Current Ramping – Most Driveability Testing Requires a Low Amp Clamp

Part 1: Getting Started – How to shop for a well designed Low Amp Clamp. In order to choose the best Low Amp Clamp it is important to know the limitations of the various Low Amp Clamps. Any limitations are usually revealed in the Manufacturer’s Spec Sheet Data.

There are several characteristics to be considered:

• Resolution
• Accuracy
• Output
• Switchable Output
• Range
• Conductor Size

Resolution: Resolution is the smallest increment of measurement of which the instrument is capable of displaying. A clamp that can measure in increments of 100mA has a Resolution of 100mA. A clamp that can measure in increments of 10mA has a Resolution of 10mA. Compared to a 100mA Resolution clamp, a 10mA Resolution clamp should be 10 times more sensitive. However, it must be determined if the Accuracy matches the Resolution.

Accuracy: Accuracy is the degree to which the measurement is accurate. Consider the example of a bathroom scale that reads in half-pounds increments. If the Accuracy is 2 percent, a 200 pound reading means the person actually weighs anywhere from 196 to 204 pounds. An Accuracy of 1/2 percent would mean the 200 pound weight is correct to within 1 pound. Accuracy is important. Repeatable Accuracy is best.

Output: The relationship of mV of Amp Clamp Output to amps detected. When using a Low Amp Clamp with a Labscope the bigger the waveform pulse the better. When using a labscope like the popular LS2000 or ADL7100 the minimum voltage sensitivity setting is 50mV/div. A Low Amp Clamp output of 10mV per amp will be 1/5 of one division per amp on the labscope display and could be too small to see well enough to reach a diagnostic conclusion. An Amp Clamp Output of 100mV per amp will yield a two division high per amp pulse which is big enough to see and interpret and make triggering easier. Clamps with switchable output can be annoying and/or confusing.

Switchable Output: Some Amp Clamps will output 100mV per amp the first few amps then switch their output to only 10mV per amp. Switching requires reconfiguring the labscope settings to match the Amp Clamp Output. For ease of use we recommend an Amp Clamp with continuous output of 100mV per amp.

Range: The maximum readable amperage should be high enough to accommodate the requirements of all underhood driveability testing. A Clamp that reaches its limit at only 2, 10 or even 15 amps will not have enough Range to meet all the requirements of a driveability technician. The minimum reading should be low enough to use with a meter to measure parasitic draw.
Parasitic draw can be as little as 30mA. For reliable readings a Low Amp Clamp that gives repeatable, accurate readings down to 20mA or less is necessary.

Conductor Size: The Amp Clamp must be able to close around the wire or cable under test. While a very small clamp opening may allow easy access to tight places, the trade-off is inability to make some measurements. A compromise of ability to accommodate varying wire sizes to overall clamp size may be best.

Hold “Zero”: For Parasitic Draw measurements an amp clamp must be able to hold “zero”. Disappointingly most low amp clamps drift substantially from zero as soon as the jaws are opened to clamp around a wire. The result is a reading that is not accurate or repeatable.