Measurement Computing PCI-DIO48H/CTR15 - Manual (Page 23)

SSRs. If you have only a few outputs to control, you may also wish to consider the
DR-OAC or DR-ODC, single point, DIN mountable SSRs. The high current outputs
of the PCI-DIO48H/CTR15 board are suitable to drive these SSRs directly.
7.3 VOLTAGE DIVIDERS
If you need to detect a signal which varies over a range greater than the maximum
input specification of a digital input, you must use a voltage divider or some other
external device to reduce the voltage of the input signal to a safe level.
Ohm's law states,
Voltage = Current * Resistance
Implied in the above is that any variation in the voltage drop for the circuit as a whole
will have a proportional variation
in all the voltage drops in the
circuit.
In a voltage divider, the voltage
across one of the resistors in a
circuit is proportional to the
resistance to the total resistance in
the circuit.
The object in using a voltage
divider is to choose two resistors
with the proper proportions relative
to the full value of the input voltage
to the desired output voltage to the
board input (Figure 7-2).
Figure 7-2. Voltage Divider Schematic
Dropping the voltage proportionally is called attenuation. The formula for
attenuation is:
For example, if the field voltage varies
between 0 and 10 volts and you wish to
detect that with a maximum board input
voltage of 5 volts, the Attenuation must
be 2:1 or simply 2.
2 = 10K+10K
10K
The variable Attenuation is the
proportional difference between the
desired output voltage (max. input
board input voltage) and the full input
voltage from the field device.
Attenuation = R1+R2
R2
19
Signal
High
Signal
Volts
Signal
Low
Vin
R1
R2
V1
V2
Vout
Board
Input
Groundt
SIMPLE VOLTAGE DIVIDER - Vin = R1+R2
Vout R2