Re: A Quick Question

"Mark Olson" <olsonm@xxxxxxxxxxxx> wrote in message news:DeOdnWqT5ZaTsc3VnZ2dnUVZ_q3inZ2d@xxxxxxxxxxxxxx
DUH. I would use a couple of relatively low value resistors
(hundreds, but not 10s of Ohms) hooked up to a switched voltage
source so that the sensed voltage is a stable fraction of the
battery voltage. Without having measured it, my guess is that
the input impedance of the sensor is relatively high (10s of K
Ohms at least). But I would do the pot thing just to find out
where the trip points were and set my divider so that I was
right in the middle of the safe zone (with the sensor hooked
up, naturally, so there's no error from input bias) to minimize
the chance of a false error light. You can certainly do that
with discrete resistors or a decade box but it's not going to
be as quick as using a pot.

Your assumption about impedance is probably valid.

My fear (and the reason I mentioned Mr. Thevenin) is that someone would, for example, use a 100K pot to approximately center the input between the trip points, then remove the pot, measure the relative resistances, and build a seemingly equivalent arrangement using 1K resistors. If there is a bit of current flowing into the electronics, you can't directly translate the pot values into the ratio of resistors to use unless the resistors sum to the value of the pot.

As I kind of said earlier, two 10K resistors don't make the same voltage divider as two 1K resistors.

And finally, to complete my offensive tutorial that you don't benefit from (but other's might):

http://en.wikipedia.org/wiki/Voltage_divider#Loading_effect

.

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