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Old 3rd May 2011, 08:37 PM   #1
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I'm designing a +-18 VDC power supply for a line level project. Basically, a simple CT transformer, rectifier pair, filter caps and a pair of LM317's for voltage regulation as has been done thousands of times.

While browsing I ran across a PS similar to the one I'm planning also using the LM317. What caught my attention is that it uses a 36 VCT 15 VA transformer, but can produce up to +-20 VDC. I've gone over the LM317 data sheets, but don't see where the LM317 can provide a higher DC output than the input. The transformer I've selected is 24 VCT 15 VA based on conventional PS design guidelines. Is there any advantage of using the 36 VCT over the 48 VCT transformer?

Next, a couple of design questions:
I've read where some designers, including John Curl, use 2.2 uF film caps in parallel to the electrolytic filter caps. They are easy and cheap enough to add, but what is the value of bypassing the filters?

The PS will be on a separate PCB from the rest of the project. Is there a rule of thumb as to how large to make the traces? or, should I make them as large as PCB area will permit?

Thanks in advance for your input
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Old 3rd May 2011, 09:15 PM   #2
agdr is offline agdr  United States
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36V CT would be 18VAC rms per secondary half. Feed that to the rectifier and filter cap and you get around (1.4)(18 - 0.7V diode drop) = 24.2VDC across the cap since it charges up to nearly the peak of the waveform, and then into your LM317. That input voltage lets the LM317 drop around 4V, a good number for that part and taking line voltage fluctuations into account, giving you 20VDC regulated out of the chip if the resistors are set for 20V.

Quote:
Originally Posted by Uncle Paul View Post
The transformer I've selected is 24 VCT 15 VA....Is there any advantage of using the 36 VCT over the 48 VCT transformer?
A typo here?

Last edited by agdr; 3rd May 2011 at 09:37 PM.
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Old 3rd May 2011, 09:56 PM   #3
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Yes, a Typo - I meant 48 VCT, not 24 VCT.

I didn't realize that output voltage was RMS and not peak - that explains it!
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Old 3rd May 2011, 10:59 PM   #4
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48VCT would result in a lot more power dissipation. For half the secondary, (24V - 0.7)(1.4) = 32.6VDC into the regulator. If you wanted 18VDC out, that would be 32.6V - 18V = 14.6V across the chip. If you were pulling 300mA, that would be (0.3A)(14.6V) = 4.4W, vs. (0.3A)(4V) = 1.2W for the 36VCT transformer.

36VCT would probably be the better choice if you are shooting for something in the +/-18VDC to +/-20VDC range.

The usual reason for bypassing big filter caps with smaller caps is lower impedance at a given frequency with the smaller 0.1uF - 1uF caps (due to lower internal series inductance), hence a better ability to pass higher frequencies. Your DC power supply looks more like a short to ground for ac across the audio frequency range (a good thing), looking back into the supply from the output port.

I'll have to leave your trace/layout question for someone more current than me.

Last edited by agdr; 3rd May 2011 at 11:22 PM.
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Old 3rd May 2011, 11:51 PM   #5
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Here is some interesting reading on capacitors on Rod Elliot's site:

Capacitor Characteristics

He makes a good case that increased inductance in larger power supply filter caps really isn't internal as much as from the leads in bad designs. From section 4.0 in that article:

"... a mere 10mm of lead length (6nH) creates a series resonant circuit at close to 2MHz. Increase the capacitance to 10,000uF, and it is now 20kHz"

You may get some opinions here that bypassing the power supply filter caps is not needed, if you do good layout work and keep the traces and leads short. I usually bypass the filters, but that is just me.

Last edited by agdr; 3rd May 2011 at 11:55 PM.
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Old 4th May 2011, 01:33 AM   #6
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Thanks for the info - truly appreciated. I'll go with the 36 VCT transformer.

I have to say that everything I've read on Mr. Elliot's site has been excellent.
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Old 4th May 2011, 03:13 AM   #7
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From the sims I've done the best ripple rejection with the LM317 seems to be when the In-out differential is 5V+ (going higher than 5V doesn't give any additional benefit, and will just result in more power dissipation in the regulator).

I tried to verify this on my breadboarded circuit, but my scope resolution was not good enough. I'm going to try a trick I saw another diy audio member do PSU RC multistage filtering which is to record the rails of the PS and use an fft to view the harmonics, that may verify/debunk the results of my sims.

I suspect the fact that all datasheets for the LM317 specify in-out differential as being 5V for their ripple rejection curves is a good indicator that it is the sweet spot though.

Also you say you are doing +- with a pair of LM317's (I'm assuming not an LM317/LM337). I'm doing this too, but was under the impression you can only do it with dual secondaries and separate rectifiers for + and - not a center tapped transformer. The reason being that the output of one reg is tied to the 0V (0V being positive) but there is a drop over the reg of a few volts, so the other side will be at a different potential and cannot share a common path to the transformer with the other reg circuit...

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Old 4th May 2011, 03:37 AM   #8
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Hi Tony,
I've flipped and flopped over dual LM317 and LM317/LM337. I have both, and will probably breadboard both for grins before I'm done. Do you have any idea if one design has any advantages over the other?

Thanks for pointing out the voltage drop issue on the dual LM317 design. It was one of the questions I almost asked.

The transformer does have dual secondaries, so I can use it either way. I'll hang onto it to make a variable lab PS and purchase a 36V/18V dual secondary for this project.

I appreciate all the input - I'm getting back into electronics after ~20 years, and can't believe how much I've forgotten.
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Old 4th May 2011, 03:56 AM   #9
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Hi Paul, my decision to go with dual LM317 over LM317/LM337 was mainly based on anecdotal observations from others, and partly because I wanted to do something different.

I've read quite a few times that the performance of the LM337 is inferior to the LM317, I don't know if this is actually true or not. Probably the thing that swayed me the most was a subjective evaluation someone did, which said that the sound was better when the +ve and -ve rails were basically identical (apart from the obvious voltage difference). This wasn't an observation made with LM3x7's and I think the poster had done extensive tests to get +ve and -ve rails with the same output impedance and noise levels. I decided the best way to try and achieve this symmetry was to use two LM317's. Whether or not it makes any difference at all doesn't really concern me, there is a bit more expense involved in going with the dual LM317's due to extra bridge and caps, but in the scheme of things I figured that it was at leas plausible it could affect performance so was worth perusing, even though it is highly unlikely that I would be able to perceive a difference.

I've gone completely overkill with my circuit and after building the prototype have realized it was even more overkill than I initially thought, but it was a learning project for me so that is what matters

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Last edited by wintermute; 4th May 2011 at 03:58 AM.
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Old 4th May 2011, 04:12 PM   #10
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Since you have the dual secondaries and could go the dual LM317 route, you might want to take a look at the LT3080, one of the more modern versions of the LM317. Beats the LM317 specs by 10x in a couple of areas, plus the LT3080 is an LDO, with a dropout voltage across it of only around 400mV:

LT3080 - Adjustable1.1A Single Resistor Low Dropout Regulator - Linear Technology

Digi-Key - LT3080ET#PBF-ND (Manufacturer - LT3080ET#PBF)

A few things to look out for vs. the LM317:
* There is a minimum output cap requirement of 2.2uF for stability since it is an LDO.
* Has a minimum load requirement of 0.5mA, so you would want to throw something like an 20k resistor on the output.
* 1.1A vs 1.5A for the LM317, but can be paralleled
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