PSU supporting both 120v and 240v. - Page 1
Forum section: electronics / beginners

author: dcbrown73
author_rank: Regular Contributor
timestamp: June 10, 2019, 05:46:53 pm
content: I've dabbled in low voltage DC electronics for a while, but I'm looking at learning and building a lot more. I intend to buildBob Heil's Pine Board AM Transmitterand part of it is building a power supply for it.That got me thinking about PC and server PSU's in how they can accept both 240v and 120v inputs. How exactly do they handle the different voltages? Does it just detect if it's 240v and use a center tap or the entire winding if it detects 120v? Sorry if this sounds like a stupid question.Thanks,Dave

author: Dave
author_rank: Super Contributor
timestamp: June 10, 2019, 09:21:06 pm
content: Unlike linear power supplies, which use hefty laminated iron core transformers, computer power supplies use smaller, high frequency switching transformers. The latter support a much wider input voltage range. There is no need for any tap switching, the regulator simply adjusts the PWM duty cycle to get the correct output.I would avoid a switchimg converter for your specific application, because your RF circuit might not interact well with the noisy power supply.If you want to learn more about switching converters, I recommend you first try to wrap your head around simpler topologies, like the buck converter.

author: IanB
author_rank: Super Contributor
timestamp: June 10, 2019, 09:31:21 pm
content: Quote from: dcbrown73 on June 10, 2019, 05:46:53 pmFor a simple linear power supply using a transformer, such as you might build yourself, this can be done using a transformer with two 120 V primary windings. You put them in parallel for 120 V mains and in series for 240 V mains. This is most simply done using a DPDT mains switch.I have a power supply where you have to open the case and manually reconfigure the wires, on the assumption that you make such configuration changes very rarely.

author: tooki
author_rank: Super Contributor
timestamp: June 10, 2019, 09:36:50 pm
content: Think of a switch-mode power supply as a device that simply takes little sips of power from the primary side and sends them to the secondary, just often enough toaverage outto the necessary output voltage. With a higher input voltage, it simply takes shorter sips of power from the primary.For example, imagine if the primary side is 120V, and you need 12V at 1A. Then (depending on the transformer winding) you might only draw power from the primary 5% of the time. If the load increases to 2A, you might need to draw power 10% of the time. At 240V, though, with the 2A load it'd only need 5% of the time.Look into PWM (pulse width modulation), as it's a really good way to get a very rough understanding of the principles.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 10, 2019, 10:05:20 pm
content: Have a look at voltage doublers:https://duckduckgo.com/?q=psu+"voltage+doubler"&iax=images&ia=imagesThe principle is quite simple.For SMPS power supllies on 230Vac the input is a simple full bridge which rectifies to about 310Vdc.Then there are 2 small modifications to have it run on 120Vac.First the elco is split in 2 elco's in series.Then the center tap between the elco's is shorted to one of the input terminals.This means the top elco is rectified to around +150Vdc, and the bottom elco is rectified to around -150Vdc, and together they are 300V, about the same as from the full bridge.Additonal advantage is that 200V elco's can be used instead of 400V elco's, Disadvantage is that somewhat bigger elco's must be used because of the (2x) half wave rectification for 120Vac.--------------------------With smaller power supplies it's more common that they simply accept a wide change of input volatage.With a bit of forethought it's not too difficult to design an SMPS for which the input voltage can have a 1:4 ratio.If you have a look at the very common small smps modules from Ali / Ebay / China with for example a LM2596 or MP1485, they have a wide input voltage, and they simply adjust the PWM duty cycle internally to regulate the output voltage.

author: tooki
author_rank: Super Contributor
timestamp: June 11, 2019, 08:16:33 am
content: Just a little FYI, native English speakers won't know what you mean by "elco"*, we just call them "electrolytics" if not calling them by their full name, or just "caps" when specifying the capacitortypeisn't actually important in context.*we stopped calling capacitors "condensers" a loooong time ago in English.

author: CJay
author_rank: Super Contributor
timestamp: June 11, 2019, 08:50:56 am
content: Native English speaker here, years of repairing Grundig and Phillips electronics and reading Elektor taught me Elco

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 11:05:05 am
content: Quote from: dcbrown73 on June 10, 2019, 05:46:53 pmMost PC and server PSUs have a voltage selector switch to be set by the operator.Small PSUs like for laptops have a wide input range, say 100 to 240 volts because they are designed to accept that range. They do not switch between voltages.I suppose you could design a PSU that detected input voltage and switched but I do not think they are common at all.

author: CJay
author_rank: Super Contributor
timestamp: June 11, 2019, 11:09:33 am
content: Quote from: soldar on June 11, 2019, 11:05:05 amAre you joking?The majority of power supplies in PCs for the past two decades have been wide input range or autoswitching, they are by far the most common type, there are even dedicated ICs for detecting line voltage and autoswitching.

