Zero-ohm link

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Zero-ohm surface-mount resistors.

A zero-ohm link or zero-ohm resistor is a wire link used to connect traces on a printed circuit board that is packaged in the same physical package format as a resistor. This format allows it to be placed on the circuit board using the same automated equipment used to place other resistors, instead of requiring a separate machine to install a jumper or other wire.[1] Zero-ohm resistors may be packaged like cylindrical resistors, or like surface-mount resistors.

One use is to allow traces on the same side of a PCB to cross: one trace has a zero-ohm resistor while the second trace runs in between the leads of the resistor, avoiding contact with the first trace.

The resistance is only approximately zero; only a maximum (typically 10–50 mΩ) is specified.[2] A percentage tolerance would not make sense, as it would be specified as a percentage of the ideal value of zero ohms (which would always be zero), so it is not specified.

An axial-lead through-hole zero-ohm resistor is generally marked with a single black band,[3] the symbol for "0" in the resistor color code. Surface-mount resistors are generally marked with a single "0" or "000".

Design considerations[edit]

In practice, zero-ohm resistors can be useful as configuration jumpers, but caution is exercised for PCB board designs which may use zero-ohm resistors to select between options which require larger trace currents in the design. For these situations, it is better design practice to specify a specific low-ohm resistance, such as a 0.001-ohm to 0.003-ohm resistor, rather than the generic "zero-ohm" resistor in which the actual resistance may be higher, and tolerance is not given. The low ohm resistors are easily obtained with 5% or 1% tolerances on a maximum specified resistance, and can be safely utilized to pass much higher currents.

For example, a surface mounted 0805 size resistor of 0.003-ohms, rated at 1/2-watt can safely pass up to 12.9-Amps of current in theory. In practice, when approaching the wattage limit for a given package it is good practice to either use an even lower ohm product (more power efficient but more expensive), or go up one package size larger (same power efficiency but cost may be kept lower. The "cost" associated with taking up more board space for the larger package may also be a consideration. In this example, for 12 amps to pass through the jumper, a lower resistance, or a bigger surface mount package (such as a 1206) are usually specified. In contrast, a "worst-case zero ohm" real-world jumper impedance of 0.05-ohms, in a similar 0805 package could only pass 3.1-amps, maximum. The use of specific tolerance resistances is a much safer design practice for higher currents than the "zero-ohm" option, although the bill-of-materials cost can be higher for low-ohm devices.

References[edit]

  1. ^ Blackwell, Glenn R.; --, James K. Hollomon, Jr. (2006). Surface-mount technology for PC boards (2nd ed. -- ed.). Australia: Thomson Delmar Learning. p. 88. ISBN 978-1418000110. 
  2. ^ Archambeault, Bruce R.; Drewniak, James (2002). PCB Design for Real-World EMI Control. Springer. p. 168. ISBN 978-1402071300. 
  3. ^ Paynter, Robert T. ; Boydell, Toby (2008). Electronics Technology Fundamentals: Conventional Flow Version (3rd ed.). Prentice Hall. p. 50. ISBN 978-0135048740. 

See also[edit]