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Using Shunt Resistors to Measure Current

The information provided is for help with Commtest Instruments products:



QUESTION

How can I measure current with the MMS3000 T6V4?

ANSWER

Place a shunt resistor in the current path and measure the voltage over the resistor as shown in the example diagram below.

Note: This technique can only be applied to DC currents, not AC mains currents.

Warnings:

Example: To measure the temperatures of both terminals of a 12 V battery it is essential to use isolated thermocouples.

Why?

Exceeding the above limits by a small amount will cause interference between the channels and hence erroneous readings. Exceeding the limits by a larger amount will burn out the input circuitry in the T6V4. For an in-depth explanation of these limits see the Application Notes in the Appendix of the Owner's Manual.

The value of the shunt resistor must be chosen carefully as follows:

Step 1. Find the maximum current

You may know this value (in Amps) already, but if not it can be found from the power rating of the motor (or whatever the load is):

(Amps) = Power (in Watts) / Voltage

In the example above: I = 300 W / 24 V = 12.5 A

Step 2. Find a suitable value for the resistance

If the resistance is too high

Too much voltage will be dropped over the resistor, taking power away from its actual purpose (in this case driving the motor). As a guide, try to avoid dropping more than 1% of the available voltage over the shunt. In the example above 1% of 24 V is 0.24 V

If the resistance is too low

The voltage seen by the T6V4 will be too low, so the current measurement will be less accurate and noisy or stepped. Avoid this by ensuring that at least 0.1 V appears over the shunt at maximum current. In the example above the voltage over the shunt should ideally be between 0.1 V and 0.24 V, when the maximum current is flowing. Experimenting with resistance values R in the formula:

Voltage(V) = I R

leads to the choice of 0.01 Ohms, which will develop 0.125 V at 12.5 A.

Step 3. Calculate the Wattage required for the resistor

We don't want the resistor to burn out, so fit a suitably high wattage one using the formula:

Power(Watts)= I2 R

In our example, Power = 12.5 x 12.5 x 0.01 = 1.56 W, so we'll fit a 2 W resistor. Once this is connected up we can configure the T6V4 readout to be scaled directly in amps instead of in voltage. This is explained in section 2.2 of the Owner's Manual.

The following picture shows the instrument screen while setting up channel 8 for the example above.

To reach this screen from the Main Menu:

Notes

The voltage range +/- 5 V has been selected instead of +/- 30 V as this will give better resolution for the small voltages expected.

The scaling 0.125 V means 12.5 Amps has been multiplied by ten in the screen shot above. This is to satisfy the T6V4 instrument's scaling requirement that the two input values should be at least 1 V apart.

FAQ ID: 11624 Last Reviewed: 27/09/2005