ACR Electronics SmartReader Plus Instrukcja Użytkownika Pobierz pdf (Strona 58)

SmartReader Data Loggers

Figure 7-10: Regulating Battery Supply Voltage

Make sure that your supply voltage to your regulator is at least two volts higher than the level

to which you want to regulate to. To do this, you can easily increase the voltage of your

batteries by hooking them up in series as shown in Figure 7-10.

If the wires between the voltage regulator, batteries, and transducers are long (more than a

few inches), the regulator may oscillate and produce an unstable voltage. You can correct for

this by connecting one 0.1 microfarad capacitor to each of the regulator's outside pins then

wiring them to the regulator's center pin. Make sure to keep the capacitor leads as short as

possible.

1.9.6 Selecting Transducers and Transmitters

A transducer is defined as a device that receives energy from one system and retransmits it, in

a different form, to another system. A transmitter is a term usually reserved for transducers in

a current loop circuit. In this section, the terms transducer and transmitter will be used

interchangeably.

For a transducer to be useful, the retransmitted signal must be compatible with standard

instrumentation. For the SmartReader 7 Logger, this can be either in the form of an analog

DC voltage or current.

Many compatible transducers exist for measuring such variables as temperature, relative

humidity, pressure, speed, PH, and more. You can obtain these from a wide variety of

manufacturers or distributors. An extensive listing of sensors and companies is published

yearly by Sensors Magazine.

To figure out what to look for when choosing transducers, it is helpful to know a few technical

terms and how they relate to use with your SmartReader 7 logger.

Excitation Voltage

This term refers to the input voltage that a transducer requires in order for it to work properly.

It is either specified as a range (for example, 9 to 30 volts DC) or as a specific voltage (for

example, 10 volts DC). When a range is specified, it means you can use an unregulated

power supply (such as a battery) as long as voltages within the range are provided. Usually, if

a specific voltage is required, it means you require a regulated power supply.

When an unregulated power supply is specified for the excitation voltage, a regulated one may

also be used. The reverse, however, is not true. Millivolt pressure transducers, for example,

usually require regulated power supplies.

Maximum Impedance

An important consideration when choosing transducers is the requirement for input or loop

impedance. This simply refers to how much resistance your instrumentation (for example, the

SmartReader 7) can have before it will begin to cause problems. For voltage inputs, a very

high impedance is usually required (usually greater than 10k ohms). This prevents the

unnecessary draining of current from the transducer circuit. The SmartReader 7's voltage

inputs are all high impedance (greater than 20k ohms).

For current loop inputs, it is advantageous to have a very low impedance (the SmartReader 7's

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ACR Electronics SmartReader Plus Instrukcja Użytkownika Strona 58