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Voltage limiting device

Discussion in 'Electronic Basics' started by The little lost angel, Jan 20, 2004.

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  1. I do lotsa tinkering lately with my PC and PSU, so getting worried
    about what will happen if it blew up :ppPP

    So I thought of building a sort of safety device for my PC that will
    limit the voltage supplied if some kind of surge occurred when the PSU
    fails. This is because there are horror stories of motherboard, hard
    drives and other components dying when the PSU exploded (or something
    to that effect).

    Would it be possible to build such a widget that allows say a max of
    13V to pass through. Anything higher is either clipped immediately or
    the device trips fast enough to prevent the harmful voltage from going
    through? If so, what kind of components and information should I be
    looking for?

    Thank you!!!

    --
    L.Angel: I'm looking for web design work.
    If you need basic to med complexity webpages at affordable rates, email me :)
    Standard HTML, SHTML, MySQL + PHP or ASP, Javascript.
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  2. Look for "crowbar overvoltage protection" in Google. Crowbar protection is a
    popular method which uses a thyristor whose gate is controlled by a Zener
    diode. When an overvoltage occurs, the thyristor is turned on and
    effectively shorts the PSU's output (which in turn blows a fuse).

    I don't know for sure, but I suspect computer PSUs already have overvoltage
    protection circuits internally, if so there's no need to worry about it.

    cheers,
    Costas
     
  3. They usually do, kinda latch (some kind of technical method I don't
    know?) the PSU to off position until AC is removed. But the problem
    I'm thinking of is when the PSU fails, kinda like the
    circuitry/caps/IC blows and sends a say 20V surge down the line to
    stuff expecting a max of say 14V.

    So I was wondering if it was possible to fix something between the PSU
    and the devices to block such stuff. It would be quite useful for
    friends who insist on using cheapo PSU, if I can make this thing for
    say $5 :ppPP

    --
    L.Angel: I'm looking for web design work.
    If you need basic to med complexity webpages at affordable rates, email me :)
    Standard HTML, SHTML, MySQL + PHP or ASP, Javascript.
    If you really want, FrontPage & DreamWeaver too.
    But keep in mind you pay extra bandwidth for their bloated code
     
  4. w_tom

    w_tom Guest

    Cheaper to buy the $80 (retail list) power supply to fix the
    missing overvoltage protection circuit and many other missing
    circuits in that cheapo power supply. You don't save money
    buying a supply based only upon price. Minimally acceptable
    supplies contain the overvoltage protection and other
    essential functions because they also provide a long list of
    numerical specs.

    Overvoltage crowbars were defacto standard even 30 years
    ago. And yet some supplies today do not have it? Yes. Today
    we have an MBA education where only price is important;
    technical facts be damned. Fix overvoltage protection - and
    still have many other problems with that cheapo supply.

    For example, overvoltage protection could kick in - and the
    supply then burns up. No acceptable supply is damaged even
    when all outputs are short circuited together! Intel specs
    even say how big the shorting wire must be. Fix the problem -
    not the symptom. Shoot the MBA; then get a real power
    supply. Build the overvoltage crowbar only to learn about
    them.
     
  5. Well, for another thing, I'm going to be fooling around with a couple
    of PSU for fun and likely to blow up a couple in the process of
    stressing them. So the device would be nice to protect my own systems
    when I do that, just thought that it would also be nice to be able to
    make the widget for friends who ignore advice to use better PSU.

    --
    L.Angel: I'm looking for web design work.
    If you need basic to med complexity webpages at affordable rates, email me :)
    Standard HTML, SHTML, MySQL + PHP or ASP, Javascript.
    If you really want, FrontPage & DreamWeaver too.
    But keep in mind you pay extra bandwidth for their bloated code
     

  6. Yes, there are ways to limit the voltages, but I don't think it will cost as
    little as $5. Don't forget, there are 4+ rails to protect. Most of all, it's
    a messy job... If you look at the PSU case, there is a whole bunch of cables
    coming out of it, some going to the motherboard, others going to the
    peripherals, etc. To do it right you'd need to open the PSU casing and add
    the circuit in there. But doing so could cause much more harm than good if
    you're not careful. Also, if your protection circuit is a little too
    sensitive and gets triggered by a voltage spike or noise, the whole PC could
    shut down on you in the middle of a job, meaning lost data, etc. I'd go for
    a high-quality PSU. I've never had a problem even with cheap PSUs running
    24/7 for years.

