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surge protection built into ordinary household electronics

Discussion in 'Electronic Design' started by Matt, Jun 6, 2007.

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  1. Matt

    Matt Guest

    I'm building a little microcontroller system to be powered by household
    current. I'm thinking it might be good to put in some kind of surge
    protection. I wonder whether and how such protection is built into
    ordinary devices such as televisions, microwave ovens, washing machines,
    stereos, etc.

    My system only has a couple of chips (costing $3 altogether), so my
    other approach is to put the chips in DIP sockets. I don't know whether
    to assume that the discrete passive components would be safe under a surge.

    How should I do this, or should I provide any protection at all?
     
  2. Marra

    Marra Guest

    All household goods have filters in now any way to pass EMC regs.
    They obviously use just enough to get through the regs as cost is
    important.
     
  3. Marra

    Marra Guest

    All household goods have filters in now any way to pass EMC regs.
    They obviously use just enough to get through the regs as cost is
    important.
     
  4. D from BC

    D from BC Guest

    I include surge suppression because it's a product selling feature..
    It's zap proof! (To a degree..see fine print..)

    I've been using these lately...
    http://www.panasonic.com/industrial/components/pdf/awa0000ce2.pdf
    D from BC
     
  5. mpm

    mpm Guest

    Wall wart?

    Otherwise: Think fuse and MOV.
     
  6. Matt

    Matt Guest

    Yep, one fuse, 3 MOVs should do the trick.
     
  7. Matt

    Matt Guest

    I'm planning to put a little transformer, a FWBR, a cap, and a 5V
    regulator on the PCB. There will be a relay to switch 20A 120VAC, so I
    need to run 120VAC to the board anyway. What would a wall wart have in it?
     
  8. Matt

    Matt Guest

    The design calls for a transformer, a full-wave bridge rectifier, and a
    capacitor, then a regulator. Should I just put that ZNR surge absorber
    across the capacitor? How do I select the size?
     
  9. Eeyore

    Eeyore Guest

    What 'surge' do you mean ? A microprocessor circxuit will likely be powered by a
    voltage regulator which will remove any line voltage fluctuations.

    Graham
     
  10. Eeyore

    Eeyore Guest

    No they don't.

    Graham
     
  11. Eeyore

    Eeyore Guest

    Then you almost certainly don't need any further 'protection' do you. What do you
    think the voltage regulator does ?

    What would be the point of that ?

    Get real.

    Graham
     
  12. Eeyore

    Eeyore Guest

    LMFAO !!!

    I hope your designs don't have to sell at a profit !

    Graham
     
  13. Eeyore

    Eeyore Guest

    Typically, a transfomer, rectifier, storage cap and maybe a regulator.

    Graham
     
  14. mpm

    mpm Guest

    Looks like Matt's just buiding some sort of 120V logic-controlled
    switch.
    (They actually have relays that do this sort of thing already, or
    PLC's, or even the stuff from www.pulizzi.com). Because he's got to
    run 120 to the box anyway, a wallwart isn't ideal.

    But to help Matt out, any surge would have to get through his
    regulator (xfmr, bridge, caps, and 7805?) to do any real damage. A
    single MOV across the AC input is a nice touch, but it's not
    essential. Matt, MOV's have issues too, mostly relating to design
    lifetime.

    If you're going to sell these things, you need to keep prices low or
    competition will eat you alive. At the same time, you'll have to make
    sure meet all the regulatory requirements (for example, conducted and
    radiated emissions if your project uses any high frequency stuff -
    even the clock crystal on a microcontroller.) And depending on where
    you want to sell them, you may need an independent safety
    certification like UL, TUV, CE, etc... Which costs money.
    And don't forget RoHS restrictions.

    Is there some special environment that these devices will be placed in
    that you're especially worried about surge protection?
     
  15. Matt

    Matt Guest

    Lightning, the kick from shutting off a sump-pump motor, stuff like that.
    Okay, obviously I am not any kind of expert, but ...

    http://www.national.com/ds.cgi/LM/LM2937.pdf
    I thought that meant that the regulator might be ruined if it gets more
    than 60V for more than 100 ms.
     
  16. mpm

    mpm Guest

    Well, "Yes", that is what it means, but think about what has to happen
    to get there.

    You have a lot of stuff in front of this spec. Your transfomer,
    filter caps, the bridge rectifier, the fuse. The 60V/ 100mS has to
    get through all that to do any damage.

    It would be a good exercise for you to look at just the transformer.
    Imagine 60V on the secondary. How much would that be on the Primary?
    Do you think this is a reasonable value you would ever expect to see
    coming out of the wall socket?

    Now of course, that's still AC at that point, but I think it's
    instructive none the less.
    Your bridge will lower the voltage even further, and the caps will
    tend to smooth out longer term events (100mS). That's why they are
    called filter caps.

    Lightning is very impulsive, and contains a lot of VHF frequencies,
    which can be somewhat difficult to avoid completely. You could spend
    a lot of money making it lightning-proof, and never sell a single
    product... A lot of people I know just like to tie a knot in the
    power cord (inside or outside the enclosure), as a lightning choke of
    sorts.

    Personally, I would use an MOV, or a Sidactor if it were a modem line
    or something...
    Good luck.
     
  17. ehsjr

    ehsjr Guest

    Yes. A wall wart charger for an NiCd powered vacuum cleaner
    attachment. The charge rate was "set" by transformer sag.
    The circuit was: secondary---diode---NiCd
    Really "extensive" engineering. :-( Cheap piece of crap.
    Two shorted cells in the pack took out the wallwart. My guess
    is that there are lot of similar "circuits" for wallwarts.

    Ed

    Ed
     
  18. D from BC

    D from BC Guest

    Actually, I've tried to cheapen some designs and ...it can get
    difficult and complicated!
    It's tough to figure out a sneaky amount of components..

    D from BC
     
  19. Eeyore

    Eeyore Guest

    If lightning gets seriously near it - it'll kill it, 'protection' or no, so forget
    that one.

    A tiny glitch on the mains ? It may not even get through a simple PSU. Have you
    checked ?

    In any event, the norm for dealing with that is filtering, not 'surge protection'
    since, a glitch isn't a surge !

    Well it won't will it !!!

    This is a 5V microprocessor circuit ? You may have 10 V on the input pin of the
    regulator. Lower possibly since it's an LDO. Do you seriously think the ac line is
    going to jump to around 5 times its normal value for a whole 100ms ?

    Have you ever *looked* at the mains ?

    Graham
     
  20. Matt

    Matt Guest

    No special environment---I'm just trying to avoid building a piece of
    junk. Just trying to conform to something like accepted practice ...
     
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