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 11:34:34 am
content: Quote from: CJay on June 11, 2019, 11:09:33 amYou made me look. I have three desktops right here and I was sure I was going to find a switch that switches to voltage doubler configuration. You know what I found instead? Stickers that say "230 volts only". On all three.I remember the 110/220 sliding switches on the older PSUs but I guess they got cheap and removed them.I went to pccomponentes.com, an online PC components vendor, and most of the PSUs are 230 volt only. Only one says "full range". None have voltage selector switches.

author: dmills
author_rank: Super Contributor
timestamp: June 11, 2019, 11:54:55 am
content: The actual trick with modern PC power supplies (at least good ones) is that the first stage of a decent supply in the reasonably high power class is generally a BOOST converter that takes the input up to somewhere around 390V or so.....This is independent of the input voltage and is done by some cleverness with a dual loop controller that makes the input current track the voltage waveform so that the load appears resistive. It is a requirement in larger supplies to control the power factor and harmonic distortion.If you have to have such a stage to meet regulations it is cheap to accommodate a wide input range, hence the common 85-265V range seen on equipment.Once you have your (more or less fixed) DC bus at 390V or thereabout you then do the main switching converter that produces the outputs and also provides the isolation.The half wave voltage multiplier (with the slide switch) trick was common before PFC became a requirement.Regards, Dan.

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 12:12:26 pm
content: Active PFC is expensive to implement and not really required on PC PSUs which can achieve harmonic reduction with passive low pass filters which are simpler and cheaper. I do not think active PFC is common in computer PSUs.Yes, I know they exist, just like 500 euro bank notes, I have heard of a guy who once talked to someone who said he had seen one.

author: CJay
author_rank: Super Contributor
timestamp: June 11, 2019, 12:54:36 pm
content: Quote from: soldar on June 11, 2019, 11:34:34 amA bit of a mystery then as only one out of the first ten PSUs I looked at on a local supplier site was 230V only (it was one of the cheap ones, I might order one to satisfy my curiosity), I can't remember ever seeing a 230V only PSU in the past ten years in any PC, all the machines I have here (site with 800 users) and at home (with the possible exception of a HP Vectra 486) are wide input range.I had thought it might be because your supplier offers high power gaming PSUs as some server PSUs derate output if operated on 110V (some HP supplies are derated to 960W from 1200W) but even that doesn't seem to be a reason for limiting to 230V as Scan offer similar and higher rating PSUs which are 100-230V input so I'm mystified as to why it would still be a thing when it's so simple to implement.(BTW, the first two PSUs your supplier offers are wide input ones)

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 12:58:51 pm
content: Maybe in the UK they have different requirements.

author: senso
author_rank: Frequent Contributor
timestamp: June 11, 2019, 01:11:27 pm
content: Dont look to the crap from Nox..Look at decent PSU's, like Seasonic, and they all state this:AC Input Voltage: 100 V - 240 VAnd, they have active PFC, who knew..https://www.pccomponentes.com/seasonic-s12ii-520w-80-plus-bronze

author: CJay
author_rank: Super Contributor
timestamp: June 11, 2019, 01:20:11 pm
content: Quote from: senso on June 11, 2019, 01:11:27 pmI've a feeling the only way to get a efficiency rating is with active PFC?

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 01:36:55 pm
content: Harmonic suppression compliance can be generally achieved with passive filters but active PFC has other benefits, mainly that it is, in itself, a pre-regulator so the following switching part is working pretty much from a constant DC voltage.

author: mariush
author_rank: Super Contributor
timestamp: June 11, 2019, 01:52:32 pm
content: 99% of 80+ bronze/gold and higher ATX power supplies are wide input range80+ "white" allows 230v only psus, which otherwise won't reach 80% effiiciency on 110v or they use small/cheap parts (ex a 4A bridge rectifier or 2-3A diodes on a 500w psu.. too low to get 500w on 110v)A linear psu can be made using transformer with 2 primary windings and parallel or series them depending on voltage/Or, you can do like atx psus AC->DC, optional boost to ~400-420v DC (actve pfc), then high frequency switching (typically at least 30kHz or higher to stay above hearing range, but 200kHz..500kHz is common)

author: wraper
author_rank: Unknown
timestamp: June 11, 2019, 02:17:53 pm
content: Quote from: CJay on June 11, 2019, 11:09:33 amThey don't do any autoswitching except very rare cases. Manually switchable input voltage is still more common than autoswitching. LOW power SMPS usually just work in wide input voltage range. Those of higher power have PFC (works in wide input voltage range as well) which converts input voltage into around 400VDC and then SMPS works from that.

author: wraper
author_rank: Unknown
timestamp: June 11, 2019, 02:26:26 pm
content: Quote from: CJay on June 11, 2019, 01:20:11 pmYou cannot legally sell them in EU without PFC. SMPS above certain power rating do have minimum power factor requirement. And simple rectifier + smoothing capacitor do not meet that.

