Andover Controls UI-8-10 - Page 36

30 Andover Controls
Assuming you know the maximum voltage reading you are trying to measure (
V
source
),
perform the following steps to determine the resistance values necessary to condition the
input for the module.
1. First, determine the current that will flow through the divider. Pick a value that your
source can supply. Let’s say 20 mA. This value will be represented by the variable
I
total
in the following equations.
2. Determine the total resistance value required of the entire divider (R1 + R2) to create
20mA given the source voltage by solving the following:
R
total
= V
source
I
total
Example, if you are measuring a source that can supply 0- 30 V:
R
total
= 30 V
20 mA
R
total
= 30 V
.02 A
R
total
= 1500
Ω
The current is constant through the two resistors. Knowing this and the fact that our
input can’t exceed 5V (or 10V) can determine the value of the resistor labeled
R
2
.
3. Determine the value of
R
2
by solving the following:
R
2
= V
input
I
total
R
2
= 5 V
(assuming you are solving for 0-5V input)
.02 A
R
2
= 250
Ω
Through subtraction, you can determine the remaining resistor value:
R
total
= R
1
+ R
2
R
1
= 1500
Ω
- 250
Ω
R
1
= 1250
Ω
UI-8-10
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