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Resistor rating

Discussion in 'Electronic Design' started by Tom Sanders, Feb 12, 2004.

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  1. Tom Sanders

    Tom Sanders Guest

    Hello,

    I am trying to put together a circuit on a breadboard and am not sure about the
    resistor ratings. I am using a two 10Mohm resistors in series with a 500V
    supply. The power ratings of the resistors are 0.25V. Is this circuit safe
    to build? The circuit would be drawing approximately 25uA of current.

    What does the voltage rating on a resistor mean?
     
  2. Ben Bradley

    Ben Bradley Guest

    In sci.electronics.design, (Tom Sanders)
    wrote:
    That would probably be 0.25W, or 0.25 watts.
    As long as you don't touch it...

    Seriously (not that I'm not serious above!), it appears you want to
    calculate the power dissipated in the resistors and compare it with
    their rated value.
    Let's see, E=I*R, 25uA * 10megohm is 250V, so two such in series is
    500V. You got the current right.
    Power (in watts) is current (in amperes) times voltage (in volts,
    of course) or P=I*E. For one resistor, that's 250V*25uA = 0.00625
    watts, which is well below the rated 0.25 watts.
    That's generally a maximum safe voltage across a resistor without
    the voltage arcing through. This is a maximum voltage even if the
    resistor is well within its power rating at that voltage.
     
  3. The power dissipated in a resistor is V^2/R. A 10 Mohm resistor
    across a 500V supply would give 500 * 500 / 10,000,000 or 0.025W. So
    a 1/4 W resistor is perfectly fine. Resistors are typically rated in
    Watts, not Volts. Maybe you're misreading them.

    --Jeff
     
  4. CFoley1064

    CFoley1064 Guest

    From: (Tom Sanders)
    Hi, Tom. If you're still with us (respond at your convenience ;-) you're over
    voltage on the breadboard, and you're probably right on the edge with the
    resistors. I wasn't able to find any information on 3M or Global protoboards.
    Try calling or emailing them tomorrow. 300V peak sounds like a maximum -- it
    might be less. I've never trusted solderless breadboards with more than 120
    VAC or VDC myself.

    Resistors have maximum voltage ratings, which usually refer to how much voltage
    can be applied across a resistor before it arcs over the body of the resistor.
    Some 1/4W are rated for 250V, some less.

    If I were putting together a 500VDC prototype circuit, I'd use a bare perfboard
    with point-to-point wiring. I'd also use 3 ea. 6.8 Meg resistors or 4 ea 4.7
    Meg in series.

    Good luck
    Chris
     
  5. At 250V/resistor that's within the voltage rating of *some* 1/4-W
    resistors. I've seen some major long-term drift in precision
    metal-film resistors exposed to high DC voltage, so if this is a
    serious application, care is called for- study the manufacturer's data
    carefully.

    Best regards,
    Spehro Pefhany
     
  6. I read in sci.electronics.design that Jeffrey Turner
    No, you are misleading. Resistors do indeed have a voltage rating, which
    is the maximum voltage that should be applied, **irrespective of power
    dissipation**. Non-SMD resistors have a voltage rating of 250 V,
    typically. SOME higher-power resistors have a higher rating, such as 350
    V, and special (small, not costly) resistors can be obtained with
    voltage ratings of several kilovolts
     
  7. Robert Baer

    Robert Baer Guest

    A 1206 SMD resistor has a 200V rating, the smaller sizes are less.
    1% metal film resistors have a 300V rating, and 5% metal oxide film
    (power) resistors have a 350V rating.
    Resistors have a voltage rating, a current rating, and a power rating;
    usually the current rating is not specified, except that a temperature
    derating curve (for current) is given.
     
  8. Robert Baer

    Robert Baer Guest

    *WRONG*
    Resistors *do* have a voltage rating; look in a DigiKey catalog as a
    quick refeernce; else look at the manufacturer's data sheets.
     
  9. Robert Baer

    Robert Baer Guest

    Solderless breadboards should bwe banned; they are utter crap: too
    much capacitive coupling between buses, too much inductance, solder and
    other bits of metal gets into the works and can cause nasty shorts, etc.
    A "jungle jim" can be superior in a number of ways; using proto boards
    with power buses and lands for parts is better, and wire-wrap (if DIPs
    are used) can be better yet, as X and Y busbars can be used along with
    appropiately placed bypass caps - giving results close to good PCB
    layout practice.
     
  10. I read in sci.electronics.design that Spehro Pefhany <[email protected]?>
    It's a very common problem with 'start-up' resistors in SMPS, which
    typically have 330 - 340 V DC applied. Even two 250 V rated resistors in
    series are not reliable. It's an application where a special part is
    needed, like a metal glaze resistor of lower value than those normally
    made, with kilovolt ratings.
     
  11. I read in sci.electronics.design that Robert Baer
    Not all of them. Check data sheets.
     
  12. Jeff

    Jeff Guest

    Scraping off the paint/ceramic coating on such failed carbon or metal film
    resistors shows the resistor is in good shape, except for a small section of
    the resistive coating which looks like it corroded away. It seems like what
    happens is a small section overheats, and looses some material, creating an
    even hotter hot spot, which gets hotter, evaporating more conductive
    coating, getting hotter, etc until the resistor fails.
     
  13. Jim Meyer

    Jim Meyer Guest

    Exactly! Resistors are not conductors like wires and they are
    not insulators like ceramic. Resistors are semiconductors and like
    all semiconductors they have voltage ratings because their
    characteristic resistance WILL become nonlinear when their voltage
    rating is exceeded.

    Jim
     
  14. Resistors' "power ratings" are in Watts, not Volts. If you have a
    resistor which is no good above 0.25 Volts then, unless it is a very
    low resistance value, you should get rid of it and replace it with a
    good resistor.

    --Jeff
     
  15. Tom Sanders

    Tom Sanders Guest

    Thank you everybody for your great advice.

    I have certain followup questions:

    1. Somebody mentioned that a solderless breadboard may not work above
    120V.
    I contacted 3M but received no response about their breadboard specs.

    If I have a high voltage dc supply of 500V but two resistors of 10Mohm
    in
    series, the current being drawn would be low. Does the low current
    make it safe and prevent the metallic surface of the board from
    melting?
     
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