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 02:55:56 pm
content: The EU mandates limits on harmonics and does not care about how you achieve that.https://en.wikipedia.org/wiki/IEC_61000-3-2A passive low pass filter can meet the requirements and is cheaper than active PFC.

author: dmills
author_rank: Super Contributor
timestamp: June 11, 2019, 03:20:50 pm
content: Quote from: soldar on June 11, 2019, 02:55:56 pmI very much doubt that it is cheaper, third harmonic is only 150Hz, so that is a LARGE inductor to keep that out, while an active PFC is like one mosfet, a gapped inductor and a small controller chip, a boost stage running at maybe 50KHz is going to be cheaper then a LC filter cutting off below 150Hz, probably just on the shipping cost never mind anything else!Certainly by the time you are looking at any kind of modern server power supply, I would be VERY surprised to see passive harmonic filtering, rather then active PFC.Regards, Dan.

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 03:46:41 pm
content: Quote from: dmills on June 11, 2019, 03:20:50 pmI have no opinion on what is cheaper but most cheap PC PSUs do not have active PFC and I assume they meet applicable regulations.My TV, computer monitors and laptop PSUs do not have active PFC either.Now that I think about it I can't think of any appliance or light in my home that has active PFC.Residential customers do not pay for reactive power so they have no incentive to improve the power factor.OTOH big businesses do pay for reactive power and have an incentive to improve PF. I remember reading about some company that had LED lighting with active PFC because they had thousands of such lights and PF was an important consideration.

author: mariush
author_rank: Super Contributor
timestamp: June 11, 2019, 03:52:43 pm
content: I think there was an exception in EU rules for psus with less than 75w output, so your monitors may lack active pfc.TV may use less than 75w, depends on backlight.if your laptop's brick is over 65w, it should have active pfc.

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 05:31:07 pm
content: Quote from: mariush on June 11, 2019, 03:52:43 pmYou think there is an exception? Why do you think that? And what exception?Because, you know, I just linked to the relevant regulation.https://en.wikipedia.org/wiki/IEC_61000-3-2Did you read it?That regulation establishes allowable harmonics current and says absolutely nothing about active PFC. And I cannot imagine why they would care how you achieve the objective.If you have better information I'm sure we'd all like to see it but "I think" doesn't cut it.All my devices are over 80 W and none have active PFC.

author: wraper
author_rank: Unknown
timestamp: June 11, 2019, 05:34:26 pm
content: Quote from: soldar on June 11, 2019, 03:46:41 pmAbout TV, maybe, if it's small or really old. As of Laptop PSU, there is 99% chance it has PFC, unless it's Chinese knockoff. How do you even know that? You cannot take apart laptop PSU in non destructive way as covers are welded together. Did you actually measure current waveform?

author: CJay
author_rank: Super Contributor
timestamp: June 11, 2019, 05:35:51 pm
content: Quote from: wraper on June 11, 2019, 02:17:53 pmAutoswitch used to be *very* common in some PC PSUs and industrial SMPS, Astec used it extensively for instance.Autoswitch was great except that it had a bit of a habit of blowing up PSUs if it got confused or the SCR went faulty.It's not common any more now that wide input range is possible and cheap.*edit*I'm quite surprised to see this device is still available, I'm pretty sure it's the one (or a very close relative) that I used to see in a lot of power supplies (because it'd glitched and gone into doubler on a 230V supply with the bang and smoke expected):https://www.st.com/en/thyristors-scr-and-ac-switches/avs10cb.html

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2019, 05:42:03 pm
content: There are two ways that common off-line switching power supplies can support both 120 and 240 volt AC inputs:1. The older method is to use a voltage doubler configuration on the input rectifier for 120 volts AC and disable the voltage doubler for 240 volts AC. The result is that the rectified DC voltage is always 340 volts. Switching is accomplished either manually or automatically. Note however that this supply does *not* support some voltage range between 120 volts and 240 volts AC.2. The more common method now is to use boost mode active power factor correction which does not care what the input voltage is as long as it is below 240 volts AC and above some minimum value. The power factor boost stage produces a constant DC output voltage slightly above 340 volts under all allowed input conditions. Unlike the voltage doubler method, this will work on any AC voltage between 120 and 240 volts.

