Measurement Computing CIO-DI48 - Manual (Page 20)

7.2 TTL TO SOLID STATE RELAYS
Many applications require digital monitoring of fairly high AC and DC input voltages.
These AC and high DC voltages cannot be read directly by the CIO-DI##.
Solid State Relays, such as those available from Measurement Computing Corporation
allow control and monitoring of AC and high DC voltages and provide 400V
isolation. So lid State Relays (SSRs) are the reco mmended method of interfacing to
AC and high DC signals.
The most convenient way to use solid state relay s and a CIO-DI## board is to
purchase a Solid State Relay Rack. SSR Racks are available from Measurement
Computing Corporation.
7.3 VOLTAGE DIVIDERS
If you wish to measure a signal which varies over a range greater than the input range
of a digital input, a voltage divider can drop the voltage of the input signal to the level
the digital input can measure.
A voltage divider takes advantage of Ohm's law, which states,
Voltage = Current * Resistance
and Kirkoff's voltage law which states,
The sum of the voltage drops around a circuit will be equal to the voltage
drop for the entire circuit.
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.
A voltage divider takes advantage of the fact that the voltage across on e of the
resistors in a ci rcuit is proportional to the voltage across the total resistance in the
circuit.
The trick to using a voltage divider is to choose two resistors with the proper
proportions relative to the full scale of the digital input and the maximum signal
voltage.
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