Improving small unregulated power supplies - Page 1
Forum section: electronics / beginners

author: soldar
author_rank: Super Contributor
timestamp: September 22, 2019, 07:15:57 pm
content: I have crates full of these old wall warts and similar PSUs. While I have many SMPS fail the old transformer-diode bridge - capacitor type seem to last forever.The main objection is that they are unregulated and they are a pain to match to the load. If the load is not stable the voltage can be all over the place.I often end up using a PSU with a lower nominal voltage because at the current I need the voltage it outputs is higher than the nominal voltage. Testing each time I need one is a pain.Series regulation is not easy or convenient because there is really not enough headroom.I have used in the past parallel regulation with a zener diode or zener plus transistor. It is a waste of energy but the amount is very small and the addition can be placed in the output cable or receptor without needing to open the enclosure which sometimes cannot be done without breaking it and there is no space anyway.I am playing with a small transformer which I guess we could call "12 V , 200 mA". The following table gives the voltages at different currents.Code:mAV019.41018.91518.62018.43017.95017.07516.010015.012514.115013.317512.620011.922511.325010.8So, let's say I want to limit the voltage to 14 V max. I would need to sink 125 mA @ 14 V which is a bit under 2 W. A zener and transistor can handle that easily.This prevents up to 20 V building up in the capacitor and being dumped suddenly into whatever device is connected.I'd like to hear thoughts and ideas on this topic.BTW, I actually measured a number of points but then I put them into Excel and I obtained the following equationV = 19.359 - 0.0492 I(mA) + 0.00006 I(mA)^2So the table is an idealized table.

author: james_s
author_rank: Super Contributor
timestamp: September 22, 2019, 07:20:11 pm
content: I typically just build a switching regulator into the project and then use whatever old wall wart is reasonably suitable. My broadband router for example has a switching regulator inside it and originally came with a 12V switching type wall wart. I think I went through 2 or 3 of those before I replaced it with an old unregulated transformer type and it has been fine for years. They are not the most efficient things but they are much more reliable than the low cost switchmode ones.

author: Zero999
author_rank: Super Contributor
timestamp: September 23, 2019, 03:09:53 pm
content: I've not noticed a significant difference in reliability. I've hardly had any problem with switched mode wall warts failing and I've also had the old transformer type fail.My advice would be to simply ditch the old transformer type of supply, especially in equipment which is left on all the time. Even if it is slightly less reliable, it will probably work out cheaper, due to the amount of power saved. If you keep having problems with switched mode power supplies failing, then it's a problem with high voltage spikes.

author: Ian.M
author_rank: Super Contributor
timestamp: September 23, 2019, 04:05:13 pm
content: I really wouldn't advise shunt regulation - if the mains supply voltage rises, something is likely to burn up and if the PSU doesn't have a thermal fuse in series with its primary, it could literally catch fire.Needing a few volts extra headroom is the cost of doing business if you need to stabilise the output from a small unregulated linear PSU.That nominal 12V unregulated PSU would make a decent feed for a 9V linear regulator. At its nominal max load current you've got 2.9V headroom, and should still have 1.7V headroom if the mains supply is 10% low.

author: soldar
author_rank: Super Contributor
timestamp: September 23, 2019, 06:33:20 pm
content: Quote from: Zero999 on September 23, 2019, 03:09:53 pmMy experience is quite different and it makes sense. SMPS are much more complex and the components are much more stressed.Just a few days ago my wife's Samsung's phone charger stopped working. It was the second one to stop working. They are tiny and have some complex charging system with two voltages. Since she spends a lot of time in the car she's just decided to charge it while she's driving and not buy a new charger for the time being. Maybe a non-Samsung, non-fancy fast charging, just regular more bulky USB would last longer. She had the same experience with her earlier tiny Apple chargers. Did not last. I have them in a box with the idea of looking into repairing them but they are tiny and it's not worth it.I wish they would make things more substantial and not try to miniaturize everything to the point where it has to fail. One of these small SMPS should be eternal.About a year ago I was woken up in the middle of the night by a small explosion in my room. A SMPS wall wart had exploded. No fire, nothing. Just a small explosion and the thing was dead.

author: Gyro
author_rank: Super Contributor
timestamp: September 23, 2019, 07:05:46 pm
content: Quote from: Zero999 on September 23, 2019, 03:09:53 pmI wouldn't bin them all. Keep some for projects needing 'quiet' supplies and ones where Y-cap leakage from SMPS types is a pain (isn't that most of them!)7.5 - 9V bricks were typically intended for 7805 regulation to 5V and tend to have good enough regulation to keep regulator dissipation relatively constant. The higher voltage ones are good for audio stuff.I have similar experience with reliability, or at least longevity. If properly rated, they tend to last forever.

