Extremely Low Resistor Values?

Discussion in 'General Electronics Chat' started by mossman, Feb 19, 2014.

  1. mossman

    Thread Starter Member

    Aug 26, 2010
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    I'm reverse-engineering a small circuit board and there are several extremely low value resistors. A couple are 4.7 ohm and there is a 1.9 ohm and a 1 ohm as well. A couple of them are connected from a microprocessor pin to ground. What is the purpose of these low value resistors and aren't they essentially a short circuit? FYI, I know for a fact that they are resistors and not inductors.
     
  2. nigelwright7557

    Senior Member

    May 10, 2008
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    I usually use small value resistors for measuring current.
     
  3. #12

    Expert

    Nov 30, 2010
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    No, they are not essentially a short circuit. 4 example, a row of NPN power transistors acting as the output of a power supply often use less than 1 ohm each to get the transistors to share the current more equally.
     
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  4. mossman

    Thread Starter Member

    Aug 26, 2010
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    Okay, but why would a microprocessor use several low value external resistors? Why not make them internal to the processor IC? Is it to dissipate heat since they are such low values?
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    What microprocessor and which pin?
     
  6. ramancini8

    Member

    Jul 18, 2012
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    Low value resistors are usually high power resistors, and they don't fit well in ICs. Mouser or Digi-Key carries them.
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    low value resistors are becoming more common, a 1 meg ohm resistor dosnt pass much current at 3.3 volts or 5 volts.
     
  8. mossman

    Thread Starter Member

    Aug 26, 2010
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    The processor is made by Samsung and there is no datasheet available so I don't know the pinout. As an example, there is a 0.6 ohm resistor connected in series between two pins in one location and a 1.8 ohm resistor connected from another pin to ground. I'll assume they are power resistors then.
     
  9. Veracohr

    Well-Known Member

    Jan 3, 2011
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    You really shouldn't assume what role the resistors have without knowing what they're connected to.
     
  10. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    They are most likely not "power" resistors. They are more than likely current sense resistors. But without a schematic and datasheets it is only a guess.

    Good luck with reverse engineering a product that you know nothing about.
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    They could be there for a number of reasons. Perhaps the pins in question need to be logically tied to ground and the choice was made to tie they through resistors for current limiting or noise considerations or who knows what else.

    Perhaps they are biasing resistors for some analog circuitry that is part of the chip.

    Why are you trying to find out?
     
  12. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    See first post. OP is trying to reverse engineer a small circuit board containing a chip to which the OP has no datasheet or schematic.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    Ah. Yes. I guess I tuned that out as my first reaction was, "Hey, I've got a board with some components on it. Can anyone tell me what they do?"
     
  14. mossman

    Thread Starter Member

    Aug 26, 2010
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    I apologize for being vague, but I have no choice but to be vague. I was just looking for a general answer as to what these extremely low value SMD resistors are typically used for.
     
  15. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I am not complaining about 'vague'.

    But it is in your interests to supply all the information you can.

    So what did this board come out of?
    Was it a preprogrammed washing machine controller?

    Do you have any idea whether this is a 4 bit, 8 bit, 16 bit micro or what?
    How many pins does it have?
    What does it look like?
    Size, pins to a side?
    Markings?
    etc etc
     
  16. wayneh

    Expert

    Sep 9, 2010
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    And let's not miss the obvious: How sure are you that you are reading the resistances properly? A photo might help folks see something you are missing.
     
  17. mossman

    Thread Starter Member

    Aug 26, 2010
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    I'm measured them using a high precision instrument (LCR meter). They are not inductors.
     
  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Could you post a photo or two? Basically all smd resistors I have ever seen have a value written on them. Could the die of the chip in question be fried and thus skewing your measurement? Have you taken some off the circuit and measured them alone?

    And more importantly, what was the purpose of that board, or where did it come from?
     
  19. mossman

    Thread Starter Member

    Aug 26, 2010
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    Again, I can't provide any details. Sorry. There are no markings. Even if they did have markings, they are so tiny that you wouldn't be able to read them without a microscope. They have all been removed from the board and measured using an LCR meter. I already know the values. I'm just curious as to what the purpose is, in general, for such low value resistors in a microprocessor application.
     
  20. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Well, if you can´t provide details, then you should not be asking questions.
    If you cannot say what the device does or any other detail apart from the fact that you found a few low value resistors, then this thread is completly pointless and you might as well close it.
     
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