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240v AC to low voltage DC in a small package. Anyone had experience with SR10? — Parallax Forums

240v AC to low voltage DC in a small package. Anyone had experience with SR10?

MicksterMickster Posts: 2,721
edited 2012-04-20 08:22 in General Discussion
Just wondering if this would work as a uController PSU? Are there better alternatives?


http://www.supertex.com/Feature_SR10.html

Mickster

Comments

  • LeonLeon Posts: 7,620
    edited 2012-04-17 06:50
    It would, if everything was insulated.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-04-17 08:52
    Insulation is of absolute importance as any device that changes the AC mains into a DC low voltage without an Isolation Transformer remains a serious shock hazard. There was a time when nobody excluded the transformer. But those days are long gone.
  • PublisonPublison Posts: 12,366
    edited 2012-04-17 09:22
    What's you anticipated current draw? Looks like the chip can supply 50ma at best.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-04-17 10:12
    The isolation matter cannot be over-emphasized. If you use that regulator, no circuitry from the powered device can be exposed to prying fingers or connected to any other devices or equipment thus exposed. Moreover, you cannot connect it to other mains-connected equipment, including a PC that might be used for programming the microcontroller. Frankly, unless you have an overwhelmingly compelling reason to go this route -- and you haven't presented one yet -- I would avoid it like the plague.

    -Phil
  • kwinnkwinn Posts: 8,697
    edited 2012-04-17 11:35
    It would be far safer and most likely less expensive to buy one of the switching AC adapters. For about 10 bucks you get an isolated output, as much as 1.5A current, and can select one with an appropriate output voltage. A lot of new equipment uses a 5V adapter and regulates that to lower voltages for circuitry that requires 1.8 to 3.3V
  • MicksterMickster Posts: 2,721
    edited 2012-04-17 13:15
    Publison wrote: »
    What's you anticipated current draw? Looks like the chip can supply 50ma at best.

    Yes, I did pick-up on the, at-best 50mA limit which already presents problem #1.

    I was mulling-over the feasibility of switching a 240v AC supply using a zero-volt switching device, triggered by something like an XBee module. The XBee would draw its power supply from the AC and the whole thing would be potted....therefore completely insulated and thanks to the radio link, completely isolated. 50mA appears to be insufficient for the XBee device from what I can tell, though.

    Thanks for the responses!

    Mickster
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-04-17 15:12
    Potting an XBee device may render it inoperative anyway. The dielectric constant of the potting material could easily be enough to detune the antenna.

    -Phil
  • MicksterMickster Posts: 2,721
    edited 2012-04-18 02:38
    Phil Pilgrim (PhiPi) wrote: »
    Potting an XBee device may render it inoperative anyway. The dielectric constant of the potting material could easily be enough to detune the antenna.

    -Phil

    Hmmm, interesting. This is definitely something that would never have occurred to me. Thinking about it, actually, this proposed circuitry would be contained in the same enclosure as the existing 240v circuitry so extra insulation would be pointless.

    Thanks again.

    Mickster
  • GadgetmanGadgetman Posts: 2,436
    edited 2012-04-18 05:51
    When working with AC, there's only 'Kill' and 'Overkill' when regarding insulation.
    You really don't want to be known for 'Kill'...
  • MicksterMickster Posts: 2,721
    edited 2012-04-18 07:25
    Gadgetman wrote: »
    When working with AC, there's only 'Kill' and 'Overkill' when regarding insulation.
    You really don't want to be known for 'Kill'...

    One should pay attention to appropriate insulation for any potentially harmful voltage level, be it AC or DC. 30mA is considered potentially lethal.

    Cheers!

    Mickster
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-04-18 09:15
    This brings up another question. Here in Taiwan, the wall warts seem to be getting smaller, lighter and more powerful. I presume that all this means that the transformer is no longer included - though I guess it could have just gotten extremely tiny.

    What would youall suggest is the best way to test for proper isolation? (NOT insulation, I mean isolation.)

