Resistors with a value of zero ?

Discussion in 'General Electronics Chat' started by deefactorial, Oct 17, 2008.

  1. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
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    I have a schematic diagram from a demo board for a microchip and it has resistors with values NL and 0 what do those values mean ?

    If I look on the physical board the ones with NL values are missing. and the Resistors with 0 are marked 000 on them. Is there actually resistors with zero resistance and why would you have a resistor with zero resistance ?

    I changed the PCB I have been modifying; the mods I made are:

    I get the power from the bulk caps now, so the ground plane is connected at two places in the power chain. The ground plane is connected at the power connector and at the end of the bulk caps for the 5.0v and the 3.3v. The power plane is connected at the end of the 3.3v Bulk caps on the positive side. the positive 5.0v power goes to the jumpers where it is selectable to power the max3232e chip with either 5.0v or 3.3v so the user selects where the jumpers go depending on the application.

    If a via is a pin-length away from the pin is that far enough away ? is there a rule of thumb for this sort of thing?

    Any last comments on the board before I get it sent away ?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    I think you will find that NL stands for "Not Loaded". In other words these components are not installed on the board.

    hgmjr
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A zero ohm resistor is used as a jumper and/or a placeholder in case a non-zero value is needed on a future modification.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Sometimes a zero-Ohm jumper is necessary to provide a bridge between apparently conflicting signals like V+, Vcc and +5V on a board, even though they may all mean the exact same thing.

    Component libraries are built by different people, who use different standards for supply nomenclature. It's a PITA for those who are tasked with developing boards - so rather than modifying the library models, they use a 0 Ohm bridge.

    It's a quick bandage for a big problem. It works. The problem is so big, it's almost too big to fix. However, 0 Ohm resistors - basically jumpers - fix the problem in the schematic and PWB so that they can get the thing into production as quickly as possible.

    If they tried to change the library models, they would create far larger problems than they would fix.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    But a jumper between two planes can be a big problem for high speed PC boards. Power and ground planes act as the reference (return) planes for high speed signal traces, which become transmission lines. In this case, the integrity of the reference plane is vital. A transmission line passing over a break in a plane causes the return current to take a detour, creating a large loop inductance with accompanying reflections, crosstalk, EMI generation, etc. If there are only a few isolated transmission lines, the problem can be handled by jumpers, but it the case of high-speed buses, The only choice may be to edit the library.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    You present points that I'm painfully aware of. We have no disagreement.

    However, if the library is modified, there is no telling what "prior art" may require retrofit.

    This is a configuration specialist's nightmare. Darned if they do, darned if they don't.

    They can't stop new models from coming into the library, because that would halt progress. However, if the new models come in... well, they may not conform to the existing models, or to the in-house standards. So then the engineers have to make compromises - with the zero-ohm jumpers.

    It's not pretty. I don't have a good solution.

    If you do, then I'm all ears.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I didn't intend to sound contentious. I was simply pointing out potential problems.
    I don't have a solution, and I also don't claim to be an expert on signal integrity (SI), since I was basically the silicon design point man between the IC design group and the SI group, who were primarily RF and EMI engineers. I did osmotically absorb a fair amount of SI knowledge in the 8 years I worked there.
    I don't recall any library issues with Vcc, etc., but I did not personally work closely with any PC board layout group.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Neither was I trying to stir the "proverbial pot."

    SI is a real problem. So is library nomenclature.

    I'm not raining on anyone's parade - just attempting to increase awareness.

    Those who are tasked with board layouts may have to make some tough choices - and 0 Ohm jumpers may be their best bet. Without 'em, they couldn't get their boards past inspection, electronic or otherwise.

    Hey, there actually is a world out there. ;)
     
  9. raybo

    Member

    Oct 18, 2008
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    I absolutely agree nomecleture is a big problem. Plus manufactures in house numbers and our goverment adding misterious numbers to common components. but there are good reasons for it. It just makes harder for us. On a final note PCB layout is a field by itself. There is no need to know electronics to lay it out just experience.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    I will agree about experience being enough to let you do a layout that makes all the connections. However, knowing electronics is necessary when working with high frequency layouts. As one example, it will make the difference between a working RF transmitter and a collection of parts that won't tune or produce an output. As another, electronics knowledge will make the difference between a board handling a small signal (microvolts and/or nanoamps) and producing a clean output and one that has noise problems.
     
  11. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
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    Then am I to assume that those 0 Ohm resistors are mere place holders for a trace, and a direct replacement of a trace line would provide a better connection.

    I've attached a schematic diagram of the demo board. It looks to me like they had an option to chose which path they wanted to power the chip through and picked one.

    I must say though for a demo board they do not explain this with clarity.
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    Ok, look at R15 and R11. R11 is present and places 3.3 volts on the pin marked "ENVREG", or regulated voltage enable. If it was not present (NL), then R15 would be there as 0 ohms, and disable the function. R9 and R16 work the same way.
     
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