Measurement Computing PC104-DI48 - Manual (Page 13)

7.2 TTL TO SOLID STATE RELAYS
Many applications require digital inputs monitor AC and DC voltages. High AC and
DC voltages cannot be applied directly by the TTL digital lines.
Solid State Relays, such as those available from Measurement Computing Corp. allow
control and monitoring of high AC and DC voltages and provide 400V isolation.
Solid State Relays (SSRs) are recommended for interfacing to high voltages.
The most convenient way to use solid state relays is to purchase a Solid State Relay
Rack. SSR Racks are available from Measurement Computing Corp.
7.3 VOLTAGE DIVIDERS
An alternative method of measuring a signal which varies over a range greater than the
input range of a digital input, is to use a voltage divider. When correctly designed, it
can drop the voltage of the input signal to a safe level the digital input can accept.
Ohm's law states:
Voltage = Current x Resistance
Kirkoff's law states:
The sum of the voltage drops around a circuit will be equal to the voltage
drop for the entire circuit.
In a voltage divider, the voltage across one resistor in a series circuit is proportional to
the total resistance divided by the one resistor (see formula below).
The object in a voltage divider is to choose two resistors having the proportions of
the maximum voltage of the input signal to the maximum allowed input voltage.
The formula for attenuation is:
For a given attenuation, pick a resistor and call it
R2, the use this formula to calculate R1. R1 = (A-1) x R2
For example, if the signal varies between 0 and 20
volts and you wish to measure that with an analog
input with a full scale range of 0 to 10 volts, the
attenuation (A) is 2:1 or just 2.
2 = 10K + 10K
10K
Attenuation = R1 + R2
R2
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