ACR Electronics SmartReader Plus Bedienungsanleitung PDF herunterladen (Seite 53)

SmartReader Data Logger

25 milliamp Channel

The 25 milliamp channel can be modified to span higher ranges (greater than 25mA DC) by

also adding a single resistor across the input terminals as shown in Figure 7-6. The value of

the resistor (R) can be calculated by the following formula, based on the desired current input

range (Iin) in mA:

R = 2,500 / (Iin - 25)

Make sure the resistor can withstand at least twice the power you will be sending it. To

calculate this, use the following equation:

RI2P

´´=

Where:

P = Minimum Power Rating of resistor (in watts)

Iini= Maximum amperage expected through resistor (in Amps)

R = Resistor value (in ohms)

For example, a 50 ohm resistor calculated to measure currents up to 75mA will require a

power rating greater or equal to 0.56 watts. Therefore, a one watt resistor will be fine.

Figure 7-6: Modifying a 25mA Channel

For increased sensitivity at lower current ranges down to 2.5 milliamps, refer to the section for

modifying the 200 mV channel.

Resistors

Resistors that can be used with the SmartReader 7 are readily available at most electronic

parts supply stores. Ask for precision 1% metal film resistors with a temperature coefficient of

less than 500ppm. The most common ones are quite small and made in the form of a cylinder

with axial leads. To show the value of the resistor, either there is:

· a color code painted on each resistor in the form of bands

· a number (followed by a letter) stamped on them

For resistors with a tolerance of 2% or looser, there are four bands or three number digits

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ACR Electronics SmartReader Plus Bedienungsanleitung Seite 53