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(Beginner) Varistor or TVS

Discussion in 'Electronic Basics' started by Ant_Magma, Feb 27, 2006.

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  1. Ant_Magma

    Ant_Magma Guest

    I'm building a power line modem.

    I need a transient protection stage btwn the plug and my circuit. Btwn
    a varistor and transient voltage suppression diode which one should i
    choose?

    Wikepedia says that a varistor degrades after repeated transient events
    http://en.wikipedia.org/wiki/Varistor

    , does it mean that i should be using a TVS diode?
     
  2. Chris

    Chris Guest


    Hello again, Ant_Magma. Ay, there's the rub. Protection from the
    slings and arrows of outrageous fortune is the very essence of why the
    idea of data transmission over power lines has never really caught on.

    You're supposed to be using a Bel coupler

    http://belfuse.com/Data/DBObject/0557-7700-XX.pdf

    with your INT5200 chipset. In the above data sheet, the manufacturer
    suggests you just put a MOV directly across the line. How quaint.

    Look, Ant. First off, this is the reason why these things always come
    to grief. There's nothing in this that suggests the whole
    HomePlug/Powerline (hey, isn't that a blog? Hi, Rich -- I know what
    you're thinking ;-D ) Power Packet setup is going to be much more
    reliable than the X10 or other means to get data across the power
    lines. Apart from the indisputable fact that home line voltage wiring
    doesn't have controlled impedances or terminations, too many evil high
    voltage high power anomalies happen on the line to seriously consider
    it as a safe or reliable data transmission medium.

    If I absolutely had to do this (and I wouldn't), I would try to find a
    non-inductive Aryton-Perry wirewound resistor of a good value to put in
    series with the line and the Bel coupler, but before the MOV (view in
    fixed font or M$ Notepad):

    | R
    | (Non-Inductive)
    | _ ___
    | .---o_/ \o-------|___|--o-----------.
    | | FU1 | - |
    | |L1 .-. / '---. ,-
    | ----- | |/ )|(
    | | | MOV )|(
    |o G /| | .---' '-
    | -/ '-' |
    | ------ | |
    | |L2 | |
    | '-----------------------o-----------'
    |
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    MOVs can be made to absorb a lot of energy (and are very economical
    compared with a TVS), but unless you have something to limit the
    current, you're depending on the MOV to absorb all. That's not very
    reasonable. And yes, MOVs are used up by powerline transients. By
    limiting the energy of the transient with a resistor, you're extending
    the life of the MOV, too.

    A series non-inductive resistor would limit the energy being absorbed
    by the MOV in the event of a lightning strike or other voltage spike or
    surge. Wirewounds are made to handle enormous overloads for brief
    periods, and might be well-suited to this application. Also, if they
    fail, they're guaranteed to fail open, which is another advantage. If
    an MOV fails, it can go either open or usually short. If short, then
    it's explosive -- be sure to fuse this (and make sure you've got a
    non-inductive fuse, too).

    You'll have to do some hunting, and try to find good non-inductives.
    Work with the manufacturer to keep total resistor inductance below 1uH.
    Try for a larger wattage resistor -- I'd be looking at 10 watts for
    better reliability.

    All of this is really to say that the problems with this setup are very
    serious, and the fact that everybody (chipset manufacturers, consumer
    electronics manufacturers, Radio Shack, coupler manufacturer) is
    pushing the buzz and pretending this problem either doesn't exist or
    isn't that big a deal just indicates that it's not ready for prime
    time. Possibly just an intellectual exercise that's doable and very
    interesting in the lab with filtered constant voltage isolation
    transformer, but is mostly just being used to pump up stock prices.

    The other part of this is that you're talking about playing with line
    voltage here. Back in the day, hobbyists came to grief trying to
    duplicate the then-emerging "next best thing", the X-10 standard for
    home appliance control. I'm not aware of anyone who was crisped, but I
    heard of a number of explosive failures, turning silicon into magma.
    Some of this came off an ill-considered DIY article in a hobbyist
    magazine in the mid-80s, with a schematic and layout that had a few
    rather sad errors.

    The questions you've been asking over the past month kind of indicate
    this might be a little too much for your level of experience and
    technical knowledge. You might want to consider putting this one off
    and doing something a little simpler until your knowledge level comes
    up a little.

    Good luck
    Chris
     
  3. w_tom

    w_tom Guest

    The varistor degradation means you next consult the numbers. Charts
    relate number of transients and size of those transients to life
    expectancy.

    TVS diodes will not degrade BUT will also provide much less transient
    conduction. Both devices must include safety provision for that
    unacceptable and catastrophic failure.

