Connect with us

Resistance in W??

Discussion in 'Electronic Basics' started by The little lost angel, Dec 15, 2003.

Scroll to continue with content
  1. Could anybody please enlighten me why is the resistance value given in
    watts instead of ohms?

    http://www.brakingresistor.com/metal.htm

    Thanks! :)

    --
    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
     
  2. John Fields

    John Fields Guest

    ---
    It isn't. Choose the resistance you need from the resistance _range_
    available and the wattage you need from the values available in the
    left-most column after you use the derating graph to determine what the
    wattage needs to be at the temperature at which the resistor will be
    operating.
     
  3. Sorry I'm slow as usual about getting this.
    The only resistance range I see is in the table and it says
    Wattage | Resistance Range (W) |
    Rating | Standard Type | Non-Inductive Type |

    Then the values are like 0.05W - 5KW

    Do you mean, I take say "Free Air 100W-150W" which is line 2
    At 100C ambient, which is 50% of rated power.

    Then resister I would need would be 300W Rating and I can get them
    from 0.00067 ohms (0.2W / 300W) to 60 Ohms (18KW /300W)?


    --
    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. DaveC

    DaveC Guest

    All resistors have an ohm value. That's their nature. In some (very few)
    applications you don't care about the ohm value of the resistor.

    In this case, it seems, these resistors are used to just dump the current
    from a motor into a load (any load) resistor so that the motor will stop
    turning (hence the "braking" in "braking resistors"). The power is wasted,
    turned into heat by the resistor.

    The resistance isn't important, just so long as it's a low value. And since
    these are all braking resistors, I assume that they're all very low values.

    You just need to be sure that the voltage and current you're trying to get
    rid of can be handled by the resistor (Power (Watts) = Voltage x Current).

    I'm not sure, however, if I had a motor to brake, how I would match one of
    these resistors to my motor... maybe measure how much voltage and current the
    motor required when running?

    Enjoy,
     
  5. John Larkin

    John Larkin Guest

    I've seen a lot of magazines and such use W to indicate ohms. This
    must be some sort of Windows-y typographic corruption of the Omega
    symbol.

    John
     
  6. The Al Bundy

    The Al Bundy Guest

    In IE 6.0.28 I see the Ohm sign instead of the 'W'. Maybe incorrect/missing
    Font settings?

    Al
     

  7. Correct. The Greek uppercase Omega in the Symbol font is in the same place
    as W in standard fonts. The reason is probably because the lowercase Omega
    looks like w.

    Costas
     
  8. DarkMatter

    DarkMatter Guest

    Perhaps because such resistors re always the same value in Ohms, and
    the only requisite for a user to select one by is dissipation ability.
     
  9. DarkMatter

    DarkMatter Guest

    Look again. Under "resistance range" Wattages are given, not a
    range of available resistance values.

    I presume that all of that class of resistor for that purpose are
    all the same value, and wattage is the only criteria a field service
    person or designer need consider for applications that involve
    "braking resistors". Probably some very low value, like an Ohm or
    such. They rely on back EMF to achieve a "braking" action from what
    would otherwise be a motor. Dead short the armature of a motor that
    is spinning with an energized field, and one will get a back emf that
    will fight the spindle speed. Braking action.
     
  10. DarkMatter

    DarkMatter Guest

    In the "Symbol" font, the "value" for the OHM symbol is a capital W.
    I have an app that distills pdfs this way, such that I had to
    actually create a bit map of the symbol to include it.

    I would think that Adobe could distill a pdf, and catch it. It is
    on my list of things to look into.
     
  11. John Fields

    John Fields Guest


    Kind of.

    Looking at the table, the wattage rating is the column on the left and
    indicates that they make resistors in this series with ratings from 60
    to 1000 watts. Next to that is a column labelled "Resistance Range
    (omega)" which is divided into two columns, one labelled "Standard Type"
    and one labelled "Non-Inductive type".

    Looking at the first row, there will be an entry in the left-hand
    column which reads "60", an entry in the center column which reads
    "0.05 omega - 5K omega", and an entry in the third column which reads
    "0.1 omega - 2.5k omega", with 'omega' meaning ohms.

