ACR Electronics SmartReader Plus Manuel d'utilisateur télécharger pdf (Page 57)

SmartReader Data Logger

The load you impose on your battery will depend on how many transducers you intend to have

in your logging circuit. The greater the number of transducers, the greater the power draw

from the battery. If you are logging from a 4 to 20mA transducer, the maximum current it will

draw will be approximately 20mA. If you power it continuously, your 500 mAh battery should

not run out until you have had at least twenty-five hours of service. Since your transducer

probably will not always draw maximum current (12mA might be a more reasonable figure),

you can probably expect up to forty-two hours of service.

The battery-saving switch contacts can increase the lifetime of your external batteries

significantly. Since the batteries will be powered only for eight seconds out of every logging

interval, the power draw on the battery will be reduced considerably.

Example

Problem: You want to log pressure once every thirty minutes from a 0 to 300 psig transducer.

The output of the transducer is 4 to 20mA. The excitation voltage is listed at between 9 and

40 volts DC.

Solution: From this information we know that a 4 mA signal will represent 0 psig and a 20 mA

signal will represent 300 psig. Since the minimum excitation voltage is 9 volts, we should

supply at least 18 volts initially from our batteries. We can do this by simply connecting two

nine volt 500 mAh batteries in series.

Without the battery-saving switch enabled, our minimum expected battery life would be

approximately twenty-five hours. With the switch enabled, and thus closing only eight out of

every 1,800 seconds (or thirty minutes), the life will be extended by a factor of 225 (or 1800/8).

This means that we should be able to log from this transducer for over 5,625 hours, or over

234 days. Actual life may even be higher depending on what pressures are actually recorded

(the lower the pressure, the lower the power draw).

Regulated Voltages

The unstable voltage characteristics of batteries makes them, by themselves, incompatible for

use with transducers that require a regulated voltage supply. You can, however, add your own

voltage regulator to your transducer circuit to compensate for this instability (see Figure 7-10).

Voltage regulators are readily available at most electronic parts supply stores.

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ACR Electronics SmartReader Plus Manuel d'utilisateur Page 57