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240->120V AC adapter horror

Discussion in 'Electronic Design' started by Spehro Pefhany, Jan 9, 2011.

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  1. An associate bought this for me (from a kind of flea market/surplus
    shop), just because he thought it looked amusingly gawd-awful. I have
    to agree.

    It's an allegedly 1600W 240->120VAC adapter for resistive loads.

    http://www.speff.com/pcb_view.jpg
    http://www.speff.com/resistor.jpg
    http://www.speff.com/side_and_cover.jpg
    http://www.speff.com/top_view.jpg

    The circuit is straightforward.. a simple preset phase control. The
    firing angle is determined by a 15K fixed resistor in series with a
    50K trimpot and a 0.1uF film cap (it should be around 115 angular
    degrees). There's a diac and a BT137 triac.

    http://www.nxp.com/documents/data_sheet/BT137_SERIES.pdf


    First thing, the cover just lightly snaps on/off; a child or careless
    adult could easily remove it. Looks like cheap non-fire-retardant
    styrene.

    The gold anodized heat sink housing is nicely made, perhaps thicker
    than it needs to be. There is no reason for the 15K resistor to be as
    large as it is.. it's just acting as a jumper. The triac is
    more-or-less held in contact with a ~1/3mm thick silicone heat pad and
    thus the heat sink, but only by the springiness of the leads. Given
    that the triac is a barely heat-sinked 8A triac.. I suppose it's a
    good thing that the fuse in series with the output is 5A. At least
    it's a 250VAC fuse. Either the insulation around the resistor or that
    on the triac could easily slide out, leaving the metal case connected
    to the mains. The bottom of the PCB has a thin (~0.004") insulator,
    and creepage distance at the pins is well under 2mm. Yet they bothered
    to put thread lock on both the nuts holding the pins to the PCB.

    Total horrorshow. The really sad thing is that they could have used
    similar materials and components and made a pretty good (and fairly
    safe) 500W converter for not much different cost.


    Best regards,
    Spehro Pefhany
     
  2. On Sat, 08 Jan 2011 20:36:30 -0500, the Spehro Pefhany

    <snip>

    That should be 500K, of course.


    Best regards,
    Spehro Pefhany
     
  3. Jamie

    Jamie Guest

    Nice converter, I wouldn't be plugging anything valuable into it.

    I am not even sure if switching supplies would operate correctly
    from a phase angle chopped source like that!..

    How ever, on a second note, I was given a Marine 1500 C/3000P watt
    converter today for parts, because it didn't output anything.. Nice
    unit, has amp and source LED graph bars. Is modified sine wave, what
    would you expect otherwise..

    After a close inspection of the GFCI duplex mounted in it, I found
    it not to reset.. Was able to use a household duplex receptacle as a
    replacement, which I had lying around :)

    Jamie..
     
  4. Phil Allison

    Phil Allison Guest

    "Spehro Pefhany"

    ** That is just a tad ambiguous.

    The CONDUCTION angle needed is 66 degrees for conversion of a sine wave at
    240 volts to 120 volts rms.

    The resulting train of pulses have peak values of +/- 310 volts - ie only
    a little under the usual peak values of a 240 volts AC supply.

    Also - if there is any electronics associated with the load presented to
    such a converter (ie timers or power controllers) - they will probably very
    soon be fried.

    BTW:

    I have seen examples of ( Canadian ) folk using one of these with their
    domestic audio gear brought out to Australia with them.


    ..... Phil
     
  5. In the power biz, firing angle is measured from the zero crossing, so
    yes you're right, the _conduction_ angle is 180 minus 66 or 114 (more
    like 113.83 if I did the math right) to get RMS output of 0.500 the
    input (ignoring triac drop etc.)
    Yup. sqrt(2) * 240 * sin(114) = 310V vs. 340V peak.
    Hosers, clearly.

    Best regards,
    Spehro Pefhany
     
  6. Nope. Clean and slippery. There's a slight embossed "tread"-like
    pattern on the triac side, while the other side is smooth.


    Best regards,
    Spehro Pefhany
     
  7. Phil Allison

    Phil Allison Guest

    "Spehro Pefhany"

    ** Had to look that unfamiliar word up - ain't Wiki wonderful or such info.

    The equivalent Aussie expression is " Bogan" :

    http://en.wikipedia.org/wiki/Bogan

    A fairly recent term, as colloquialisms go.


    BTW

    As I recall, the particular Canadians were of Irish extraction.

    Double whammy.......



    ...... Phil
     
  8. Maybe a copy of a copy of something that once worked okay?



    Best regards,
    Spehro Pefhany
     
  9. Phil Allison

    Phil Allison Guest

    "Spehro Pefhany"

    ** Makes the * rms * output current 13.3 amps at 120 VAC.

    But the *average rectified value* is just over half that number

    - meaning that an 8 amp rated triac is OK.

    Should dissipate about 10 to 12 watts - at most.



    ..... Phil
     
  10. Yes. The voltage drop across the triac is more-or-less fixed,so
    average current (mostly) determines power dissipation.
    1) There's a 5A glass fuse in series with the output, so we
    don't need to worry about much more than 6~7A RMS
    for any length of time (AGC5's take ~10 seconds to go at 7A)

    http://www.cooperbussmann.com/pdf/b040436b-cbc7-495d-95a6-99fe85202786.pdf

    2) With that idiotic "heat sink" setup, it
    looks like a toss-up to me whether the triac fails
    before the fuse or the other way around. When the
    triac goes, it puts 240VAC across the output.. and
    hopefully the fuse then goes, but that depends on the load,
    obviously). The heat sink might be barely adequate for 5-10W
    if the triac was actually firmly attached to it.


    Best regards,
    Spehro Pefhany
     
  11. A rectifier would will put 41% more RMS voltage on the load than it is
    designed for. Hence the phase control.
    At 170VAC on a 120V lamp, not for long.



    Best regards,
    Spehro Pefhany
     
  12. Phil Allison

    Phil Allison Guest

    "Spehro Pefhany"

    ** Anyone see a problem here ??



    ..... Phil
     
  13. Rich Grise

    Rich Grise Guest

    Marketing.

    Cheers!
    Rich
     
  14. ehsjr

    ehsjr Guest

    Well, not quite a _total_ horrowshow. There's one potentially
    good thing about it: it could give Joerg an opportunity for
    sound effects. :)

    Ed
     
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