Hello again,
Just be careful if you use a pot instead of two resistors because you can
turn the pot up too high and blow out the logger input.
Here's an ascii representation:
Code:
[FONT=Courier New]0 to 150v in o------+
|
|
RTop
|
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+------o Logger input
|
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RBottom
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Ground o------------+------o Logger ground
[/FONT]
The formula for the input/output relationship is:
Vout=Vin*(RBottom)/(RTop+RBottom)
where
Vin is the 0 to 150v input
and
Vout is the output to the logger.
The power in any resistor will be:
P=V*V/R
and a good design rule is to use a resistor rated for twice
the power as calculated above, or:
Pr=2*V*V/R
With a max input of 150v the upper resistor will see
120v with a 5 to 1 divide ratio, and the lower
resistor will see a max of 30v. If you want to use
1/2 watt resistors then you should keep RTop above
57.6k, and keep RBottom above 3.6k.
With RTop=100k and RBottom=25k this works out pretty
well.
If you want to go with a 10 to 1 ratio, then you can use
either 100k and 11.1k, or 90k and 10k resistors for RTop
and RBottom respectively.
If you have a little noise in the system a small capacitor
across the lower (RBottom) resistor helps too when
measuring DC voltages. A 0.1uf is good, up to around
10uf if your measurements dont have to be really fast.
Voltage rating for the cap should be 50vdc.
The repeatability will be very good.