On October 8, 2005 12:20 pm, John Nall wrote:
> Gerhard Fiedler wrote:
> >> John, did you check what the spec says about the temp coeff of
> >> your
> >
> >resistors? Many are spec'ed with +- ranges, but some are spec'ed
> > with 0...+ ranges, for example 0...350 ppm/K (pardon me the use of
> > a SI unit here :)
> >
> >350 ppm/K would be 3.5% over 100 K -- positive. In that case, it
> > would make sense that they are a bit below center at room
> > temperature; at least if we assume that the target temperature
> > range is mostly above room temperature.
>
> No, I did not check that.  Makes sense, though.  I feel sure there is
> a rational explanation.  Although I am going to check the calibration
> of my ohmmeter, I really do not think that it is out of spec because
> all three of them give the same readings.  Which does not mean that
> they are not all reading low, of course, but it decreases the
> probability.   At any rate, I do not want to make a federal case out
> of it -- just wanted to share what I thought was an interesting
> observation.  (I try to share such interesting observations with my
> wife, but she has developed this annoying habit of using the remote
> control to turn up the volume of the TV whenever I try and share such
> things with her.  :-(

FYI, with some wrist watches, the crystal is calibrated to run about the 
right speed while the watch is on your arm, or about 27degC versus 
running at room temp.
You may have hit the odd statistic where all your resistors are indeed 
low, but, like Gerhard is describing, you may want to figure out what 
it would read if you got running hot, let's say it's a 1/4 watt 
resistor, so maybe run try it at 1/8watt and try reading the temp 
afterwards...
1/8watt is V=sqrt(wr) = sqrt(1/8watt x 1000ohm) = 11v
So, if it's a 1/4 resistor, see what happens after you run it at 12v for 
a moment.

cheers!


> John

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