LM317 Dual Shunt Regulator - Page 1
Forum section: electronics / beginners

author: thyself
author_rank: Contributor
timestamp: January 12, 2023, 11:45:40 am
content: I have recently discovered these schematics showing the use of LM317 as a Shunt Regulator.I haven't tested it yet, but I wonder If anyone here have used this approach and would like to comment?

author: jwet
author_rank: Frequent Contributor
timestamp: January 18, 2023, 05:36:24 am
content: That's a strange circuit. Any time I see 470 uF caps on low level signals, its a clue that this is not very conventional. I can't see any special benefit to the circuit. The first 317's, are set up as a current source and the downstream 317's are shunt regulators basically multiplying the output current of a zener diodes but really just acting like a protected transistor. The overall accuracy will be no better than these zeners. It might offer some benefits in cross regulation but a straight 7815, 7915 would probably be better overall.

author: inse
author_rank: Super Contributor
timestamp: January 18, 2023, 06:47:43 am
content: The circuit is also known as rail splitter to generate a virtual ground.I have this as add-on to my lab supply, slightly different to deal with variable voltage.

author: MadScientist
author_rank: Frequent Contributor
timestamp: January 18, 2023, 08:25:58 am
content: Quote from: jwet on January 18, 2023, 05:36:24 amWith respect your analysis is incorrect , this is quite a standard application and has appeared in many an application note

author: jwet
author_rank: Frequent Contributor
timestamp: January 19, 2023, 06:27:31 pm
content: Never seen in it and I was an FAE and Apps manager at Maxim for 25 years! I'm talking specifically about the first cirucit with the cross coupling between the two regs. The other circuits are pretty conventional. Can you point me to an app note number or a reference circuit A?. Maybe I'm just not recognizing it in the way its drawn- doesn't look familiar or especially useful to me?

author: wraper
author_rank: Unknown
timestamp: January 19, 2023, 06:46:51 pm
content: Quote from: inse on January 18, 2023, 06:47:43 amThe problem is that first circuit has a secondary winding with a center tap which is connected to the virtual ground outputSecond circuit is similar but ground is not actually virtual at all.

author: inse
author_rank: Super Contributor
timestamp: January 19, 2023, 07:32:28 pm
content: Of course you’re right, I concentrated on the last circuit…

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 07:58:20 pm
content: I believe the third circuit is incorrect. It is missing the source resistor or some kind of a CCS. It should be like that:

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 08:08:56 pm
content: I understand the first LM317 is configured as a CCS to provide the source current for the zeners.I don't understand why the zeners are not referred to the ground?instead to the opposite rail.

author: inse
author_rank: Super Contributor
timestamp: January 19, 2023, 08:28:13 pm
content: This is the rail splitter I built (courtesy of goldpt.com).https://www.goldpt.com/virtual%20ground%20goldpoint%20basic.jpg

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 08:57:37 pm
content: I understand the rail splitter concept. What I don't understand about the use of 317 as a shunt regulator is what function it performs, especially if we already have a zenner regulator (even with CCS on top) a front of it? The zenners connected to the adjust pin just lift the ref voltage up, what else is there?

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2023, 09:30:31 pm
content: Quote from: thyself on January 19, 2023, 08:08:56 pmConsider that the LM3x7 is working 'backwards'. Its basic function is to maintain it's OUT pin at 1.25V above (or below for the LM337) the ADJ pin. It does this by conducting if that voltage is below 1.25V and not conducting if it is above. Now look at the circuit and imagine the voltages are such that the ADJ pin is either more or less than 1.25V below the OUT pin (of the LM317) and see what happens in each case.QuoteIn the first circuit, the CCS will be putting out ~300mA. If you just used a 15V zener as a shunt regulator, it would dissipate 4.5W.The first two circuits seem pointless (as a whole) and I'm not sure what the reason would be for using a shunt regulator where you have anything other than an isolated DC source to deal with. They look like they were designed by an audiophool with OCD, especially the first. 10mF filter caps!

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 09:45:02 pm
content: Does that mean the 317 as a shunt after the zeners will "straighten" the residual ripple of the zeners?

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 09:51:18 pm
content: I am asking because currently I have a project where I need to bring 40V after capacitance multiplier down to 16V for powering 20 OpAmps, and I wonder which route to take: zener (simple, cascaded, bridge, etc.), transistor (serial, shunt, etc.), LinReg (LM317-723 etc.), OpAmp+Mosfet, LDO, SMPS, etc...!?

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2023, 09:52:40 pm
content: Quote from: wraper on January 19, 2023, 06:46:51 pmThe first two are not rail-splitters at all, except maybe the first is sort of by accident. And it would be interesting to see what happens at the junction of the two huge 10mF filter caps since whoever drew that omitted a connection to ground for that section.

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2023, 09:55:11 pm
content: Quote from: thyself on January 19, 2023, 09:51:18 pmSingle ended, not split/bipolar?