    Asia is an awkward environment for engineering fundamentals and quality control. While the average Asian student is much harder working than an American; Americans and other Westerners in general are more willing to DIY and far more aware of what should be good standard safety processure. I have had one transformerless gel cell charger go up in flames while charging. The same item gave me a rather bad jolt even though it was supposed to be only a 6VDC charger (at 3amps).

    I had one student working in IC production here that asked me for help with a letter to there supplier of epoxy asking how they could use the Part B to thin the viscosity of the Part A component. He was quite surprised when I explained what a 'hot batch' was and that he really needed a thinning solvent - not try to adjust the ratio of two parts.

    I have lots of other anecdotes about how 'high technology' in Asia is rather hazardous in the hands of newcomers, but I am really most concerned with this transformerless power supply becoming a hazard to hobbyist such as Parallaxians.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-04-18 10:18
    Loopy Byteloose wrote:
    ...the wall warts seem to be getting smaller, lighter and more powerful. I presume that all this means that the transformer is no longer included...
    No, it's because they use high frequency switching ahead of the transformer, which permits the use of much smaller magnetics.

    -Phil
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-04-18 22:56
    @Phil
    Okay, maybe you are right - but I'd still like to be able to check for myself. Do I use a VOM and measure the AC voltage on each output (the DC + and -) with the other side of the meter connected to ground?

    I also would like to point out that these do NOT have any UL logo or other international safety certifier. They do have a lable that states "Made in Taiwan" and one side has a CAUTION lable that says, "Risk of electric shock. Dry location use only. Indoor Use only."

    The actual wall wart is not much larger than a normal AC plug - 3/4" x 1 1/4" x 1 7/8" for 5VDC with 600ma output. (Input is 115VAC .11A)

    I do like these as I have found the voltage regulation is quite stable and I can power 5VDC without added regulation. There are similar, but a bit larger ones that work both with 220 and 110VAC at 50/60cycles. Similar lack of UL certification.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-04-19 09:52
    Loopy Bytleloose wrote:
    The actual wall wart is not much larger than a normal AC plug - 3/4" x 1 1/4" x 1 7/8" for 5VDC with 600ma output. (Input is 115VAC .11A)
    That's pretty typical for the latest 5V switching power supplies. They have to be isolated. Nobody could sell one on the open market that wasn't -- even without the UL tag.

    -Phil
  • RDL2004RDL2004 Posts: 2,554
    edited 2012-04-19 10:03
    Apple USB Power Adapter
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-04-20 07:45
    @Phil
    You are awfully sure. I've been too often duped in this side of the world, so I will run some tests. After all, the shock hazard warnings on the side of the unit seem to contradict the idea that the unit is fully isolated.

    Beside testing the +/- to neutral with a VOM for AC; I will turn around the pull in and see if that changes the results. I suspect it may as it is 110VAC unit. That Apple USB supply seems to have a proper plug that cannot be reversed AND the housing is fully covering any electrical conductors. So it too may not include an isolation transformer.

    I'll post the results when I get done. My impression of electronics in Taiwan is that they DO NOT have to be isolated or that enforcement of such is very lax. Items exported to the 3rd world countries may be similarly lacking and the cost of manufacture is cheaper, so is the cost of shipping.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-04-20 08:22
    Loopy,

    In these kinds of wall warts, the AC is rectified to DC then converted back to PWM'd AC at maybe 1 MHz. This is a high enough frequency to get significant amounts of power through a very small isolation transformer, perhaps just a tiny toroid. On the isolated side, the AC is re-rectified and regulated to the chosen DC level. The regulation can involve feedback across the isolation barrier to the PWM controller, sometimes via an optocoupler. Here's a diagram from this article:

    SMPS_Block_Diagram.png

    And, yes, all of that circuitry will fit into a tiny cube like the Apple USB supply. Apple simply could not sell a non-isolated supply. If they tried, and if our vast regulatory machinery did not come down on them, a hoard of tort lawyers would be storming their gates with torches, pitchforks, and class-action lawsuits.

    -Phil
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