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Monday, May 1, 2017

Color Code Violation -- Almost as Bad as Breaking Ohm's Law!


A cautionary tale:   How would you read the color code on that little resistor?    It was plucked from a strip marked 2.2 ohms.   And indeed it is red-red-gold.  2.2 ohms, right?   But no,  my friends. On the DVM it reads 2.2 kilo ohms.  2200 ohms.  Believe me, that makes a big difference when it is in the Vcc line of your driver!  It took me a while to find out WHY that driver wasn't driving very well.  

This turned out to be one of those very satisfying trouble-shoots --  the problem was elusive and it wasn't all my fault.  But I should have MEASURED the resistor value before soldering it in. 

Here is the scary part:  The next resistor from the same strip was marked the same way, but measured 2.2 ohms.  Be careful out there. 

5 comments:

  1. This can happen when you get resistors from cheap overseas sources. It's also possible that it can happen with U.S. manufacturers, but generally only if the resistor changes value (not real likely when not in a circuit).

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  2. A 2.2 ohm resistor? over the years I've only seen one call for that value: Its the power input resistor for the 2N2219A driver stage in the current Bitx board from Ashhar.

    So, are you building a power amp using an IRF510?

    bob-N7SUR

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    Replies
    1. Good eye Bob. Yes indeed, I put the exceptionally stable BITX 40 Module amp circuit into my NE602 transceiver.

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  4. That's a twist on the "wrong bin" purity syndrome when grabbing loose bulk off the shelf. Off a strip is weird!

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