    Note conduction. Suppressor devices do not work by absorbing. They
    are shunt mode devices that make temporary electrical connections.

    Conduction also means one must consider multiple types of transients.
    Install an MOV or avalanche diode for differential transients. That
    protector then provides a typically more destructive common mode
    transient with more paths through electronics.
     
  4. Ant_Magma

    Ant_Magma Guest

    Thx w_tom & chris.

    Chris, thx4 ur concern but this is a project i have to do, if i want to
    graduate from university.

    Maybe i'll post my coupling designs 4 u guys 2 verify before i turn on
    the power?
     
  5. Ant_Magma

    Ant_Magma Guest

    Thx w_tom & chris.

    Chris, thx4 ur concern but this is a project i have to do, if i want to
    graduate from university.

    Maybe i'll post my coupling designs 4 u guys 2 verify before i turn on
    the power?
     
  6. Chris

    Chris Guest

    Hi, Ant. Just out of curiosity, could you answer a few questions?

    * What's your major?

    * Frosh, Soph, Jr., Sr?

    * Is this the senior project?

    * When's it due?

    Thanks
    Chris
     
  7. John Fields

    John Fields Guest

     
  8. Ant_Magma

    Ant_Magma Guest

    Electronics major
    Senior year
    Yes
    Approximately 2 months time

    John, pardon me. Thank you for your gentle reminder =)
     
  9. Chris

    Chris Guest

    Chris wrote:
    Hi, Ant. Just out of curiosity, could you answer a few questions?

    * What's your major?

    * Frosh, Soph, Jr., Sr?

    * Is this the senior project?

    * When's it due?

    Thanks
    Chris


    Ant_Magma wrote:
    Electronics major
    Senior year
    Yes
    Approximately 2 months time

    John, pardon me. Thank you for your gentle reminder =)

    Hi, Ant_Magma. You're out of time. You definitely need more help than
    you can get on s.e.b., in a timely manner, especially at this stage of
    the project. I'd suggest looking around for a graduate student or
    another senior who might be able to give you a dozen hours of
    concentrated time over the next couple of weeks, and help you get your
    project on track. This will, of course, be at standard tutoring rates.
    But if you're willing to pay, you should be able to get help.

    I would guess it's too late to back out of your project. If it isn't,
    google s.e.d. for a number of good threads on worthwhile senior
    projects.

    But if it is too late, and for one reason or another you can't withdraw
    from class or get an extension, get a hold of a small 240VAC : 120VAC
    isolation transformer (240VA) immediately, both for safety and also to
    help with line transients. Use any unregulated wall wart for your
    power supply, preferrably with a single 9VDC @ 1A or more output. Use
    an LM317 with 10uF caps on input and output side, and with 120 ohm and
    220 ohm resistors to set output voltage. It will need a 6 watt heat
    sink. Spend the money and get these things immediately, as well as
    lining up some help. If your samples haven't arrived, just buy them
    today, and have them FedExed to you.

    Waste anything but time.

    Good luck
    Chris
     
  10. Ant_Magma

    Ant_Magma Guest

    Thanks for you advice Chris. There were a few problems that had me
    started with this project too late which i won't elaborate further.

    If i follow your method, won't i have 2 plug heads (don't know what's
    the proper term)? 1 plug brings in the signal to the transformer the
    other brings in the power supply from the wall wart?

    I have already ordered Bel's powerline signal coupler, can i try and
    build it like the schematic drawn in the datasheet? The live and
    neutral wires are connected parallelly to the coupler and the PSU
    (step-down transformer). Since the coupler only passess the Homeplug
    frequency of 4Mhz-20Mhz filtering out the 50/60Hz power freqeuncy i
    should be safe right? For the step-down transformer i'm thinking of
    using any encapsulated PCB transformer available on Farnell.

    Please advice.
     
  11. Ant_Magma

    Ant_Magma Guest

    Thanks for you advice Chris. There were a few problems that had me
    started with this project too late which i won't elaborate further.

    If i follow your method, won't i have 2 plug heads (don't know what's
    the proper term)? 1 plug brings in the signal to the transformer then
    to the coupler and the other brings in the power supply from the wall
    wart?

    I have already ordered Bel's powerline signal coupler, can i try and
    build it like the schematic drawn in the datasheet? The live and
    neutral wires are connected parallelly to the coupler and the PSU
    (step-down transformer). Since the coupler only passess the Homeplug
    frequency of 4Mhz-20Mhz filtering out the 50/60Hz power freqeuncy i
    should be safe right? For the step-down transformer i'm thinking of
    using any encapsulated PCB transformer available on Farnell.

