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.

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 two different current clamps:

Model		Ranges
Current clamp TP-CC80		20 A / 80 A switchable range, AC and DC
Current clamp TP-CC600		200 A / 600 A switchable range, AC and DC

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.

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.

Both the Current clamp TP-CC80 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
	80 A		100 A = 1 V		100
Current clamp TP-CC600	200 A		1000 A = 1 V		1000
	600 A		10000 A = 1 V		10000

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

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.

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

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.

Correct functioning of an engine depends on many things like e.g. correct sensor readings and good functioning actuators. But the engine itself also needs to be in good condition. With a relative compression test can easily be determined whether all cylinders of an engine have the same compression.

This relative compression test uses the starter motor current to determine the compression values of all cylinders. The advantage of a relative compression test is that no pressure sensors are needed to check each individual cylinder, all cylinders can be tested at once with just a lab scope and a current clamp.

Before performing the relative compression test the engine needs to be prepared such it will not start during cranking.

To perform a relative compression test with a Current clamp TP-CC600, first download the file below.

Measurement	File
Relative compression test

Connect the Current clamp TP-CC600 to the lab scope Automotive scope ATS5004D with a Measure lead TP-C812B and clamp the current clamp around the wire from the battery to the starter motor, see figure 1. The direction of the wire through the clamp must be such that the starter current will introduce a positive output voltage on the clamp.

Figure 1: Setup the current clamp and lab scope for a relative compression test

Open the file in Multi Channel oscilloscope software and proceed as follows:

1. Switch on the current clamp and zero the current clamp.
2. Start the measurement with the start button or press hotkey S.
3. Crank the engine for 2 to 4 seconds to perform the relative compression test.

The relative compression test is now ready and will look like figure 2.

Figure 2: Result of a relative compression test

The relative compression test in figure 2 is done on a 4 cylinder engine in healthy condition. Some variations in the result of the relative compression test is acceptable.