Mains Supply options for diy (semi-professional) electronics - Page 1
Forum section: electronics / beginners

author: csheldon
author_rank: Contributor
timestamp: October 30, 2018, 12:12:41 pm
content: Hi all,I wanted to know what is the common approach and best option when trying to design professional consumer electronics that need to workon mains supply voltage. it is low power supply, say to around 100-200mA of secondary current (usually some mcu's and peripherals).Problem with mains transformers + graetz + linear regulator is that it is relatively heavy and bulky.SMPS is a potentially good option, but where to get SMD SMPS transformers? what IC's to use?What is the ussual approach to this problem when designing diy (semi-proffessional to professional devices)

author: dmills
author_rank: Super Contributor
timestamp: October 30, 2018, 12:28:31 pm
content: The usual solution is to use an external off the shelf power supply module, makes all the mains safety certification pain go away.I would be very reluctant to bother designing my own off line switcher for a product, especially for something jellybean like a micro plus peripherals sort of thing, the costs of certifying it for every interesting market would be such that just buying an off the shelf line lump is highly attractive (And if 5V @ 500mA will do you then, USB?).Even if I was designing something that needed an internal power supply, I would still be thinking in terms of a brought in module, Meanwell IRM series or whatever, problem solved, onto more interesting things...If you really wanted to do your own flyback for some reason then Coilcraft, Wurth and others have transformers for specific chipsets, but it is seldom worth the pain.Regards, Dan.

author: jeroen79
author_rank: Frequent Contributor
timestamp: October 30, 2018, 01:01:32 pm
content: Seconded.Either use an external powersupply or some drop in module.You can simply solder these onto the PCB.

author: Benta
author_rank: Super Contributor
timestamp: October 30, 2018, 01:23:48 pm
content: It's a really good idea to use a universal SMPS, meaning it should have an input voltage range of 90...265 VAC.

author: GeoffreyF
author_rank: Regular Contributor
timestamp: October 30, 2018, 01:28:45 pm
content: For a consumer product the design decisions have a very great deal to do with insurance liability. To manage that risk, the entire "box" into which mains go has to be certified safe by an organization such as UL, CE, ETL. The most expedient thing is to OEM a device that has these certifications and delivers low voltages to the product. Certifications are often needed for that product because of rechargeable batteries and so forth, but they are simpler to achieve.The certifications are very strict for something with mains voltage if the consumer might be holding or touching the thing while it is plugged in.Over the years, the requirements for certification have become more strict.For someone who is building something for use by others, in the home or someone else's home, it's also wise to build it around a certified power supply that delivers low voltage from the mains. You could be liable for injury as well.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 30, 2018, 01:38:07 pm
content: This is one of those questions where "if you have to ask, you probably shouldn't consider doing it".Even companies with 100x or 10000x your resources (budget, engineering talent, etc.) solve the power issue by simply BUYING commodity power supplies, whether internal or external. Especially for low-power loads like what you are describing. And EXTERNAL solutions completely sidestep most of the problems of not just engineering but also of legal risk/liability, licensing, etc. etc.Even if you were an experienced power supply designer with a great design, the legal issues would still be a primary factor to avoid making your own. Unless you are talking about a mass-market product where the expense of certification, etc. makes it economically viable.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 30, 2018, 03:05:58 pm
content: Maybe it's different in the US, but here in the UK, using a 3rd party PSU absolutely won't absolve you of any liability at all in the event that your product electrocutes someone or starts a fire.Having your product tested for CE compliance by a 3rd party, and being able to produce a test report, will work in your favour in court. It'll probably keep you out of jail, but won't stop you being found guilty of selling an unsafe product, if indeed that's what you've done.It'll also make your product liability insurance cheaper, and you'd be a fool not to have this if you're selling mains powered equipment. This insurance is what stops you going bankrupt in the event of a major incident.Mandatory CE marking might be an expensive PITA, but it is what keeps a lot of badly made (even if well-intentioned) crap from ever seeing the light of day. That's a good thing.

author: Benta
author_rank: Super Contributor
timestamp: October 30, 2018, 05:20:09 pm
content: Quote from: AndyC_772 on October 30, 2018, 03:05:58 pmThat's partially true.If you integrate the power supply into your equipment, you're responsible.It's a bit different with an external supply, eg, brick or wall wart. If the external supply is certified by the manufacturer, you're at least off the hook when it comes to electrocution or fire in the PSU.Your own equipment still needs to comply with EMC, fire, RF etc. regulations.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 30, 2018, 05:32:33 pm
content: You chose it, determined it to be suitable, and supplied it to the end customer.If you incur losses as as result, then you might well have a case against the manufacturer, but you can't just point your customer at the OEM and walk away.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 30, 2018, 06:04:29 pm
content: Quote from: AndyC_772 on October 30, 2018, 05:32:33 pmOnly if you supply it with your product. If you sell your product without a power supply and the end-user buys it directly, you are out of the loop.Assuming you have designed your product to fail safely if they use the wrong power supply.

author: Benta
author_rank: Super Contributor
timestamp: October 30, 2018, 06:42:45 pm
content: Quote from: AndyC_772 on October 30, 2018, 05:32:33 pmCorrect. But as the OP keeps it a great secret where he's located, this discussion is difficult.If we're talking EU here, it's essential that the PSU supplier has a legal entity in the EU, that's your backup.

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 30, 2018, 07:16:22 pm
content: Another advantage of using a commodity power supply is that in case you start selling them to a different country with different mains voltage/frequency, you don't have to re-design the whole thing.And last but not least.......the different plug configurations used in different countries.All the major external PSU vendors already offer most, if not all, the worldwide plug configurations.

author: csheldon
author_rank: Contributor
timestamp: December 22, 2018, 11:29:02 pm
content: For example, see the schematic of Sonnof 4ch pro...https://www.itead.cc/wiki/File:Sonoff_4CH_Pro_Schematic.pdfIt is perfect example of what I want to achieve...SMPS with input voltage of 90-250V, and small transformer, but I can't seem to find ee14 assembled ferrite transformer, this iw1700 IC seems to be obsolete...does anyone here have recommendations for some IC and ferrite transformer SMPS soultuion for which parts could be easily order from ebay?If so, please , could you share the knowledge?

author: tszaboo
author_rank: Super Contributor
timestamp: December 23, 2018, 09:13:12 am
content: Sonoff is the worst example for this. it is neither safe, nor certified. And they are writing stuff on the labels, that are over the limit of the actual components. If you want to place something on a DIN rail, then just buy a din rail power supply (Its 13 EUR from Meanwell)

author: james_s
author_rank: Super Contributor
timestamp: December 25, 2018, 11:12:54 pm
content: Why would you even consider designing your own power supply for something like this? There are numerous off the shelf solutions, both external "wall wart" types and power supplies designed to be integrated into equipment.As someone else said, if you have to ask how to design a power supply, you really shouldn't be doing it unless it's a hobby project to learn about power supply design.

author: Mr. Scram
author_rank: Super Contributor
timestamp: December 25, 2018, 11:25:43 pm
content: Just comparing the costs of the two should tell you enough. Plenty of companies make the mistake of spending vast resources on engineering, testing and certification while a cheap COTS solution exists. Are you prepared to go through all of that to end up with what's probably going to be a less cost optimised and more expensive power supply in the end? That's if you actually know what you're doing. If you don't know about power supply design, it's exponentially worse.