    Please advice.
     
  12. Ant_Magma

    Ant_Magma Guest

    Btw, Chris i guessed you have read the datasheet (however brief) it is
    of the Bel's powerline signal coupler. At the last couple of pages with
    the schematics, at the capacitor it writes 4.7n x2 class. What does x2
    class mean? And the 2 resistors 200k, can't they just use a 400k
    resistor?
     
  13. Ant_Magma

    Ant_Magma Guest

    Btw, Chris i guessed you have read the datasheet (however brief) it is
    of the Bel's powerline signal coupler. At the last couple of pages with
    the schematics, at the capacitor it writes 4.7n x2 class. What does x2
    class mean? And the 2 resistors 200k, can't they just use a 400k
    resistor?

    At the Electrical specifications part of the Bel coupler, the
    parameters of the coupler has this Hi Pot @60Hz 1mA and a value of
    2000V. What does it mean? It rejects the 60Hz?
     
  14. Chris

    Chris Guest

    Hi, Ant_Magma. One thing at a time.

    A real engineering prototype would have one line cord, with the split
    to the powerline modem and the power supply occurring inside the box.
    For a proof-of-concept prototype, and for initial work, you can have
    two plugs. You can always add a small linear or switching power supply
    later, after you have the thing working. Points off for
    proof-of-concept as opposed to full engineering prototype, but no
    points if it doesn't work. You need to get something going NOW.

    Whenever you're working with line voltage in an engineering lab
    environment, you should use an isolation transformer. This is first of
    all for safety. As an extra benefit, you can realistically step down
    from 240VAC to 120VAC, which should provide additional safety. I'd
    rather work with 120VAC than 240VAC. The thing is, you apparently have
    very little non-SPICE experience, and you're dealing with potentially
    lethal voltages. You need an isolation transformer for your personal
    safety. This is also why you need an advisor who has worked with line
    voltage before, and can keep a watchful eye on this part of the circuit
    (once you've got it done, you can enclose it and be done with it).

    According to the coupler datasheet, it provides 2KV of isolation
    between the line primary and the secondary. This is pretty much
    standard for transformers. On paper, you should be fine. The X-10
    looked good on paper, too. But I have a vision of your getting to the
    last couple of days, and a line transient jumps up and smokes your
    works. Pobre Ant_Magma. Line voltage accidents are usually explosive.
    Apart from any electrical safety considerations, you don't and won't
    have the time to fix this if a line transient smokes your prototype in
    the last week. Do everything you can to keep your prototype safe, and
    only use it and the other piece of comm equipment with an isolation
    transformer.

    By the way, here's what happens with a hipot transformer test. It
    measures the coupling between the transformer primary and secondary,
    and is required for proving isolation from line voltage. This is a
    standard safety test which is used to get listing from regulatory
    agencies like UL and CSA.

    | .------. ,------.
    | | )|( |
    | | )|( |
    | o------' '------o ^
    | | DUT | |
    | | | |I
    | | | |
    | | |
    | / \ / \
    | (2KV) ( A )
    | \_/ \_/
    | | |
    | | |
    | === ===
    | GND GND
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    The 4.7nF cap they're talking about is one rated for line voltage use.
    In the event of overvoltage, it's made to be self-healing instead of
    just shorting out. Don't use a cap which isn't rated for the line
    voltage you're using. And 200K is a standard resistor value. Also,
    resistors have a voltage rating as well as a wattage rating. If you
    apply more than the rated voltage, it may arc over the surface of the
    resistor, which would cause failure. Using two resistors means you can
    halve the voltage across each resistor, which might be a good thing --
    they're not self-healing. ;-)

    On a personal note, I had the pleasure in days of yore to work in a
    place where they liked to hire one or two undergrads and work them to
    the bone part-time for peanuts. As part of the deal, they had
    unofficial use of lab facilities after hours, and the use of all the
    kibbitzing they could wheedle out of the staff for their senior
    project. I tried to give them a hand when I could, and had quite a bit
    of fun working with them. It's satisfying to see students finally "get
    it".

    I'm not qualified to help you here. I've never done a component-level,
    "from-scratch" network interface. And in order to get an answer to two
    or three basic questions here, you had to wait over 10 hours. If your
    project is due in 9 weeks, you've only got about 1500 hours left. You
    need to get that loop lag down, and get your questions answered now, in
    quantity, by someone who knows what they're doing. If you wait for the
    kindnesses of strangers, you'll still be talking at deadline. Get some
    help, you're out of time.