author: soldar
author_rank: Super Contributor
timestamp: September 23, 2019, 07:10:30 pm
content: Quote from: Ian.M on September 23, 2019, 04:05:13 pmI have used shunt regulation for very small power supplies and never had a problem. I have never seen serious overvoltage here. If it did happen I bet a lot of appliances would be damaged and my small circuits would be the least of my worries. A very small transformer has substantial output resistance and just a zener works well. It only conducts during a very small part of the cycle and dissipates little power. Again, this is for very small transformers with high output resistance. I am talking about, say, under 12 V 200 mA. It does not make any sense once you get into higher power.Quote from: Ian.M on September 23, 2019, 04:05:13 pmWell, I totally agree and if I design anything from scratch I will do it that way.The most common nominal voltage of the PSUs and of the devices is 12 V. I could stabilize the PSUs to a lower voltage but I do not have that much need for lower voltages. The issue is that I have several crates of the old transformer type power supplies and I try to use them but every time it is a pain having to measure currents and voltages and trying to get something that matches.The ideal receptor has stable current consumption and an unregulated PSU can be matched but if the receptor's use varies then you need an active regulator. In the past I have had this issue with circuits that switch relays and the coil of the relay uses much more current than the controlling circuit.As I say, I am mainly trying to use the PSUs I already have; not trying to design or build new ones.

author: Zero999
author_rank: Super Contributor
timestamp: September 23, 2019, 08:30:02 pm
content: Quote from: soldar on September 23, 2019, 06:33:20 pmNot necessarily. A properly designed switched mode power supply can be more reliable than the old transformer type, because it won't run as hot. It will also have over current protection/limiting, so won't overheat, like the old transformer types do resulting in the single shot thermal fuse failing.QuoteThat sounds very strange. I thought all modern phones just worked at 5V?QuoteJust stick to decent branded USB chargers and you should be fine. The 12V ones used to power routers, I've also found to be very reliable.QuoteThat was probably the smoothing capacitor and there's one of those in the old transformer type of power supplies too. I've not experienced it myself, but I've heard stories about old transformer power supplies melting down. Probably because they had too little iron and copper and didn't conform to the appropriate safety standards i.e. the lack of a thermal fuse. Cheap, poorly designed junk won't last long irrespective of what technology it's based on. One of the best old transformer wall warts I've had was a Tandy/Radioshack brand. It was the one with the voltage selector switch and had a resettable bi-metal strip thermal cut-out, for safety, as well as reliability.Quote from: Gyro on September 23, 2019, 07:05:46 pmYes, I suppose the old transformer power supplies still have their uses. A low drop-out regulator can be used for minimal power dissipation. The AC output type are also very handy because they can be used with a voltage doubler circuit to get a dual rail supply for powering op-amp circuits.One issue I've found is the smoothing capacitor is almost always far too small. It's often something stupid like 2200μF in for a 1A unit, which will result in a huge ripple. I slap in a 4700μF capacitor before the regulator in anything run from one of those old transformer PSUs and the capacitor should be rated to 25V, for reliability's sake.

author: rcbuck
author_rank: Frequent Contributor
timestamp: September 23, 2019, 08:36:04 pm
content: I would just use a resistor, zener, and capacitor on the base of the transistor and run it in pass mode. That has the advantage of the transistor multiplying the effective capacitor filtering value.

author: Gyro
author_rank: Super Contributor
timestamp: September 23, 2019, 09:32:25 pm
content: Quote from: Zero999 on September 23, 2019, 08:30:02 pmOh yes, I place a premium on the ones that have case fixing screws rather than being ultrasonically welded shut. Quite a surprising number (of UK ones) do. Always worth checking when scanning ebay.

author: james_s
author_rank: Super Contributor
timestamp: September 23, 2019, 09:32:35 pm
content: Most of the small consumer grade switching wall warts are not what id call "properly designed", they work ok but they rarely last me more than a couple years in 24/7 service and they tend to be RF bombs. Yes some are ok, but far from all of them.I have had probably a dozen of them fail over the last decade compared to zero failures of the iron transformer type in my lifetime. These days I save all the transformer ones because they're hard to get anymore.

