Current; Measuring Temperature - Arcom RC210 Hardware Manual

Current

Measuring current requires a circuit which develops a voltage proportional to current. This can easily be accomplished with a
small value current sensing resistor and a differential type operational amplifier and 4 resistors. The output of the op amp is
equal to the current times the value of the sensing resistor, times the voltage gain of the amplifier. The value of the sensing
resistor that should be used depends on the maximum load current, since the4 voltage drop across the resistor reduces the
voltage to the load. BE sure to calculate the worst caser power dissipation of the resistor (I²R) and use an appropriately rated
resistor.
Ideally, a power supply with remote sensing would be used, with the sense return after the sensing resistor. This way, the
voltage drop through the sensing resistor would be compensated for by the power supply.
The 4 resistors used in the op amp circuit should be 1% metal film types. Be sure that the common mode input voltage range of
the op amp will accommodate the operating voltages that result from the resistor/gain selection. For example, an LM324 or
LM358 op amp, operating at a single ended supply of +12 will operate properly with input voltages between 0 and 10 volts.
The following shows how to measure current

Measuring Temperature

It's a simple matter to measure temperature at the repeater site, using an LM335 Precision Temperature Sensor. The LM335 is
electrically like a zener diode with a precision temperature/voltage characteristic. It will provide a resolution of ±2 degrees.
There are spaces for installing resistors (R13 & R14) on the PC board that act as the reference voltage source when using an
LM355 on ADC channels 1 & 2. This allows you to eliminate some external parts by installing 4.7K resistors right on the board.
These resistors are not provided but if you install them, there are only 2 connections between the sensor and RC210 necessary.
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