    Actually, the real lesson of the senior project is how to do projects.
    By doing something enormous by yourself, you get a practical baptism of
    fire in the gentle art of working under brutal pressure. It's a
    required part of the engineering skillset.

    Good luck
    Chris
     
  15. Ant_Magma

    Ant_Magma Guest

    Thx Chris for your advice. Sad to say i'm currently on my own. About
    the 240/120 isolation transformer i'll have to check with the lab. But
    i'll definitely put my own safety as the first priority =)

    I did some designs on my own, taking into account your advice and some
    other resources that i've managed to find and i was wondering if you
    could give your opinion on it:
    http://geocities.com/Antonio_Magma/transient.jpg

    I decided to use a 3-stage protection circuit with a gas discharge
    tube, a resistor and a MOV (as stated in the Bel coupler datasheet).
    For the MOV i followed the coupler datasheet and i chose the LITTLEFUSE
    Varistor 390V 130J (V250LA40B). The gas discharge tube, the EPCOS 350V
    (EC350X). These parts i chose based on the availability of it in my
    area.

    However, for the series resistor i'm not sure how do i choose the
    value, that's where i hope you can help me.

    Next, i was thinking since my signal is transmitted at 4Mhz-21Mhz, i
    could probably design a high-pass filter to filtering out everything
    else. Thus, i designed a 4pF capacitor and 10kΩ resistor to form my
    high-pass filter.

    For the PSU, i decided to take your advice and i'll search for a linear
    power supply of maybe 10V output then use a LM317 to bring it further
    down to my operating voltage of 3.3V.

    What do you think?
     
  16. Chris

    Chris Guest

    Hi, Ant. If you don't have an isolation transformer, you're not making
    safety your first priority.

    Don't worry about the resistor at this point. The MOV alone will do
    for proof of concept stage. Just do exactly what's in the data sheet
    (4.7nF cap in parallel with 2 ea. 200K resistors).
    L1 L2
    | |
    | |
    | |
    o | 2X200K
    ( | ___ ___
    ) | .--|___|--|___|----.
    o | | |
    | | | || |
    o-----)-------o-------||---------o---. ,-
    | | .-. | || )|(
    | | | |/ 4.7nF )|(
    | | | |MOV .----' '-
    | | /| | |
    | | | '-' |
    | o-------o-----------------'
    | |
    | | .-------------.
    | | | |
    o-----)---------------o |
    | | | PS |
    | | | |
    '---------------o |
    '-------------'
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    A 4pF series cap won't work. Your signal will be gone.

    Any 6V to 9VDC wall wart that can provide at least 1 amp will give you
    something you can regulate down to 3.3V with an LM317. If you have
    time, you can put a small linear or switching power supply on board
    later.

    Please look for another source for help besides newsgroups.

    Good luck
    Chris
     
  17. Ant_Magma

    Ant_Magma Guest

    You mentioned it would not work with the 4pF capacitor. Can you
    elaborate further why?
     
  18. Chris

    Chris Guest

    Try it and see. Actually, you're not taking the load into account. I
    know this because the manufacturer uses a 4700pF cap.

    Chris
     
  19. Rich Grise

    Rich Grise Guest

    I've worked with varistors before (they called them MOVs), and they're
    crap. During the period of time where I worked at that place, they
    completely converted over - they quit buying varistors and replaced
    them all with transzorbs.

    So, yes, you should be using a TVS diode. I'd recommend a bidirectional
    one, of course. :)

    Good Luck!
    Rich
     
  20. w_tom

    w_tom Guest

    The Transzorb or Transil devices should only be used IF properly
    fused. For example, I would never use a 1.5KE220CA without a fast blo
    type fuse of 2 amps or less.

    MOVs, avalanche diodes, gas discharge tubes, etc all have their
    place; all are effective. But that means the human must understand the
    principles behind each device. It is why most all 'accidents' are
    directly traceable to human failure.

    I don't understand why the OP wants or needs protection. If for
    human safety, then he is missing the point. His reasons for protection
    do not come with numbers - which was one important fact that I was
    watching for. Without such parameters, I cannot comment on what is and
    is not effective protection. I still don't yet see the underlying
    reasons - the problem - he wants to solve.

    BTW, MOVs have highter capacitance. This is why the telcos did not
    use MOVs in pre-1985 phone line protectors. Other avalanche diode type
    protectors also address this 'excessive capacitance' problem. Numerous
    solutions exist - although all may be beyond the scope of that project.
     
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