Is a zener better than a Trimpot in some LM317 application? - Page 1
Forum section: electronics / projects

author: fc3000
author_rank: Regular Contributor
timestamp: January 15, 2021, 12:22:06 pm
content: Hi all,I was designing with an LM317.I was thinking. Is it a better option to use a Zener instead of a trim-pot?A zener many times is less expensive than a pot.A pot can crack and gives you an open circuit situation so that Vout=Vin.I am assuming you can get the correct zener for your application.I would like to hear your thought on the subject.RegardsFrancescoC

author: fcb
author_rank: Super Contributor
timestamp: January 15, 2021, 12:35:22 pm
content: Do you need to change the Vout? Pot's only necessary if you need to adjust or trim the output.If not then fixed 1% resistors will be better than a zener.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 15, 2021, 01:04:51 pm
content: What use you could possibly think for a zener? A schematic would help?The whole point of LM317 is that it includes a voltage reference, far more stable and precise than a zener.Output voltage is set with fixed resistors. Only rarely you need trimmable voltage. In this case, it's a good idea to invest in a known good brand, a bit more expensive trimpot. Sometimes you can do with DIP switches / pin headers + jumpers switching fixed resistors in/out of circuit for a few voltage levels.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 15, 2021, 04:55:11 pm
content: As above, and you can limit the problems caused by pot failure by using surrounding resistors to limit the range to just where you need it, and insure that an open pot doesn't allow the voltage to change excessively.

author: ledtester
author_rank: Super Contributor
timestamp: January 15, 2021, 05:42:29 pm
content: You should show exactly how you intend to use the zener.Note that the zener voltage is not very precise and the zener voltage depends on the current going through the device.Looking at Mouser a 2V 1% zener costs $1.32 and the minimum ordering quantity is 100.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2021, 05:43:36 pm
content: If you want more precison, use the TL431, but as mentioned above, there's no point in using a zener, over a plain old resistor. Just because the schematic on the LM317 data sheet shows a potentiometer, it doesn't mean it can't be replaced with a fixed resistor. The same goes in just about every circuit, unless it needs to be adjustable, obviously.

author: wizard69
author_rank: Super Contributor
timestamp: January 16, 2021, 12:08:57 am
content: Quote from: fc3000 on January 15, 2021, 12:22:06 pmA zener would give you one possible solution to safe guard against this though I have to think about how it would actually work. But it doesn't seem to make sense as others have pointed out the regulator already contains a high precision reference. Further is an excessive high output voltage is a concern then you likely would want some other safety measure beyond designing for this failure point.

author: MarkF
author_rank: Super Contributor
timestamp: January 16, 2021, 12:58:26 am
content: Here is a LM317 configuration I found online where the outputgoes to minimum voltage if the potentiometer wiper opens.Never had a chance to wire it up.Let me know how it works if anyone tries it.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 16, 2021, 09:30:40 am
content: Well, a device failure is often allowed to... drumroll... cause a device failure!Often, if one component fails, the whole thing is turned inoperable,anyway.Any solution whichsomewhatlimits the voltage in case of single-component failure means you then need to analyse thoroughly that the circuit still works with that voltage. In which case the question arises, why do you need the regulator at all?I see two fundamental ways to design:1) Make each part as reliable as possible. Avoid excess circuitry; simplicity makes it easier to make a non-failing design. Avoid excess safeguards. If some part fails, it is likely it takes something else with it. This obviously affects repairability;somefailures cascade, causing multi-part failures.2) Safeguard as much as possible. If one part fails, safeguards may be able to save other parts from failing. But, the product will usually still fail as a whole. Repairability may be better, but may not; it's practically impossible to safeguard everything against everything.The problem is, 2) requires surprisingly lot of resources. In any case, it adds a lot of complexity and parts, each of which then again is a source of another potential problem.The big question is, what is this regulator supplying?If it's just some simple and non-critical circuitry, it's likely best to just minimize overall risk of problems, by minimizing places where something can go wrong. Then accept that if something still fails, the whole product is replaced. When you see a problem, solve it permanently, so further units do not fail for the same reason.OTOH, if the regulator is supplying some super-expensive, super-sensitive space technology thing that costs you $10000 to replace, then I'd add safeguards to the regulator. But try to make the protection as generic as possible. Now you have found one possible weak link, the trimpot. But something else can go wrong as well; typical failure mode for LM317 is short across IN to OUT. Instead of strengthening the feedback circuit, I'd add acrowbarto the output. It can be designed separately and will catch all overvoltage incidents, caused by any reason. Now a failure still makes the unit non-operational, but the failure isn't cascading into expensive parts.Finally, if the regulator is supplying a life support device, or something similar that needs to stayoperationalduring failure, then you would, say, make two copies of the circuit, with said crowbars, and diode-OR the outputs together. And stupidly oversize that diode so it doesn't become a new weak link.But I prefer the way of simplicity, least number of parts, analysis, and finally, upon failures, root cause analysis so that the problem never arises again. I believe in the process of doing high-quality, well through out design, then do on-field repairs by swapping complete products or relatively large modules, andimportantly, ship failed unit backs to the manufacturer forroot cause analysisso that the next revision does not fail. This is the only way to achieve both low cost and very high reliability."Repairable" products on the other hand tend to be lazily designed, same issues remain decade to decade, and failure rate is high. Yet people who repair these are happy, it brings food to their table. For this reason, be careful with the opinions of "repair people" when it comes to product reliability!Unless you arereally reallyexperienced designer, initial guesses of the potential failure modes of your products tend to be completely wrong.

