110VAC to 240VAC SMPS 12V 5W - Page 1
Forum section: electronics / beginners

author: rentner
author_rank: Regular Contributor
timestamp: May 06, 2013, 08:19:34 pm
content: Is there a "simple" way, to get mains voltage down to 12V without using a 50/60hz transformer? I don't want to use anything, that can cause a bigger 50/60hz magnetic field, than just the traces of the PCB, because it can and will kill my project. I want to have a high density, so the required distance from a transformer to the rest of the PCB would be to big for me. 500mA to 1A is definitely enough. Very important: it definitely needs galvanic isolation from the mains. Not even 100MEG is acceptable. It needs to be isolated like air.I don't want to use a ready PSU, because DIY will give me a better look in how these small suckers work. I thought, I could just find an IC for that, but that did not happen.Well, well. What solutions do you offer? Or IS there actually an IC for that purpose, which will not cost too much?

author: Unknown
author_rank: Unknown
timestamp: May 06, 2013, 08:40:47 pm
content: I can't see how your going to do this job without a PSU or Battery source ? what is the project for it to be so sensitive to PSU noise.500 mA to 1 Amp is a demanding current for this project.SMPS is going to produce one hell of a lot of high frequency hash, along with the 50/60 Htz mains near the project input, if at high Z... and if a linear transformer is going to prove to be a magnetic field problem too; your going to either have a separate PSU DC cable feed off-board, or just battery powered on-off-board !At the end of the day, the engineering has to make it's compromise between wanted and what's feasible.

author: IanB
author_rank: Super Contributor
timestamp: May 06, 2013, 08:46:14 pm
content: If you don't use a mains frequency transformer you will need a switching supply operating at a higher frequency and consequently a small high frequency transformer. Since higher frequencies tend to have more energy than low frequencies your interference problems may be worse. (Do you have problems with magnetic fields, electric fields, or electromagnetic radiation?)As far as isolation is concerned, a switching supply will have Y capacitors for safety, and those capacitors will couple the mains into your isolated side.I don't think you are going to get complete isolation without a large and expensive design, or a battery.

author: c4757p
author_rank: Super Contributor
timestamp: May 06, 2013, 08:59:46 pm
content: Low noise, direct mains input, compact size. Choose two.Quote from: rentner on May 06, 2013, 08:19:34 pmNot even 100 Meg? Why?

author: ejeffrey
author_rank: Super Contributor
timestamp: May 06, 2013, 09:09:08 pm
content: Quote from: IanB on May 06, 2013, 08:46:14 pm<nitpick> the capacitors are there to meet emission requirements. They are class Y for safety.</nitpick>

author: rentner
author_rank: Regular Contributor
timestamp: May 06, 2013, 09:15:14 pm
content: I know, that this definitely requires High frequency. In fact, the HF Magnetic field is not a problem at all, because 25khz or more is not audible. It is just for a small Amplifier. Anything below 20khz can cause the problem to be heared. That's the reason, why I don't want a 50/60khz transformer. For regulation i was thinking of simply an opto isolator.

author: c4757p
author_rank: Super Contributor
timestamp: May 06, 2013, 09:18:19 pm
content: Just audio? God, most "audio people" would have you excommunicated for ever suggesting putting an SMPS within twenty feet of an audio circuit! Just avoid having very high impedances (shield them if you must), ground properly and you'll be fine.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 06, 2013, 09:33:44 pm
content: A 50/60 Hz transformer is more than adequate for audio applications. The magnetic fields created by the transformer will not have any effect on a halfway decently designed audio amplifier. In case it is, you can use a toroidal or magnetically shielded transformer.An SMPS will almost certainly have worse performance in every respect, including 60 Hz hum. (from the PWM duty cycle variation, as well as the necessary class-Y caps). Also, it is a mistake to think that because the switching frequency is ~50+ kHz that it can't affect audio quality. While that frequency is not directly audible, high frequency interference can cause audio frequency artifacts. Try holding a GSM phone near a pair of cheap computer speakers sometime to see what I mean.The advantages of a SMPS is that it is smaller, lighter, more efficient, and cheaper (at mass production quantity) than a 60 Hz transformer. Low noise is not generally considered one of its virtues. It is possible to make a low noise SMPS if you need the cost/weight/size advantages, but if you are making a hobby project, start with a good linear supply.Finally, making a safe line operated isolated SMPS is not a good DIY project. If you do decide to go with a SMPS, buy an off-the-shelf supply and then add external filtering or post-regulation. You can get converters in a number of form factors including PCB plug-in modules that look like oversized ICs.

author: IanB
author_rank: Super Contributor
timestamp: May 06, 2013, 09:46:17 pm
content: Quote from: ejeffrey on May 06, 2013, 09:09:08 pmOK, that makes sense.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 07, 2013, 03:13:02 am
content: Quote from: c4757p on May 06, 2013, 09:18:19 pmMy homemade Hybrid Digital amplifier is powered by an old laptop PSU and it works great. And a lot of newer (pure) digital amplifiers are using switching supplies to do the dynamic supply voltage.

author: c4757p
author_rank: Super Contributor
timestamp: May 07, 2013, 04:03:51 am
content: Oh, of course. I've done tubes (valves?) with a DC-DC before (two actually, one for the filaments!), all the while laughing with glee at how much it might piss off the old farts.Just saying that there is no reason to bother avoiding mains transformers, even the audio "purists" are OK with them.