author: wraper
author_rank: Unknown
timestamp: June 11, 2019, 05:48:09 pm
content: Quote from: David Hess on June 11, 2019, 05:42:03 pm3. Just design so it works in wide voltage range. Quite easy with say flyback or forward converter topology.

author: Nerull
author_rank: Frequent Contributor
timestamp: June 11, 2019, 06:04:34 pm
content: You have to go really cheap these days to find a consumer PSU that has either a voltage select switch or passive PFC.One wonders how many things someone can be wrong about while still acting as an authority on a subject.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2019, 06:28:27 pm
content: Quote from: wraper on June 11, 2019, 05:48:09 pmThat comes at a cost and performance disadvantage compared to doubling or not doubling the voltage at an earlier stage. Some low power converters work that way though and of course universal input power factor correction stages do exactly what you suggest at high powers but they are not dealing with high transformation ratios.

author: wraper
author_rank: Unknown
timestamp: June 11, 2019, 06:32:21 pm
content: Quote from: David Hess on June 11, 2019, 06:28:27 pmVirtually all prone chargers work this way.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2019, 06:41:50 pm
content: Quote from: wraper on June 11, 2019, 06:32:21 pmPhone chargers are low power.

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 06:51:56 pm
content: Quote from: wraper on June 11, 2019, 05:34:26 pmYou are not responding to my questions. You are changing the subject. Again: Where is the regulation that says appliances over 75W need to have active PFC? Where? Because I am not saying it does not exist. I am saying the regulation that I have already cited twice does not require it. And between IEC_61000-3-2 and your vague and unsupported assertions I am afraid I have to go with the former.And, yes, I have opened my Dell 90W power brick as I discussed recently in another thread. You can find it easily.Again, look, it seems you just want to argue for the sake of arguing. I am not saying I am right. I am just showing the evidence that I have but you contradict it without any evidence whatsoever. Just show me where it says active PFC is mandatory in the EU. I will be glad to see it and learn. Please show us the evidence. What regulation is it?

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2019, 07:09:29 pm
content: Quote from: soldar on June 11, 2019, 06:51:56 pmThe regulations do not specifically require PFC but instead specify a maximum input harmonic distortion for different power levels and classes. Usually the most economical way to meet the specifications is to use active PFC although sometimes passive PFC is sufficient.https://en.wikipedia.org/wiki/IEC_61000-3-2#ScopeIncidentally, industrial uses may be charged extra based on their power factor but doing this has been considered impractical for consumers so far. So instead they regulate it on the supply side now. It was not a big deal until switching power supplies became ubiquitous.

author: wraper
author_rank: Unknown
timestamp: June 11, 2019, 07:16:45 pm
content: Quote from: soldar on June 11, 2019, 06:51:56 pmI did not mean regulation requires PFC. Well, I was wrong about it being about power factor, it was about harmonics. PFC is a way how to do it relatively cheap and easy, maybe wording was not the best. Also it makes easier to get good performance out of SMPS part.QuoteToo much ego on your side. IMO it was already clear what regulation was about. Yet I guess it's not enough attention for you to say about it only twice.QuoteWhich questions even vaguely addressed to me?I did not see a single one. I cannot read your thoughts.

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 07:38:20 pm
content: Quote from: David Hess on June 11, 2019, 07:09:29 pmYes, that's pretty much what I said.I just realized I had not thought about my inverter split air conditioner units which I am pretty certain they would have active PFC.To get back to the OP subject. Low power units like wall warts can have a wide ranging voltage input because efficiency is not of great concern.Even laptop PSUs may not have active PFC because of space and weight constraints. My Dell PSU does not even have an inrush limiter and I have blown a few fuses in the UK until I inserted a PTC directly in series in the cable itself.Cheap PSUs for PCs used to have the voltage doubler switch and now are just for a single voltage and do not have active PFC...Because if they did the would be:Higher range PSUs have active PFC which raises the voltage to a set voltage and this is in fact a pre-regulator and therefore admit a higher range of inputs (full-range).Note also that Triac power controllers for incandescent lighting have some filtering but not what I would consider a lot.

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 07:46:32 pm
content: Quote from: wraper on June 11, 2019, 07:16:45 pmLook, I do not enjoy arguing, with you or with anybody else.Read the thread. For example post 24.Again, I do not want to argue but you asserted some things and even gave specific numbers.There is nothing wrong with me asking for a cite supporting that.I never even said you were wrong. I just asked for confirmation and you ignored y question.And you say I'm the one with ego? Sheesh.

author: dcbrown73
author_rank: Regular Contributor
timestamp: June 11, 2019, 07:50:31 pm
content: Thanks for the enlightening debate (argument?) I learned a lot from my simple question!