    Thinking about it, I suppose you could add some sort of passive protection
    to the rails by adding transient voltage suppressors in parallel with each
    rail, rated just above the rails' voltages. These are basically high-power
    Zener diodes that only conduct if the voltage exceeds a given threshold.
    This solution could cost less that $5, but again, I don't think there is a
    real need for it.

    Don't worry too much about it! Backing up your data regularly is a far safer
    method than extra overvoltage protection.

    Regards,
    Costas
    _________________________________________________
    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying
     
  7. w_tom

    w_tom Guest

    The overvoltage crowbar as described previously costs far
    less than high power zener diodes AND has been industry
    standard for over 30 years.

    No reasons to think about it. Any acceptable ATX power
    supply already has this protection circuit. Those with basic
    power supply knowledge knows this circuit to be so important
    that even Intel specs demand it:
    Suggest you seek a good source on power supply design to
    learn the simple, less expensive, and more powerful
    overvoltage crowbar circuit - an industry standard circuit
    that uses 'low' power (much less expensive) zener diodes.
     
  8. w_tom

    w_tom Guest

    I ever so strongly, with emphasis, encourage you to build
    and test these circuits. Don't be afraid to make smoke. Part
    can be obtained even in Radio Shack. And don't worry about
    blowing up supplies. Only defective supplies will be
    damaged. Then continue on to find out WHY power supply was
    defective (with a warning about 300+ volts stored on some
    electrolytic capacitors inside that power supply - which is
    why power supply capacitors are shorted out before applying
    hands). There is great power in electrically dirty hands.
    Dirty hands - something that too many 'newsgroup experts about
    power supplies' don't bother to do.
     

  9. I know about crowbars... Read my previous posts in this thread. I've already
    suggested it to the OP. The point is that there are 4 rails to protect, each
    requiring an SCR and Zener. Plus it's an unnecessary job as most PSUs
    probably already have them built-in.

    cheers,
    Costas
     
  10. w_tom

    w_tom Guest

    Sorry. I did not understand that was your point.
     
  11. I'm not worried about blowing up the supplies, I'm more worried about
    blowing up the system connected to the supplies. It's easy enough to
    replace a cheapo supply if I blow it up during a stress load, but NOT
    the computer system!

    --
    L.Angel: I'm looking for web design work.
    If you need basic to med complexity webpages at affordable rates, email me :)
    Standard HTML, SHTML, MySQL + PHP or ASP, Javascript.
    If you really want, FrontPage & DreamWeaver too.
    But keep in mind you pay extra bandwidth for their bloated code
     
  12. w_tom

    w_tom Guest

    An overvoltage protector circuit will not damage any other
    device as long as (and this is important), ground integrity to
    the OVP is maintained. In a multivoltage system, ground is
    essential to all peripheral safety. Importance of ground is
    why better busses design peripheral interface connectors with
    grounds on each end of card; so that ground will always be
    first to make and last to break.

    OVP will not cause any power supply voltage to exceed
    rating. And low voltage does not damage electronics; just
    creates strange intermittents.
     
  13. Erm, I think you're misunderstanding me. I don't mean to say the OVP
    will cause any damage. I want an OVP device to prevent damage.

    The damage likely to come from an overloaded supply blowing up. There
    have been more than a few reports of supplies taking out motherboards
    and harddisks when they die.

    --
    L.Angel: I'm looking for web design work.
    If you need basic to med complexity webpages at affordable rates, email me :)
    Standard HTML, SHTML, MySQL + PHP or ASP, Javascript.
    If you really want, FrontPage & DreamWeaver too.
    But keep in mind you pay extra bandwidth for their bloated code
     
  14. Bill Vajk

    Bill Vajk Guest

    Massive zener diodes.
     
  15. w_tom

    w_tom Guest

    OVP will limit voltage IF properly sized. Size is why high
    power zeners are not used - as was true 30 years ago. Current
    is why silicon controlled rectifiers (thyristors) are used.
    Each SCR sized sufficient to shunt maximum current for each
    supply voltage.

    A supply overloaded by an overvoltage crowbar shunt must not
    be damaged; as required by design specs. But then, even if
    supply is damaged, the OVP continues to shunt; guarantees no
    damage to computer.

    BTW, power supply does not explode. Internal components
    simply fail - most often without external, visual indication.
    This completely transparent to the load because the OVP
    exists.

    If in doubt, test with power supply disconnected from
    computer. Power supplies running without connection to
    motherboard easily accomplished.
     
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