author: soldar
author_rank: Super Contributor
timestamp: September 23, 2019, 10:44:36 pm
content: Quote from: Zero999 on September 23, 2019, 08:30:02 pmI disagree. The old type are simpler and more rugged. I have linear power supplies that have been going for decades. It seems your experience is unusual.I have no doubt SMPS can be built to last but the small ones are not built like that, they are built to minimize size and budget. The public prefers a tiny charger even though unreliable to a bulkier but reliable one.Quote from: Zero999 on September 23, 2019, 08:30:02 pmUm, no.https://www.digitaltrends.com/mobile/how-does-fast-charging-work/Now, build a complex SMPS and make itthis sizeand it is a recipe for unreliability.Quote from: Zero999 on September 23, 2019, 08:30:02 pmThe ones that failed were original Samsung and Apple. Again, look at the design and size of the thing.Your experience that SMPS warts are more reliable seems to be contrary to everybody elses's, inclusing mine.

author: Zero999
author_rank: Super Contributor
timestamp: September 24, 2019, 11:26:14 am
content: Quote from: Gyro on September 23, 2019, 09:32:25 pmI'm talking about the unregulated type. No need to open it up, just put the capacitor on the PCB of the device you're designing.Quote from: soldar on September 23, 2019, 10:44:36 pmI'd hardly call a thread with a few people commenting concrete proof either type of PSU being more reliable. It's anecdotal evidence which means nothing. To prove one way or another, we need hard, reliable, mean time before failure data.QuoteIt's probably more to do with energy saving legislation. The old transformer PSUs have too higher idle current to meet the current EU regulations, hence why switched mode power supply power supplies are now the norm.QuoteI've no experience with Samsung, but with Apple it tends to be the cable which fails, nothing to do with the actual power supply; I've had that happen to me several times.QuoteNo, I didn't say that. I've found modern switched mode power supplies to be no better, nor no worse, than the old transformer type. In my experience they are equal, reliability wise.

author: soldar
author_rank: Super Contributor
timestamp: September 24, 2019, 01:11:59 pm
content: Quote from: Zero999 on September 24, 2019, 11:26:14 amRight now I got in my hand something hard, reliable and mean telling me all I need to know.Quote from: Zero999 on September 24, 2019, 11:26:14 amI was not comparing SMPS with old style. What I meant is that I have no doubt they could design and build a SMPS for a phone which would last for many years. What is apparent is they cannot do that and make it fit in a one inch cube and between reliability and miniaturization they choose the one that sells.Quote from: Zero999 on September 24, 2019, 11:26:14 amNope, I have fixed many cables but I have a few "one inch cube" SMPS which failed. I don't know why I keep them because I am pretty sure I will never attempt to repair them.

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2019, 05:29:03 am
content: Efficiency regulations is the reason the iron transformers have gone away. The small iron wall warts are not very efficient, only about 50% IIRC, they are "impedance protected" by having thin wire windings that cannot supply enough energy to catch fire if there is a short but this also leads to high losses and significant sag under load.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: September 26, 2019, 07:04:14 am
content: The biggest problem for me is the absence of ground out of the WW. This has caused me problems in making small signal generators for the bench. For instance I made a ovenized 10 MHz source a couple of years ago and used what I thought was a good WW to power it. It did power it well since the design had 7X05/7X12 inboard. The problem was that the output signal had some 60 Hz added to it that when connected to other equipment would bleed 60Hz and harms into my counter/scope/audio spec an. etc.Now I am making a synthesized signal generator to replace it and will use a 3-wire grounded cord and a linear supply with Xfrmr to try to avoid that.It bugs me that I cannot justify using up a spare WW even when I have one with suitable V/I output and I have about 25 lbs of the things in a box doing nothing.

author: soldar
author_rank: Super Contributor
timestamp: September 26, 2019, 07:50:51 am
content: Quote from: james_s on September 26, 2019, 05:29:03 amI do not think "efficiency regulations" have much, if anything, to do with it. The losses in very small units, say under 5 or 10 W, are negligible. More powerful units get too big and heavy. A 50 or 100 W SMPS is very manageable but it would be too heavy with a mains transformer.In the very small units range I believe the main reason is economic. SMPS are cheaper and provide stable regulated output. If you want to add a linear regulator to a conventional PSU then it gets too big, too heavy and too expensive.Another point in favor of SMPS is wide range voltage input.In summary, in the very small power range, say under 10 W, I believe market forces are the reason SMPS have replaced conventional PSUs and not any regulations (which I have not seen or heard of).

author: soldar
author_rank: Super Contributor
timestamp: September 26, 2019, 07:53:16 am
content: Quote from: basinstreetdesign on September 26, 2019, 07:04:14 amI do not understand this. Adding a ground to a floating PSU is trivial. Am I missing sumpting?

author: Gyro
author_rank: Super Contributor
timestamp: September 26, 2019, 09:00:03 am
content: Quote from: Zero999 on September 24, 2019, 11:26:14 amI disagree, there's no point in losing an unregulated PSU brick just because its cheap overstressed internal electrolytic (is about to) spill its guts. You can put snubber caps on the rectifiers too - although I've found that most reputable ones already have them fitted.Also, you have the opportunity to convert them to AC/AC, sometimes centre tapped.

