Re: EOL's

[Nomen Nescio <nobody@xxxxxxxxx>]

Doug L. said:

>The bottom line is that the value of a resistor can change over a period of
>time, even someone as obtuse as RLB should be able to understand that if you
>start with a resistor that is 20% off its nominal value, it will take less
>of a change to put its value outside the tolerance range of the control
>panel than a resistor that starts off within 5% of its nominal value. This
>in a nutshell is what I believe Al originally meant in the first place.

Doug has hit the nail on the head.  Most of the comments have been about
the tolerance of the resistor, but what really counts is the tolerance of
the control panel.  UL used to require an alarm if the flow of current in a
protection circuit increased or decreased by 50%, a carryover from the old
direct wire days, no doubt.  Putting this in terms of EOLs, a variation of
20% from nominal values should not cause a problem.  Obviously, this will
depend on the design parameters of the control panel circuitry.  Anyone who
really cares can connect a potentiometer across the zone terminals of a
panel and determine the values at which the zone goes into trouble or
alarm.

A little bit of internet research will convince even the most inexperienced
technician that carbon resistors can change value over time, and that
carbon resistors are more likely to do this than carbon film resistors or
other types of resistor construction.

However, the tolerance marking on a resistor is not a lifetime guarantee
that the resistor will never change by more than the designated amount.  It
is indeed a manufacturing tolerance, a representation by the manufacturer
that the actual resistance will not vary by more than the designated
tolerance from the marked value -- at the time of manufacture.  The
resistor manufacturer obviously isn't going to guarantee that the resistor
will never, ever change by more than 5%, 10%, or whatever.

- badenov