Amphenol Wilcoxon TN14 Dépannage des installations - Page 2

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Figure 1 is a diagram representing the performance

of the circuit. The line represented by "Instrument

Power" and BOV is one conductor that has two

functions. So even though the power supply is pro-

viding a higher input voltage, the BOV is the mea-

sured output voltage level on the cable connecting

the accelerometer to the data collector or analyzer.

The BOV carries the dynamic vibration signal to the

analyzer. The AC signal swings high and low from

the BOV level and is limited by the power supply

level and ground. For example, if the power supply

level is 24 volts, the swing of the AC signal would

be limited to no more than 24 volts and no less than

ground (0 volts). These are theoretical limits. In re-

ality, the limits to this swing occur at about 1.5 volts

above ground and about 1.5 volts below the power

supply level, as shown in figure 2.

Most portable data collectors and online systems

supply 24 volts to the sensor. The sensor should

have a nominal BOV of approximately one half of

the power supply voltage, to maximize the amount

of swing in the positive and negative directions.

Most two-wire sensors produce an 8 to 14 volt bias.

When the signal amplitude runs into the supply voltage or ground, clipping occurs. Clipping the vibration signal

distorts the waveform. In other words, a clipped signal is no longer a true analog representation of the vibration

the sensor is attempting to measure.

Measuring the BOV

The constant current diode (CCD) limits the current supplied to the sensor. It provides a constant current to the

sensor regardless of the supply voltage because the use of unlimited power supply current will damage most

internally amplified sensors. For this reason, most commercially available data collectors and vibration moni-

tors have power supply circuits that include a CCD to regulate the power supplied to the sensor.

Figure 3: Schematic of a sensor power

Figure 3: Schematic of a sensor power supply

supply containing a CCD

Figure 1: Sensor amplifier

Figure 1: Sensor amplifier circuits

circuit

Figure 2: AC signal limits

Most battery power supplies contain a 2 mA CCD to ensure

long battery life. Line powered supplies (where power con-

sumption is not a concern) should contain 6 to 10 mA CCDs.

This offers the ability to drive long cables. For operation above

100° C, it is best to limit the current to less than 6 mA to re-

duce self heating. Most data collectors supply 2 mA of cur-

rent to the sensor; most online systems supply 4 to 6 mA. If a

power supply that is not current limited is used, a CCD should

be placed in series with the voltage output of the supply. En-

sure that proper diode polarity is observed. If a current limited

power supply is probed with a voltmeter the supply voltage will

be measured before the CCD. The bias voltage will be mea-

sured on the side of the CCD connected to the sensor. Figure

3 shows a schematic of a sensor power supply containing a

fixed CCD between 2 and 10 mA.

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