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Zero ohm resistors, why??

Discussion in 'Electronic Components' started by Sal Brisindi, Jun 23, 2004.

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  1. Sal Brisindi

    Sal Brisindi Guest

  2. They can be inserted by the same machines that insert resistors- note
    the tape. They are also easy to insert by hand if you buy the bulk
    kind, if you keep to the usual spacing. I designed the things into
    dozens of products..

    Of course the "1/4W" is an indirect reference to the body size.

    Best regards,
    Spehro Pefhany
     
  3. Dave Platt

    Dave Platt Guest

    Zero-ohm resistors are made in the same physical sizes as
    small-wattage non-zero-ohm resistors. They can be handled by
    automatic pick&place equipment, independent of the fact that they've
    got zero ohms inside. The bulk of the "resistor" body gives the
    automated equipment something to grab onto (usually using a vaccum
    hold, I believe).

    Wire jumpers would either require manual placement, or a different
    sort of automated-placement tool.

    I'd guess that the slight additional cost involved in purchasing "zero
    ohm resistor" parts, is usually more than compensated for by the fact
    that they don't have to be treated specially during automated assembly.
     
  4. I heard that "Mil spec" circuit boards have to lack wires jumping from
    one point to another on the board. My guess is that wires could get
    snagged while removing boards to service them or otherwise while servicing
    them.
    But there are no rules banning use of components on a circuit board.
    And if they say a zero ohm resistor is a wire, then they open the wormcan
    of saying a 1 ohm resistor is a wire if you use it where a zero ohm one
    would achieve essentialy the same results, and that a 100 or a 143 ohm
    resistor in series with a high impedance signal path at least in many
    cases is a wire, and how do they go about regulating usage of 100 and
    143 ohm resistors?

    At this point, I would guess that wires (or preferably cables) can only
    connect to a "mil spec" board via connectors...

    - Don Klipstein Jr. ()
     
  5. If the amount of current that results in dissipating 1/4 watt into the
    "resistor" does not significantly heat the copper foils that it is
    soldered to, then it appears to me that this is a safe current.

    Than again, in typical usage this is .7 inch of wire, and it appears to
    me that the leads of a 1/4 watt resistor are 24 AWG (or .5 mm dia.) tinned
    copper, resistance at 100 degrees C is approx. 1.8, maybe 1.9 milliohms...
    Current that results in 1/4 watt dissipation here is around 11-11.8 amps
    - keep in mind guidelines for what the foil traces can hangle - I suspect
    that will be the limiting factor.

    Also - power ratings of mil-spec resistors are often half that of
    commercial grade resistors of the same size. This limits a "1/4 watt"
    "zero" to somewhere around 8 amps (or the amount of current that the
    circuit board traces can handle, whichever is less).

    **********************

    Power rating of a resistor may depend on heatsinking effects of either
    the leads or *near-resistor-body* circuit board traces that are oversized
    for the amount of current actually being conducted. This may mean that a
    1/4 watt zero ohm resistor may only safely pass 8-11 amps when its ends
    are in close contact with conductors that don't have much of a temperature
    rise at this amount of current.

    Keep in mind that my "measurement" of the wire size of a "1/w4
    watt" resistor might be "off by a size", and that I am not especially
    certain that 1/4 watt zeros aren't made with AWG 26 (.4 mm dia.) wire. A
    ..7 inch length of that dissipates 1/4 watt at 100 degrees C at somewhere
    around 9-9.5 amps, and 1/8 watt at 100 degrees C at somewhere around
    6.25-6.7 amps.

    Also, unless you have circumstances unusually favorable to 24 gauge
    wire (such as heatsinking of a very short piece by something much
    thicker), the safe current carrying capacity will be closer to whatever
    the "norm" is for 24 gauge wire. I do not know this figure for sure, but
    I believe it is somewhere around 2 amps.
    (Transformer windings with AWG 24 may be safe only at much lower
    currents like around .5 amp due to a large assembly of adjacent turns all
    producing heat, heat from other windings, and heat from the core.)

    - Don Klipstein (Jr.) ()
     
  6. Guest

    Some other reasons:

    1) so you can measure current easily during development
    2) to allow the board to have optional current paths, in case there are
    other devices that must change and cause the wiring to change
    3) to allow you to connect two nodes together (Pwr Gnd and Analog Gnd)
    in a CAD system, and still allow full checking
     
  7. I also doubt they are carbon film....

    A few years ago, a listing in the Farnell catalogue for these read :

    Resistance : 0 ohms
    Tolerance : 5%
     
  8. Some kind of nickel alloy, IIRC, from the manufacturer's data. But so
    are low values of "carbon film" resistors!
    I design instruments and controls a lot- if only I could get a
    precision 1% version...

    Best regards,
    Spehro Pefhany
     
  9. On Wed, 23 Jun 2004 02:57:50 +0000 (UTC), the renowned
    W is a dissipation, not a current. Zero ohm resistors have a rating in
    current. You have to read the actual manufacturer's data sheet to find
    this, not some catalog hacked together by a distributor.

    The body has a thin layer of metal alloy over a ceramic core- it's NOT
    a wire staight through. The whole thing is made by resistor
    manufacturing equipment.

    Best regards,
    Spehro Pefhany
     
  10. Sal Brisindi

    Sal Brisindi Guest

    Thank you all for your input. Now I know why there is a zero ohm resistor.

    Regards,
    Sal Brisindi
    http://www.numitron.com
     
  11. Ken Finney

    Ken Finney Guest

    Milspec boards are allowed to have jumper wires. There is typically
    a limit based on the surface area of the board, but you almost never
    approach it, because after only a couple, it is cheaper to re-layout
    the board to incorporate the jumpers and/or use zero ohm chips.
     
  12. Boris Mohar

    Boris Mohar Guest

    You would still have to throw half of them out because they would have
    negative resistance.
     
  13. cpemma

    cpemma Guest

    I'd keep those for the perpetual motion machine. ;)
     
  14. John Miller

    John Miller Guest

    No, no, no.

    Those, you connect in series and use as a perpetual source of energy.

    --
    John Miller
    Email address: domain, n4vu.com; username, jsm

    "Die? I should say not, dear fellow. No Barrymore would allow such a
    conventional thing to happen to him."
    -John Barrymore's dying words
     
  15. Greysky

    Greysky Guest


    Now you've gone and done it. The Govt. was hoping that the secret to
    infinite energy would never leak out, because it turns out to be so bloody
    easy! Everyone who has participated in this particular discussion will
    disappear over time. The secret must never be allowed to be divulged....
    ;-(
     
  16. hotkey

    hotkey Guest

    insertion machines (robots) can handle resistors, not wire bridges
     
  17. Dave VanHorn

    Dave VanHorn Guest

    Good also for options, and points where you might need to measure current in
    a prototype, or tying grounds together at a single point.
     
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