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Old 16th January 2013, 12:31 PM   #191
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My understanding is that the power supply will be less able to maintain proper regluation at the frequency of resonance. The lower the output impedance the harder it is for the load to modulate the supply. Thus if you have a resonant peak the load will be more likely to be able to cause the PS to modulate at that frequency, and I guess if it is bad enough it will oscillate.

Tony.
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Old 16th January 2013, 12:39 PM   #192
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Originally Posted by wintermute View Post
good questions, and I can't give you a straight answer! I'd not thought about any capcitance on the load, and the potential effects of that. Perhaps when it is far enough away (assuming a few cm of wire between powersupply and the load) the problem is reduced.

The cap on the output is generally there to ensure stability of the regulator. The datasheet recommends using tantalum capacitors, these generally are not low esr. It is the use of low esr caps that makes addition of additional series resistance necessary.

a typical 1uF solid tantalum capacitor as specified in the datasheet probably has enough resistance to damp it to the point where it is not an issue. However if you were to use a 1uF film cap on the output it would be a very different story.

Yes the model does change with current. The resonant peak shifts when the current is varied (up to a point).

Tony.
I see. I just wonder, in cases where a transient current is needed by the load, will it draw it from this capacitor? Which is in general the reason power supply caps are there, I suppose. Or will it draw it directly from the LM317?
Should it draw it from the cap, the peak wattage on the series resistor could be close to 2 watts if I see well. Not that it should introduce any problems, though. I feel the assumption about its proper wattage that we made above is still valid.

It would be quite enlightening to include a variable load in your simulations. I don't know if this is possible - little do I know about simulations yet. A variable current source would be a quite demanding load, and should give appropriate answers. Something like a 20kHz 0.6A rms value.
Again, not sure if anything like that is possible!
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Old 16th January 2013, 12:41 PM   #193
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I've attached a zip file that should hopefully be a standalone directory with various LM317 circuits in it. You should be able to load any of them and run them without adding anything into your spice libraries (let me know if it complains that anything is missing).

There are a few different models that I have been playing with for doing different things. Hopefully they are of some use to people

The yarps powersupply I have actually built (I'm not 100% certain, but I think that is the final actual circuit). It was based on Fred's one transistor upgrade earlier in this thread, and the full gory details are available on my blog It was/is an excercise in overkill. My measurement gear is not really good enough for me to have formed a concrete oppinion as to whether or not it was a success, however from a point of view of it delivering +- 10V it was Sims have indicated using 6.6 ohms in one leg only, gives the same result as using 3.3ohms in both legs. if you feel uncomfortable with the resistors in the return line then feel free to ignore them

Tony.
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Old 16th January 2013, 02:35 PM   #194
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That's great. I want to try a few things out.
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Old 16th January 2013, 08:23 PM   #195
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I see. I just wonder, in cases where a transient current is needed by the load, will it draw it from this capacitor? Which is in general the reason power supply caps are there, I suppose. Or will it draw it directly from the LM317?
Should it draw it from the cap, the peak wattage on the series resistor could be close to 2 watts if I see well. Not that it should introduce any problems, though. I feel the assumption about its proper wattage that we made above is still valid.

It would be quite enlightening to include a variable load in your simulations. I don't know if this is possible - little do I know about simulations yet. A variable current source would be a quite demanding load, and should give appropriate answers. Something like a 20kHz 0.6A rms value.
Again, not sure if anything like that is possible!
First a dsiclaimer I'm just a guy with pretty rudimentary electronics knowledge who has read a LOT of datasheets, articles and threads on the LM317. has done LOTS of simulation, and quite a bit of real world testing of LM317 circuits. I don't have an EE background

So with regards to your question. I don't think that there will be any significant current draw from the output capacitor, provided that the circuit is not transiently trying to draw more current than the LM317 and upstream components can deliver. The filter caps before the reg are providing this service more. The reason they are necessary is because of the pulsed nature of the rectification. The diodes only conduct for a brief period of time not continuously, and the capacitors tend to smooth this out, allowing current to be drawn from the cap at the times when the retifiers are not conducting. If you run the spice model for yarps and poke around looking at waveforms at various points such as straight after the bridge, then after the first cap etc you will see what is happening.

wth respect to the variable load, see the Voltage_vs_current model in the attached zip file

@Bonsai note that the LM317D (D for discrete) used in those circuits is only good to about 600mA, if you push it past that it will start to give you erronious results. It is the one I posted a bit earlier in the thread that I modified the transistors to get a more accurate voltage reference.

Tony.
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Old 17th January 2013, 01:45 PM   #196
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Linear Tech showed this circuit in the LT1085 datasheet -- easily applied to the LM317. The fixed voltage versions of LT1963A (which I prefer to the LM317) has a "sense" pin which gives you something approaching Kelvin connection.
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Old 17th January 2013, 09:04 PM   #197
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Yes, I can't remember which ones but some of the LM317 datasheets also show this. Along with the suggestion that the sense resistor (R1) above should be should be connected directly to the case of the regulator. The selection of the ground point for R2 also apparently makes quite a difference.

ok attachment is from the TI datasheet (it has a good explanation as to why), I think that the original Natsemi or possibly the Linear Technology one have a diagram almost identical to jackinnj's one. (actually I guess it was the LT one because jakinnj's one is an LT one )

I've read elsewhere, that this is probably the single most important thing for getting max performance out of an LM317.

Tony.
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Old 12th September 2013, 12:01 AM   #198
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Hello people,

what a really interesting thread, which I've been reading for about 2 hours now. As it exceeds my electronical and english language competencies I couldn't understand all of it. I'm happy to see people with a lot of knowledge participating their results with others. Unfortunatelly I've seen a lot of circuits during the last 20 pages and I cannot say, which is the best performning schemata. As the thread seems not to be carried forward anymore, please could somebody help me? I'm looking for a best performance regulated power supply for my amps bias and other works.

Many thanks,

Stammheim
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Old 12th September 2013, 01:38 PM   #199
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The basic LM317 circuit does not have many alterations, really. If something is better is the use of a preregulator. Other than that, bypassing the adjust pin and using output caps for stability is all you can do, plus doing a proper layout design based on the above posts.

There are other regulators that - supposedly - perform better than LM317 regulators. You must specify the degree of quality, stability, noise, cost etc of your builds, to get further help.
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Old 12th September 2013, 03:52 PM   #200
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Thanks for your answer!
OK, what I need is 12 - 16V out / 2A, alternatively I would use the LM350 with it's 3A for reserve. Do you other / better suggestions?

Many thanks!
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