaviour

Hi all,

This is a noddy question, but I thought I'd ask it anyway.
I've built a smallish PSU to feed a DAC I'm building.
The PSU itself has 4 voltages out of it, 2 regulated by LM317s, and the other 2 by LM337s.

When finally powered up, it all seemed to work fine. I could trim the output voltages to what I required, and it seemed okay.

However, I noticed, after a few minutes, all four of the output voltages started to creep up (well, the negative ones crept down really). I don't think it was due to temperature drift, the regulators are using the chassis as a heatsink. I do wonder if the regulators see a slight conductance to ground due to maybe not perfect insulation from the chassis, but I'm probably just being paranoid (surely It wouldn't work at all?).

My only other theory is its because the regulator outputs are unloaded (except for capacitance), and this is causing the behaviour I am seeing.

I will start loading the PSU this evening to investigate this further, but would this be expected behaviour?

I've built PSUs before, but only using 78XX regulators, and I don't believe I've ever seen this before (or maybe, because they were always loaded with at least power on LEDs, I didn't see the issue?).

Someone please, put my mind at rest

Cheers,
Phil
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Old 19th June 2007, 10:02 AM   #2
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The datasheets specify clearly that the regulators may need up to 10 mA of load current to maintain regulation (or 5 mA for the L versions). They are not supposed to be used unloaded.
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Old 19th June 2007, 10:05 AM   #3
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Question Drift

How much they creep Phill? A small drift is normal.
Christer's hint is right on target. You must use loading and also recommended caps around the reg!
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Old 19th June 2007, 10:30 AM   #4
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That's good news, thanks for your replies!
Its creeping maybe a couple of volts over 5 minutes.

I am using essentially the recommended circuit for them in the datasheet (the one for improved ripple rejection - also seen in Horowitz and Hill), so I'm happy the capacitances are fine, I'm just not loading it (I've only just finished building it).

I will begin using them tonight, so I hope it will all come good.

Thanks again for the reassurance.

Cheers,
Phil
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Old 19th June 2007, 10:53 AM   #5
Dave is offline Dave  New Zealand
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Yes, they need a minimum load current and also output capacitance for stability.
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Old 20th June 2007, 06:28 AM   #6
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Hi all,

Well, last night I loaded the PSU with a TL431 regulator stage, and the voltage out of the LM317 stayed constant - as I had hoped, and as you all successfully predicted.
Thanks for the reassurance!

Now, on with building the DAC....

Cheers,
Phil
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Old 23rd June 2007, 10:18 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
the resistor from output to adjust is usually set to 120r.
With the 1.2 to 1.25V that the regulator maintains between these two pins you will find that the 10mA requirement is met by the 120r resistor. Then the regulators will work open circuit or loaded.

What voltage is on the inputs when the outputs are unloaded?
Could the ripple at the input be overloading the regulators?
Could excessive heat in the regulators be causing the drift problem?
Have you fitted adequate heatsinks for the worst combinations of mains input voltage, transformer regulation and maximum output current demand?
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Old 23rd June 2007, 10:56 AM   #8
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To Andrew's point -- if you measure the input-output differential and multiply by the current drawn you know the amount of heat to be dissipated by the regulator --

Work backwards to determine the thermal impedance of the heat sink -- fwiw I usually use a clip on TO-220 type sink with the LM317/LM337 and haven't had a problem. Over a few watts dissipation I usually use an external pass transistor.
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Old 23rd June 2007, 11:12 AM   #9
sandyK is offline sandyK  Australia
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Default Unloaded LM317/337 behaviour

Andrew
Many designs specify a much higher value than 120R. This is usually to allow voltages such as 15V etc. to be achieved using standard 1% resistor values.
SandyK
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Old 23rd June 2007, 11:31 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi SandyK,
no, the resistor across the output to adjust pins always carries the voltage drop sent out by the regulator.
The lower leg resistor (adjust to ground) is scaled to suit the output voltage.
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