author: magic
author_rank: Super Contributor
timestamp: January 16, 2021, 09:52:27 am
content: Quote from: MarkF on January 16, 2021, 12:58:26 amThat's neat and I'm sure it will work.One thing to watch out for is the 50µA typical / 100µA maximum adjustment pin current, which will flow through 5kΩ resistance of the pot when output voltage is set low. This increases minimum output voltage from 1.25V to 1.5V typical, if I got it right. Furthermore, the current can vary 0.2µA typical / 5µA maximum over temperature and load conditions, resulting in 1mV / 25mV output variation.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 16, 2021, 09:56:16 am
content: Also before including a trimpot in a design, I want to be like 250% sure Ireallyneed analog trimming in that circuit.

author: srb1954
author_rank: Super Contributor
timestamp: January 17, 2021, 02:20:57 am
content: Quote from: fc3000 on January 15, 2021, 12:22:06 pmIf you require trimming to better than 5% a typical Zener is not going to improve your accuracy.A better approach to trimming an LM317's output is detailed in National Semiconductor application note LB-46. This uses several additional cheap resistors connected around the LM317's programming resistors where these are selectively clipped out of circuit or shorting links are selectively clipped out to adjust the voltage to the required accuracy.This method is cheaper than either a Zener or trim-pot albeit at the expense of some additional PCB space. It also doesn't suffer the over-voltage problem from open-circuit trimmers and is likely to be more stable long-term.

author: Peabody
author_rank: Super Contributor
timestamp: January 17, 2021, 03:26:45 pm
content: Could someone explain to me how a zener would be used with an LM317? How exactly do you get regulation using a zener?

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2021, 03:32:53 pm
content: Quote from: Peabody on January 17, 2021, 03:26:45 pmIt could go in place of R2, in the usual circuit, with the zener.

author: Peabody
author_rank: Super Contributor
timestamp: January 17, 2021, 04:24:35 pm
content: I guess I don't understand how the LM317 works. If you have a zener, then it seems the voltage at Adj would be fixed at the zener voltage no matter what the output voltage is. How do you get any feedback that provides regulation? Is that feedback inside the LM317?

author: magic
author_rank: Super Contributor
timestamp: January 17, 2021, 04:43:08 pm
content: LM317 regulates its output to be 1.25V above the adjust pin. The usual configuration sets adjust to a fraction of the output voltage wrt ground and hence the output becomes a corresponding multiply of 1.25V, but you can just apply any voltage to adjust and the output will be 1.25V higher.

author: bdunham7
author_rank: Super Contributor
timestamp: January 17, 2021, 07:45:36 pm
content: Quote from: Siwastaja on January 16, 2021, 09:30:40 amIt's amazing that one drive-by post with no follow-up can send us all atwitter on various subjects. I would assume the OP may have seen various techniques of usingfixedVRs with zeners in the ADJ circuit to float them up. This works well in some cases, for example a 7808 with a 5.8V zener will give a pretty stable 13.8 volts. I've seen this relatively crude method used in some surprising places. You're right, of course, that it makes no sense with an adjustable regulator. Perhaps a zener makes sense as a simple crowbar with a fuse, but how and why you would use it in place of a trimpot isAs far as robustness and repairability, I simply don't see the negative correlation that you talk about. Some very simple, straightforward, repairable designs are also very robust. Some complex, poorly-implemented designs fail often but get binned because they are unrepairable. I've seen all 4 corners of that diagram, both in electronics and cars. For power supplies, my expectations are simple--I expect it to last forever in normal use and to blow a single protection device if abused beyond reasonable expectations. Anything short of that means it isn't very good, no matter how easy it might be to repair.

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2021, 07:49:44 pm
content: I suppose a zener might make sense if another voltage reference, 1.25 lower than the positive voltage rail is required. It would be better to use a zener, rather than a resistor, because the loading would have less effect on the reference and output voltage.

author: coppercone2
author_rank: Super Contributor
timestamp: January 18, 2021, 10:55:02 am
content: if you cannot achieve the range or performance (linearity) you want with parallel resistors to act as a safety then the zener or other methods look like a good optionall people will give you conventionally is the least possible parts for most profit, usually you wonder if they should get a rabies shot when they try to explain that philosophy to youI personally had the pass element on lab supplies short out before from benign things and would have preferred that not to happen because it trashed complicated things. With anything but like mass testing of cheap bullshit usually the thing you are working on is more expenisve then the supply when you factor in your time.Getting 80V from a +15 hooked up to a custom circuit with a bunch of precision op amps = pissed the fuck off. Or a weird hard to get RF thing.

author: coppercone2
author_rank: Super Contributor
timestamp: January 18, 2021, 11:06:02 am
content: Quote from: bdunham7 on January 17, 2021, 07:45:36 pmthose modules cost HOW much vs a several dollar repair with parts possibly owned? AH yes that is like a 90's+ tektronix manual making everything into a module. 4 screws and a zip tie are a fucking module with them. Great find some 001231 assy Rev 4 that was never gonna fail since Rev 1. Then when you look at it you wonder what the fuck they were thinking (try a CRT driver old revision on expensive stuff). No one seems to tell you anything anymore because they can push another module.Ahh 34470A service manual. Full 6 pages with 5 photos on how to replace a battery but they can't give you anything else. Keep bleeding the big guys, guys. And what does it take like 30 minutes to find some chinese guy that can reverse engineer the entire thing in like a week? Its because.. they want to sell some assys and... modules. Might highlight some cut corners too (these things grow in the dark).When you ask its usually well uhh *unintelligible MBA made up fear mongering bullshit* *translation* we think its more profitable for us *translation* yea don't mess with that manager making these changes hes got interest from other companies on linkedin!!!