Measuring with a current clamp

Introduction

To measure a current with a TiePie engineering measuring instrument with voltage inputs, the current has to be converted to a voltage related to the current. One way to do this, is to add a known resistor in the current path and measure the voltage over the resistor. This method has several drawbacks:

  • The existing circuit has to be broken to add the resistor
  • Adding a resistor to the circuit may affect the circuit
  • Unless a differential input is used, it is not always possible to measure directly over a resistor in a circuit.

A better way to measure a current is using a current clamp, since it does not have these disadvantages.

Current clamps

A current clamp offers a way to measure the current in a conductor without the need to break the circuit. The current can be measured while maintaining a galvanic isolation between the measuring instrument and the circuit under test.

TiePie engineering offers three different current clamps:

Model Ranges
Current clamp TP-CC80 Current clamp TP-CC80: 0 - 20 A and 0 - 80 A input ranges 20 A / 80 A switchable range, AC and DC
Current clamp TP-CC400 Current clamp TP-CC400: 0 - 40 A and 0 - 400 A input ranges 40 A / 400 A switchable range, AC and DC
Current clamp TP-CC600 Current clamp TP-CC600: 0 - 200 A and 0 - 600 A input ranges 200 A / 600 A switchable range, AC and DC

Connecting the current clamp

To measure a current using a current clamp, connect the current clamp to one of the inputs of the oscilloscope. The TiePie engineering current clamps have 4 mm female banana sockets. Connect the red socket to the positive terminal of the input and the black socket to the negative terminal.

Set the selector switch from the OFF position to the required input range of the current clamp. The power LED will light to indicate that the current clamp is switched on.

While the current clamp jaws are closed, press the ZERO button to reset the current clamp to zero. This will remove any offset, caused by residual magnetism remains in the core of the jaws. During zeroing, no current should flow through the clamp, so remove any wires from the clamp, or make sure no current is flowing through them.

Clamp the jaws around the current carrying conductor(s). Always make sure the jaws are tightly closed.

Setting up the input channel

The current clamp outputs a voltage that is related to the current it measures. The channel's probe gain setting can be used to convert the measured voltage to the original current value.

Setting up the Probe settings

To change the input channel settings, right-click the channel in the object tree or on the channel toolbar. This will bring up a menu with all available channel settings. To enter the relation between current and output voltage for the current clamp, select the entry Probe settings. This will bring up another menu, with some pre defined settings and a User defined... setting, which will show an input dialog in which an arbitrary value can be entered.

Probe setting gain dialog

The Current clamp TP-CC80, Current clamp TP-CC400 and the Current clamp TP-CC600 can be used in two different current ranges, which each require their own Probe gain setting:

Model Range Current - output voltage relation Required Probe gain Setting file
Current clamp TP-CC80 20 A 10 A = 1 V 10 Download
80 A 100 A = 1 V 100 Download
Current clamp TP-CC400 40 A 100 A = 1 V 100 Download
400 A 1000 A = 1 V 1000 Download
Current clamp TP-CC600 200 A 1000 A = 1 V 1000 Download
600 A 10000 A = 1 V 10000 Download

Enter the required value from the table. The table also lists a settings file in TPS format with the appropriate settings for each current clamp, for each current range. You can download the TPS file and open it in the Multi Channel oscilloscope software to have your USB instrument setup correctly for the current clamp. These TPS files assume the current clamp is connected to input channel 1.

Changing the measurement unit to Amperes

Since current will be measured, the unit of the input channel will need to be changed. In the channel settings menu, select the entry Set unit.... This will show a dialog box in which the proper measurement unit can be entered:

Unit dialog

Enter an "A" here.

Measuring DC current

When measuring DC current, it is important to zero the current clamp before clamping it around the conductor. Always press the Zero button with fully closed jaws.

In case of measuring a DC current, the polarity of the output voltage depends on the current direction through the jaw of the current clamp.

Model Current direction1 Output voltage
Current clamp TP-CC80 Top to bottom Positive
Current clamp TP-CC400 Bottom to top Positive
Current clamp TP-CC600 Bottom to top Positive

1. Top of the current clamp is the side with the range selector switch.

Leakage detection

When more than one conductor is placed within the jaws of the current clamp, the sum of all currents flowing through the conductors is measured. Since a current clamp is direction sensitive, currents flowing in the opposite direction will be subtracted.

This makes it possible to detect a current leakage in a device. Place the clamp around all power conductors going to the device, except the earth. The sum of all currents should be zero. When the sum is not zero, there must be a current leak to earth in the device.