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Troubles with Texas Instruments LM337LZ
Troubles with Texas Instruments LM337LZ
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Old 23rd February 2019, 04:33 AM   #1
rsdio is offline rsdio  United States
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Default Troubles with Texas Instruments LM337LZ [solved]

I just built a bipolar power supply based on Doug Self's Small Signal Audio Design book.

The positive supply is regulated by an ON Semi LM317LZ, and the associated output voltage is dead on.

The negative supply is regulated by a Texas Instruments LM337LZ, but the output voltage is only about 72% of what it should be. Looking at the controlling node, which should be a nominal 1.25 V, I'm measuring only 0.9 V! I thought this particular 337 might have been damaged by connecting a transformer with too much AC voltage, so I replaced it with a new LM337LZ and used a smaller AC voltage. Same results. I then thought that it might be the protection diode across that node, so I removed the diode. Still 0.9 V!

What could possibly cause this?

Looking at the data sheet, the reference voltage should fall between 1.20 V and 1.30 V, never as little as 0.9 V.

I noticed that the ON Semi and TI data sheets suggest (require?) a 240 Ω or 249 Ω resistor across the voltage reference. Doug has a 100 Ω resistor in this position. The voltage calculation formula is still the same, but I wonder whether the 100 Ω resistor that I'm using is somehow not presenting enough of a load.

I'm testing the power supply with no load at the moment. That doesn't seem to be a problem for the positive supply. Maybe I need to connect the 100 F caps to the outputs?

Another detail is that I'm using a two-pin AC wall wart rather than a raw transformer. Since this doesn't provide a center tap, I'm using a capacitor voltage doubler to separately feed the positive and negative regulators. Unfortunately, the 16 VAC adaptor that I started with produces too much voltage : around 40 V. That's right at the limit of the regulators. So, after replacing the LM337, I have only connected a 12 VAC adaptor that produces only about 28 V at the regulator inputs. That should be safe. I'm planning to buy a 9 VAC adaptor since I really only need about 20 V to feed my +/-17V supply.

I can attach a schematic, if that would help. I'm pairing 1.3 kΩ resistors with the 100 Ω resistors to adjust each regulator to 17.5 V.

Last edited by rsdio; 25th February 2019 at 07:02 AM. Reason: changing the title to reflect solved status
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Old 23rd February 2019, 04:45 AM   #2
PRR is offline PRR  United States
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The pin-out (In, Out, Adj) may be *different* between the positive and negative forms of "similar" regulators. (This has bit me before.)
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Old 23rd February 2019, 07:33 AM   #3
googlyone is offline googlyone  Australia
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Troubles with Texas Instruments LM337LZ
Schematics and maybe a photo will help.

You are using one of the worlds most common and bulletproof regs, so I suspect something is astray in his you are applying it.
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Old 23rd February 2019, 08:09 AM   #4
FauxFrench is offline FauxFrench  France
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100 Ohm sounds very small. When working that would be a bypass current of 12.5mA!
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Old 23rd February 2019, 08:41 PM   #5
Vovk Z is online now Vovk Z  Ukraine
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Troubles with Texas Instruments LM337LZ
Negative integral regulators usually are more skittish-notional-capricious. They need that everything was as in the dataseet - they need all bypass caps, load not less than and so on.
100 Ohm is ok.

Last edited by Vovk Z; 23rd February 2019 at 08:55 PM.
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Old 24th February 2019, 08:36 AM   #6
rsdio is offline rsdio  United States
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Quote:
Originally Posted by Vovk Z View Post
Negative integral regulators usually are more skittish-notional-capricious. They need that everything was as in the dataseet - they need all bypass caps, load not less than and so on.
100 Ohm is ok.
That was it!

I finally got around to connecting the output cap. My power supply board plugs in to the audio board, and I thought it made more sense to put the output caps "local" to the audio board where they connect directly to the power planes. Unfortunately, this means that when I tested the power supply PCB by itself, the regulators had no load capacitance at all. I should have known better, since the data sheet basically requires a 1 F output capacitor.

The positive regulator may not have had a problem with this (no) load, but as you said the capricious negative regulator was very unhappy.

Thanks!
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Old 24th February 2019, 07:47 PM   #7
JensH is offline JensH  Denmark
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Troubles with Texas Instruments LM337LZ
Be carefull with the output capacitor!
If you look at the datasheet a solid tantalum or electrolytic is recommended.

I once had an unstable regulator using the LM337L. I had used a ceramic output capacitor (due to some stringent design requirements). The LM337L didn't like that. Inserting a 1 ohm resistor in series with the output capacitor solved the problem. The ESR of the capacitor should not be too low!
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Old 24th February 2019, 11:13 PM   #8
rsdio is offline rsdio  United States
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Here's the schematic for the power supply board. It's designed to work with an external wall wart that outputs AC on a standard barrel connector.

Pin 2 on the jumper brings in 12 VAC (and I might reduce that to 9 VAC).
Pins 3 and 4 return +17.5 V (16.8 V to 18.2 V) and -17.5 V to the main board.

There are 100 F capacitors on the main board, where I thought they would be more effective, but that basically means I can't test this power supply board without a load attached. Actually, I tested with 47 F capacitors because that's what fit (limited space for larger diameters).

Hmm, I should check the current draw of my main board to make sure it's under 100 mA...
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Old 25th February 2019, 08:32 PM   #9
Vovk Z is online now Vovk Z  Ukraine
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Troubles with Texas Instruments LM337LZ
Output bypass cap (and input cap too) should be not far than several santimeters from regulator.
Your circuit is ok if the wires between this regulator and load are not then 10 santimeters long. (Now you know that).
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Old 25th February 2019, 08:35 PM   #10
Vovk Z is online now Vovk Z  Ukraine
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Troubles with Texas Instruments LM337LZ
You can add any small electrolytes right on the output of your regulator board, for example 10 uF x 50 V, or 22uFx25V, to be shure it will be stable in any situation.

Last edited by Vovk Z; 25th February 2019 at 08:38 PM.
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