Low amp current probes have become a popular technician purchase in the last 10 years or so. There have been many good articles on the usefulness of this tool in our daily diagnostic adventures. Through visits to hundreds of shops over the years as a mobile diagnostics professional and trainer, I have found that one piece of the current probe training is usually missing - how to set up the probe before using it for diagnostics on a particular vehicle circuit.
What the current probe does
Before we address setup or scaling of the current probe, we must understand how it measures current in a circuit. A current probe is what is known as a transducer. A transducer is a measuring device that reads one type of quantity and outputs information directly proportional to that measurement.
A simple alcohol thermometer is a transducer. It takes the increase in volume of liquid in a marked tube to be a direct correlation of heat applied to the liquid. Whenever current flows through a wire, a magnetic field radiates at a right angle to that wire. The greater the current flow, the stronger the magnetic field.
The current probe is designed to absorb some of this magnetic field through the ferrous metal pickups mounted in its plastic jaws. Once the magnetic field enters the probe circuitry, the probe outputs a voltage that is directly proportional to the strength of that magnetic field. This voltage output by the probe is to be measured with a digital multimeter or oscilloscope as shown in Figure 1. So, more current flowing in a wire equals a greater magnetic field, which, in turn, creates a greater voltage output to be measured by a meter or a scope.
How to scale the probe output
A current probe typically is powered by an internal 9-volt battery. Some of the available 9 volts is used by the probe to process the measurements, and some of the voltage is used to output a signal to your chosen measuring device. Most current probes will output around 2 volts peak to peak, meaning 1 volt above ground reference and 1 volt below ground (or negative 1 volt).
Automotive low current probes can measure current in both directions. This is why they output 1 volt in each direction. This makes them useful in circuits like window motors and throttle-by-wire systems where current can flow in either direction in a given wire. The scaling you choose must divide this possible 1 volt current probe output into small enough segments to have a sufficient number of them to represent the total amperage limit you intend to measure.
Here's an example. If you intend to measure current in a circuit that carries up to 100A, you would have to divide that available 1 volt probe output by 100. This means that each 10mV output by the probe represents 1A of current flow in the circuit you are measuring, or a 10mV = 1A scale. One hundred times 10mV = the 1 volt probe output. One hundred times 1A = 100Amps.
If you intend to measure current in a circuit that carries up to 10A, then the available 1 volt probe output should be divided by 10. This would mean that each 100mV output by the probe represents 1A of current flow in the circuit you are measuring, or 100mV = 1A scale. Ten times 100mV = the 1 volt probe output. Ten times 1A = 10Amps.
How do you choose scaling on your probe before hooking up to the circuit? Simply check the amperage rating of the fuse used to protect that circuit. The size of the fuse that protects the circuit is a good indication of the amount of current that flows through the circuit. Typically, the fuse will be double the amount of steady state current in any circuit. The initial inrush of current can be as high as the fuse rating itself depending on the type of circuit you are testing.
For example, if you are measuring the current flowing in an electric motor circuit, such as a fan or fuel pump that is protected by a 15A fuse, then you can expect about 15A of current draw for the first thousandths of a second the motor begins to spin. After that, a normal electric motor will pull around half the fuse rating amount. If you are unsure of the current in the circuit, start on the highest scale and scale down as needed.
Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.