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2019, 04:03:12 pm
content: Gyro, I see your point, but it's not worth my time or effort opening up a linear wall wart to replace the capacitor.Quote from: basinstreetdesign on September 26, 2019, 07:04:14 amWas it the transformer type or SMPS? Presumably the latter, in which case, it sounds like you've had problems with the leakage through the Y capacitor. This shouldn't be an issue for low impedance nodes, but can be a problem for low level high impedance stuff. The easiest way to resolve this is to connect the 0V of the circuit you're powering from the wall wart, to the other devices connected to its output. It does surprise me you had problems with a signal generator, because the wall wart's ground would normally go the the output ground, which would be connected to the device under test.

author: soldar
author_rank: Super Contributor
timestamp: September 26, 2019, 07:41:49 pm
content: Oh, I forgot to mention that I have used a few transformer PSUs to build simple battery chargers. Provide a battery holder and a resistor and you have a battery charger. I have built a few of these for friends when they complained their fast & furious & intelligent chargers were damaging their batteries. A simple slow charger is much better.

author: james_s
author_rank: Super Contributor
timestamp: September 27, 2019, 06:25:51 pm
content: Quote from: soldar on September 26, 2019, 07:50:51 amIt is absolutely about regulations, at least in the USA. The old iron transformer type is typically around 50% which is not compliant.https://www.eesi.org/articles/view/new-energy-efficiency-standards-for-small-power-adapters-will-make-a-big-di

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2019, 08:18:56 pm
content: The same is true in the EU. Old transformer based power supplies use too much power when unloaded to meet the new efficiency requirements.https://ec.europa.eu/info/energy-climate-change-environment/standards-tools-and-labels/products-labelling-rules-and-requirements/energy-label-and-ecodesign/energy-efficient-products/external-power-supplies_en

author: soldar
author_rank: Super Contributor
timestamp: October 13, 2019, 09:20:06 am
content: I am seriously thinking of buying a bunch ofthese regulatorsI saw mentioned in another thread and just making them into regulated PSUs at whatever voltage each one can handle.

author: Zero999
author_rank: Super Contributor
timestamp: October 15, 2019, 11:17:47 am
content: Quote from: soldar on October 13, 2019, 09:20:06 amWhat about reliability?

author: soldar
author_rank: Super Contributor
timestamp: October 15, 2019, 12:03:33 pm
content: Quote from: Zero999 on October 15, 2019, 11:17:47 amI am not sure I understand. What about it? You think those regulators are not reliable?

author: Zero999
author_rank: Super Contributor
timestamp: October 15, 2019, 12:31:05 pm
content: Quote from: soldar on October 15, 2019, 12:03:33 pmYou previously said you had problems with switched mode power supplies being unreliable, so I'm surprised you want to add one to the output.You might as well just use a reliable, decent quality, switched mode wall wart in the first place.

author: soldar
author_rank: Super Contributor
timestamp: October 15, 2019, 01:43:42 pm
content: The purpose of this is to use the unregulated PSUs I already have.And I don't think I could find SMPS for the price of those regulators.In the photo two Samsung PSUs which have failed within one year of each other. I keep saying I am going to look into them even if it is just to determine the cause of failure.

author: soldar
author_rank: Super Contributor
timestamp: October 15, 2019, 01:51:44 pm
content: Come to think about it the ideal regulator for an unregulated mains supply would be a buck-boost, SEPIC or Cuk which can output higher or lower than the input. That way there is no problem even if the input voltage sags a bit under the output voltage.Otherwise I have to choose an output voltage which is lower than the input might sag.E.T.A.: I had a look in Aliexpress and it is amusing how sellers throw in words just to attract views.Buck boost buckboost stepup stepdown step up voltage converter module power supply ...You have no idea what you might be buying, but, hey, for under a dollar what would expect?

author: Gyro
author_rank: Super Contributor
timestamp: October 15, 2019, 03:22:58 pm
content: Surely one of the two main reason for an unregulated brick is quietness (that and reliability). Once you've accepted the lower efficiency, aren't you most likely to want a nice quiet linear regulator?