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 09:57:01 pm
content: Quote from: bdunham7 on January 19, 2023, 09:55:11 pmBipolar +-40V to +-16V.I thought of using LM317-337HV, which I have in stock, but I am not sure about PD, i.e. 4.8W (24V drop*0.2A) at least.In the datasheet the PDmax is missing, it shows just "internally limited"

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2023, 10:16:00 pm
content: Quote from: thyself on January 19, 2023, 09:57:01 pmYou wouldn't actually need the HV part, but it would give you a bit of safety margin in case of certain malfunctions. I didn't know there was an LM337HV available in TO-220.As for the power dissipation, you just need a 100R 5W dropping resistor before each regulator. That way at full load, the resistor will take 20V of the drop and still leave you 4V for good operation of the regulator. That gives you 0.8W, which is handled by a small solder-in finned heatsink. If you want extra filtering, just use 3x 33W 5W resistors in series with a 470uF capacitor at each stage.PDmax being internally limited just means that it will fold back if it gets too hot and the actual PDmax is determined by ambient temp and how well you heat sink it. You don't want to be trying to keep an LM3x7 going with 4.8W dissipated as this would take heroic levels of heat-sinking. Resistors like that sort of thing, semis don't.

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 10:22:21 pm
content: Thank you for the advice bdunham7!What about a Zener shunt after the 100R/5W as a pre-regulator to the 317?20V zener will provide 200mA, and 4V remain for the 317.Would it be beneficial in terms of noise?

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 19, 2023, 10:23:58 pm
content: I am going to bring, and perhaps kill, the elephant in the room: would you consider using a SMPS? And perhaps follow it with a linear regulator?-ducking for cover…

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 10:27:28 pm
content: Quote from: schmitt trigger on January 19, 2023, 10:23:58 pmWell it is a PSU of a guitar amplifier, maybe it would bring noise and harmonic artifacts...

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2023, 10:33:41 pm
content: Quote from: thyself on January 19, 2023, 10:22:21 pmYou could do that, although I'd at least use 3x ~6.4V 5V zeners in series to keep them from cooking. The only way I see it as having any benefit is it would keep your RC filter loaded if you were using the 3x 33R + 3x 470µF I suggested. With a straight resistor I don't see how it would help. You could also use the zener(s) directly as a dropper instead of the resistor.However, in either case I'm dubious of using zeners like this because I've seen examples like this having reliability issues and any noise benefits are questionable, as in unproven. The LM317 is really, really good at filtering out ripple, especially if you use the recommened additional components like a small cap right at the input and an ADJ pin capacitor.

author: thyself
author_rank: Contributor
timestamp: January 19, 2023, 11:12:31 pm
content: Is that what you mean?

author: bdunham7
author_rank: Super Contributor
timestamp: January 20, 2023, 01:04:33 am
content: Quote from: thyself on January 19, 2023, 11:12:31 pmSomething like that, except I don't see why you want to use D1 and D2 instead of a ~2.6k resistor? And I don't think you need C4 unless your layout is unusual.I neglected to ask what the exact source of the 40V is or how much ripple it has, nor how quiet you need the output to be. Many op-amp circuits will have pretty good PSRR anyway. So the RC thing may be overkill, but it also may not cost that much to implement since you can probably use 33R 2W resistors. Don't spend extra for non-inductive ones!

author: thyself
author_rank: Contributor
timestamp: January 20, 2023, 01:50:42 am
content: Corrected

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 20, 2023, 03:37:18 pm
content: Quote from: thyself on January 19, 2023, 10:27:28 pmAgain, not attempting to flog a dead horse, but that is the reason for the linear post regulator. Anyways, it was only an idea.I DO GET your point. I started electronics almost 50 years ago by repairing guitar amps. Indeed the harmonic artifacts, can really mess up a guitar sound. The only acceptable noise in a guitar amp is 120 Hz hum.It adds warmth to the sound.

author: David Hess
author_rank: Super Contributor
timestamp: January 20, 2023, 04:23:32 pm
content: Quote from: thyself on January 19, 2023, 08:08:56 pmThe opposite rail is just used to provide the current to bias the zener diodes into conduction. Ground could be used with a resistor but the voltage would be the 1.25 volt reference of the 317, which might be preferable.In this circuit the zener diodes add to the 1.25 volt 317 reference to make a power shunt regulator. The only advantage of this circuit that I see is that it is a simple way to provide a fast responding current limited precision voltage output without separate current and voltage control loops with operational amplifiers. I would have done it with operational amplifiers and one 317 per side.

author: thyself
author_rank: Contributor
timestamp: January 20, 2023, 10:06:30 pm
content: Quote from: schmitt trigger on January 20, 2023, 03:37:18 pmI will try both approaches (SMPS->LinReg and RCRCRC->LinReg) and share the observations.

author: thyself
author_rank: Contributor
timestamp: January 21, 2023, 06:49:45 am
content: Quote from: David Hess on January 20, 2023, 04:23:32 pmCan you please post/ref to a schematic?

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2023, 11:24:57 pm
content: Quote from: thyself on January 21, 2023, 06:49:45 amIt is a little tricky because the current shunt cannot be on the positive side, for a 317, without affecting the regulation, and if it is on the input side or ground side, then the 317's quiescent current affects the current regulation, but except in precision applications that does not matter.I am in the middle of moving and do not have access to my application library and did not find any good examples online. It comes down to using an operational amplifier to pull the adjust pin negative through a diode when the current limit is reached.I do not currently have access to my application notes but I would do something like this, but a little simpler.https://forum.allaboutcircuits.com/ubs/a-16-16v-1-67a-constant-voltage-constant-current-lab-psu.480/