Value of resistor
  • Hello,
    Is this a problem if a resistor given to 68 ohm measured at 70 to the ohmmeter.?
  • It depends… Where is it used?

  • Is it Blue? Then it should read something between 67,32 and 68,68 – theoretically. Practically you most likely have a “cheap” Multimeter whose Accuracy is way worse that the Accuracy of the Resistor. And it might be influenced by the Batteries Power/Voltage/Freshness. SO you can’t tell if its a off Resistor or a bad MultiMeter….

    Thankfully the 68R is used as a Protection for the OpAmp if you accidentally short your Output so the actual Value isn’t that Critical. It would well work with 71 Ohms ;-)

    P.S.: The correct Value is about 0,082€ ;-)

  • Thanx,
    My multimeter is a FLUKE with new batterie !!! i think is not a bad multimeter, but the cables ?
    Lindeborg, it's the R3 resistor of the SMR4 filter board, so, fcd72 say The 68R is used as a protection for the OpAmp.
    Therefore i can used it ?
    For example, the R5 value is 220, and my multimeter display 221.3 !!!
  • Yes, you can use. If it were 10 ohm, or 200 ohm, it would work :)

  • Ok here we go again
    In fact, pichenettes, i receive the LT1054 CI today, thanx
  • Even a FLUKE is a “Cheap” Multimeter compared to the Precision with which Resistors are made nowadays. Certainly its not Bad – Heck, i wish i had one instead of the cheap MrVoltcraft I’m am using – but its bound to the same Physical limitations. A quick glance a Reichelt tells me a Basic Fluke measures Resistance with ±0,9%. If the Resistor is 1% up (True Value 68,68R) then the Fluke could easily show 69,29R and everything is within Tolerance. If you peek deeper you will have to understand that the 1% Tolerance for the Resistor is an average Tolerance (so its a certain percentage out of the whole batch will be inside the 1%, there might as well be some outside) and the actual Resistance is a Function of the Temperature etc pp…. So many Things might add up to an Error that reads +2R for this particular Resistor.

    As with every Measuring equipment its as important to know how and what and with which precision you are measuring as the actual reading… this to be told by an Engineer ;-)

    The Purpose of this Resistor is to limit the Current the OpAmp delivers when tied to GND within the Limits of what the OpAmp can handle at max without being destroyed. With 68R you could solder a Bridge to GND and the only thing happen would be the OpAmp getting a bit Warm – for a momentary Short you could go without a Resistor (in fact many Devices do….), but Olivier likes to Design everything fail Save, Rugged, MilSpec and certified for the Dumbest possible User.

  • Thank you fcd72 for these precision
  • Now you Knowledge matches your fine measuring instruments ;-)