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2019, 09:06:23 am
content: Quote from: Gyro on October 15, 2019, 03:22:58 pmYes, it does seem daft to do that. You might as well go switched mode for the whole thing.I haven't had any problems with noise from switched mode power supplies which weren't fixable, with a bit of filtering. I've just completed a project with an audio amplifier running off a switched mode power supply. It's for an intercom system that uses dynamic microphones. Firstly, the low level microphone pre-amplifier part was powered from a linear regulator, powered by the switched mode, which directly powered the power amplifier. The biggest problem was hum, due to leakage across the Y capacitor, which was solved by connecting both power supply rails to the chassis with 10µF capacitors and the squeaky noise was removed with a common mode choke and a big inductor in series with the power supply.

author: Ian.M
author_rank: Super Contributor
timestamp: October 16, 2019, 10:18:14 am
content: OTOH if you need multiple rails for an item of testgear you are designing, and want it to be fully floating, high frequency DC-DC converters (for ease of filtering) running off a line frequency transformer wallwart may well be ideal if you need to minimize line frequency leakage current.

author: soldar
author_rank: Super Contributor
timestamp: October 16, 2019, 11:02:57 am
content: Quote from: Gyro on October 15, 2019, 03:22:58 pmYes, I have done that in the past. a single transistor and a zener. The main issue is the headroom needed. But, yes, I have done it and will do it again as needed.The thing with those switching regulators is (1) they are incredibly cheap, (2) more efficient and (3) very simple.There is no way I can get SMPS for that price. And, at the risk of repeating myself, the purpose is to use the wall warts I already have.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2019, 01:51:55 pm
content: Quote from: Ian.M on October 16, 2019, 10:18:14 amBut by the time you've done that, the money would have been better spent on a better switched mode power supply, with lower leakage current.

author: Ian.M
author_rank: Super Contributor
timestamp: October 16, 2019, 02:01:53 pm
content: Availability!You *TRY* finding a SMPSU that has a grounded inter-winding screen and absolutely no secondary side grounding at a half-way reasonable price. Then see how much extra such a PSU with say three outputs is. e.g. +/-15V for your analog circuits and +5V for your logic.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2019, 03:37:29 pm
content: Quote from: Ian.M on October 16, 2019, 02:01:53 pmYou're right that a wall wart with an AC output is the most convenient choice for +/-15V.You don't need a grounded inter-winding screen, just a PSU with a decent Y capacitor between the secondary and earth, which is quite common with laptop power supplies.Grounding the secondary is a good idea and doesn't cause any problems, as long as you don't connect it in series with another grounded PSU.

author: Ian.M
author_rank: Super Contributor
timestamp: October 16, 2019, 06:32:02 pm
content: A 'Y' capacitor*from secondary to earth would be rather undesirable for a floating item of test equipment. When you initially make a connection from secondary side 0V to a ground referenced circuit, there will be a nice zap of current to charge/discharge that capacitor to the new potential. Not exactly desirable for a supposedly floating instrument.That's why I specified an inter-winding screen, as the only way to return the HF leakage current from the inter-winding capacitance between the secondary and the 'hot' (switched) end of the primary whence it came without compromises#is to intercept it with screening before it reaches the secondary. Once you've got the screening, it might as well be grounded to minimize line frequency leakage current. Unfortunately inter-winding screens cost core window area so push up the size, weight and cost of the magnetics so are usually only seen in medical rated equipment, and purpose designed PSUs for precision equipment, where the extra cost is a minor factor.* Is it a Y capacitor when it doesn't have to stand off line voltage, even if the part you use is rated as one?# See above, & also, the leakage current causes bad things happen to sensitive devices under test if your suggested grounded secondary side, or grounded Y capacitor SMPSU ever looses its ground connection.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2019, 10:02:08 pm
content: Quote from: Ian.M on October 16, 2019, 06:32:02 pmThe value of that capacitor is tiny (the order of a few nF) and there'll only be a zap if the capacitor is charged, otherwise nothing will happen. In normal conditions, the capacitor shouldn't be charged, as it's only coupling the secondary side to earth.QuoteYes, that does sound expensive.QuoteIf it's just connecting the secondary to earth, then I suppose it doesn't have to be a safety rated capacitor, but the switched mode power supplies I've seen do use a Y capacitor. I suppose it's to ensure the secondary can float at high voltages, relative to earth, if necessary. I don't see why it's a problem. Any decent switched mode lab power supply will have a Y capacitor connecting the output to earth. It's never caused be any trouble and I've not heard any stories about it creating problems for anyone else.Anyway, I forgot the original poster is only interested in this for hobby purposes, not a professionally designed project. To answer the question: yes, a switched mode converter can be added to the output of an unregulated wall wart.