    The first entry in the center column corresponds to the first entry in
    the "Wattage Rating" column and indicates that if you need a standard
    (not non-inductive) 60 watt resistor you can get them with resistances
    anywhere between 0.05 ohm and 5000 ohms. The first entry in the third
    column indicates that they can supply non-inductive 60 watt resistors
    with resistances anywhere between 0.1 ohm and 2500 ohms. The rest of
    the table follows suit, and serves to show what resistances in standard
    and non-inductively wound resistors they can supply at the wattage
    ratings they offer.

    Their "% of Rated Power" VS "Ambient Temperature" graph is pretty
    straightforward and indicates the derating that must be done with the
    resistors either on a proper heat sink or in free air at different
    ambient temperatures. Let's say for instance that you do your
    calculations and find that your resistor will need to dissipate 100W in
    free air at an ambient temp of 25°C. Since the second plot down
    corresponds to 100W ~ 150W, draw a line vertically starting at 1/4 of
    the way from zero to 100°C until it hits the second line and then
    continue the line you're drawing horizontally from that point until it
    hits the 0 to 100% line. It'll fall somewhere a little bit higher than
    70%, so if you use 70% you'll be safe. Now, since you need for the
    resistor to dissipate 100 watts and the graph shows that the resistor
    needs to be derated to 70% of its rating to dissipate 100W at 25°C, that
    means you have to start out with about a 143 watt resistor, so 150 watts
    would be a good choice. Now go back and look at the range of
    resistances available with a 150W rating and you'll find that they have
    from 0.1ohm to 12000 ohms available, so it's probably likely they'll
    have what you need. OK?
     
  12. DarkMatter

    DarkMatter Guest

    Yes, except that that is not how it appears. To most, the "omega"
    you refer to appears as a capital W. Easily confused for the vendor
    supplying wattage ranges.

    However... it is all clear to me now.
     
  13. DarkMatter

    DarkMatter Guest

    You must have a LOT of free time to have actually sat there, typing
    away, at several huge paragraphs of re-iterated information.

    Quite informative. Not the post... the free time, and the fact
    that you felt Usenet was worth that much of it. Life must be boring
    for you.
     
  14. John Fields

    John Fields Guest

     
  15. John Fields

    John Fields Guest

    ---
    Not at all. On the contrary, it delights me to be able to give someone
    knowledge that they didn't have before. I don't suppose you've ever
    seen the lights go on in someone's eyes when loose ends that they had in
    their mind suddenly get connected, because if you had you certainly
    wouldn't say making that happen was a waste of time.
     
  16. John Fields

    John Fields Guest

     
  17. Ah, so it's my browser/font that was throwing me off totally.
    Thanks for the clarification and I appreciate the detailed explanation
    on how to read/use those charts. :)

    --
    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
     
  18. DarkMatter

    DarkMatter Guest

    It is obvious, however, that you are clearly ignoring the fact that
    the orig poster, and many others see the W symbol.
     
  19. DarkMatter

    DarkMatter Guest

    Not really. Banks of resistors can be set on a conductor rail, and
    hooked up in sequence, in parallel, according to the braking load
    desired. A giant rheostat.

    Most trolley brakes I've seen (Siemens) are a sliding rheostat
    "looking" arm on the operator's control console. Were there banks of
    a given value resistor hooked up, they would each dissipate the same
    heat as each was brought into use during braking, and the brake
    operator only engages as many of the resistors as are needed to effect
    the smooth stop curve needed to keep that last second jolt out of the
    picture for the passengers.
     
  20. DarkMatter

    DarkMatter Guest

    What I actually said was that you redundantly repeated what was
    already on the site, and STILL didn't "fix" the "problem".

    The answer was:

    "The capital W is shown that way, because your browser did not
    catch the use of the other font utilized in the document. The capitol
    W in the SYMBOL font is the upper case Omega symbol, widely used in
    the electronics industry, as it stands for a basic electronic unit of
    measure.

    Replace the "resistance range" "W"s with the term Ohms, and all
    should be clarified, or save and edit the page, and attempt to point
    to the font usage yourself.

    Much shorter. Still it was a very good job, and fine effort, as
    that is all any of us should need.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-