Current Readings with the vb Instrument

The information provided is for help with Commtest Instruments products:

• vb instrument

QUESTION

Can I take current readings with my vb instrument?

ANSWER

Yes. vb3000 instruments using firmware version 4.30 and higher can accept current sensors as well as other sensor types.

Other vb instruments such as the vb2000, vb1000 and any vb3000 instruments running firmware versions prior to version 4.30 only have an accelerometer input. However, they can be fooled into taking current measurements. You will need a clamp style ammeter with a voltage output suitable for plugging into a multi-meter, and a cable to allow connection from the ammeter to the BNC connector on the vb instrument.

Setup

• From the Set Parameters Menu set the Mode to acceleration.
• Turn drive current OFF and adjust the sensitivity to match the ammeter output. To do this select option [9] from the Set Parameters Menu. This will display the Sensor Setup Menu where you can adjust the sensor settings.

When using an accelerometer to measure velocity or displacement the instrument must perform a transformation on the input signal. When measuring acceleration no signal integration is required so the displayed chart is a good representation of the input signal.

The drive current is used to drive the accelerometers but will not be needed by the ammeter. Although unlikely, the drive current could cause false readings or even damage to the ammeter if left on. Typically these meters give an output of 10 mV/A. By setting the accelerometer sensitivity to 10 mV/g there will be a direct ratio of mV to g. The display on the vb instrument will read 'g' instead of 'A', but the values will be correct. At 50 or 60 Hz line frequencies the instrument will be able to read up to approximately 35 A rms with these settings.

Some ammeters give an output of 1 mV/A. In this case you should still set the accelerometer sensitivity to 10 mV/g. Now each 'g' displayed on the vb screen will represent 10 A. For example 0.10 g = 1 A. The instrument is able to measure up to approximately 350 A rms with these settings.

Most current readings are measured in rms so you should also set this to be able to compare readings with other ammeters.

The above description can be used with other voltage-output devices, subject to the following restrictions:

• The input to the vb instrument is AC coupled - it will only read down to 1 Hz.
• The maximum voltage swing permissible on the instrument input is 1 V peak-peak at 50 to 60 Hz. This increases to 16 V peak-peak at 1 kHz.

FAQ ID: 11644 Last Reviewed: 01/11/2005