Vernier Tech Info Library TIL #1139

Question

How can I measure currents outside the range of the Vernier Current Probe?

Answer

There are two basic approaches to consider:

1. Modify the DCP-BTA: It is fairly easy to modify the one we have to increase the range. You do this by just putting a 0.1-ohm resistor in parallel between the terminals around the outside of the box. This has the advantage of being visible and reminding the user that something is different about the sensor. The sensor then reads half the current it should read. The range is now 1.2 amps. You now need to make up a new calculated column of double voltage or, you could recalibrate and burn in the new calibration. The first option is far more simple.

You could put two 0.1-ohm resistors in parallel and make the reading one third and the range -1.8 to +1.8 amps.

2. Use our Instrumentation Amplifier and a resistor:
The basic idea of most current sensors is to measure the voltage drop across a small resistor placed in series with the circuit in question. In our Current Sensor (DCP-BTA) we use a 0.1 resistor. We measure the small voltage drop across that resistor, amplify it and then output that as a voltage that varies with the current. After calibration, you have a current sensor. The same idea works easily with other current ranges. Rather than modify our DCP current sensor, another solution would be to use our Instrumentation Amplifier, order code INA-BTA, $59.00. It is a general-purpose amplifier for this kind of thing. It has 6 ranges (from 0-20mV to +/- 1V). Just put a small resistor across the two banana plugs on the Instrumentation Amplifier and you have a current sensor. For example, if you want to measure up to a 10 ampere current, you could use a 0.1 ohm resistor. At max current the voltage drop across the resistor would be 10 * 0.1 = 1V. Set the Instrumentation Amplifier to read +/- 1V and you have a current sensor for that range.

Note that this method should not be used in 110 volt or 220 volt circuits. We do not recommend connecting any of our equipment to AC mains. There are inductive based current probes from Radio Shack, Omega, and Extech that work in high voltage circuits. Sometimes these units have a DVM output that could be connected to a Vernier interface.

Created by: dvernier on January 21 2003
Last updated by: jhopkins on August 31 2009