Current Essentials

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Table of Contents

Current Overview

Current is the actual flow of electrons through a conductor and is directly proportional to the source voltage. If the source voltage goes up, the current will go up. Current is inversely proportional to resistance. If the circuit resistance goes up, the current will go down.

Units of Measure

Base UnitSmall Units
Pronouncedampere (Amp)milliamp
Table 1: Units of Measure for the Ampere

Effects of Current

It is the flow of current that ultimately does “work” by either producing heat/light or magnetism.

Heat / Light

When current flows through specific materials heat, light, or both are generated. Examples of this are:

  • Incandescent light bulbs (light and heat)
  • LED light (light)
  • Conventional stovetop heating elements
  • Electric hair dryers
  • Plug in space heaters


When current flows through a conductor, a spiral-like magnetic field is created around the outside of a conductor (as shown in the top of Illustration 1 below). The higher the current flow, the stronger the magnetic field.

When a conductor is wrapped into multiple loops, the magnetic fields of each wire combine to make a single stronger magnetic field (as shown on the bottom of Illustration 1 above). If the coils are wrapped around an iron core the magnetic field is further strengthened.

Electromagnetism principles are used in at least the following automotive components:

ComponentHow Electromagnetism Works
AlternatorA winding is placed inside the alternator’s rotor that creates a rotating magnetic field. This rotating magnetic field ultimately induces even more electrical power in the stator’s windings. The stator produces the output that meets the electrical needs of the entire car.
StartersAn electromagnetic field is created inside the starter’s armature. This magnetic field in the armature reacts with another magnet field in the starter’s shell to create large amounts of torque that cranks the engine.
RelaysA relay is a control device that can control large amounts of current remotely with a small amount of current. The relay is activated by a small amount of current that creates a magnetic field that literally pulls the relay’s switch closed.
ActuatorsActuators are used to create a pulling/pushing force and are often used to lock/unlock doors or other such tasks. It operates by current flowing through a winding that creates a magnetic field. The magnetic field pulls on a steel plunger to create the pulling/pushing force.
Ignition SystemsAn automotive ignition system operates at a high voltage level (approx. 40kV) and is often called secondary voltage. The ignition coil uses the inductive quality of coils and magnetism to create the high secondary voltages.
Power Windows / SeatsMost power seats and windows use little DC electric motors that work on magnetism much like all other electric motors do.
Ways Electromagnetism is Used in a Modern Car

Measuring Current

With Ammeter

Current is measured by an ammeter which is a standard component of a digital multimeter (DMM). When measuring current, the DMM leads must be changed to use the “A” port and “Com” port.

The current goes into the meter and through an internal shunt with virtually no resistance then back out. With this configuration, the meter will have no internal resistance and will not skew the current which flows straight through the meter. A sensor mounted beside the shunt measures the strength of the magnetic field and translates that to a useable number to display.

Attempting to measure voltage while the DMM is set up to measure current (no internal resistance) will blow the internal meter fuse. Meter fuses are special fuses that are relatively expensive and often hard to find locally.

An ammeter must be placed into a circuit
(in series) to measure current.

When measuring current the DMM must go in series with the circuit so the circuit current literally goes through the meter as shown in the illustration above.

With Inductive Clamp

Measuring current with an ammeter has a few limitations and disadvantages, such as:

  • Meter Must Be in Series – The meter has to be placed in series in the circuit. This means the circuit must be physically opened up so the meter can be connected in series to the circuit (all the circuit current must go through the meter).
  • Maximum Current Limit – The typical DMM has a maximum capacity of 10A with a few DMMs going up to 20A. With larger currents an alternative method of measuring current, such as an inductive amp clamp, is necessary.

Due to the limitations of a DMM and extensive setup time, most techs measure current with an inductive amp clamp, as shown in the image below.

20A Current Clamp – Produces a Voltage For Meter Input

This is a low current amp clamp (up to 20A DC) that is used for the majority of current measurements. A large capacity current probe is available for measuring currents such as the starter draw.

The current probe measures the strength of the magnetic field produced by current flowing through the wire. The probe then produces a voltage signal that is connected to a DMM for display. For example, the meter shown in the image produces 100mV/A.

Measuring Current With an Amp Clamp (Video)

Measuring current with an amp clamp | Duration 5:45

Diagnosing Parasitic Draws


In the past when you turned off the ignition switch, no more current would flow out of the battery, for the most part, unless a door jamb, glove box, or trunk lid switch failed. However, with the proliferation of electronic control modules used on current cars, a whole new skill set has emerged, diagnosing parasitic draw. Parasitic draw is a current that slowly discharges the battery when the car is turned off, just like a parasite draws blood from a host animal. In extreme cases, the parasite can kill the host (animal or battery).

Parasitic draw issues are becoming more common due to the ever-increasing use of electronic control modules. A lot of engineering goes into how each module goes “asleep” how much current it will draw when it is asleep, and what wakes it up. Engineers try to keep the parasitic draw to under 50mA so a car could sit close to 30 days and still crank.

For an excellent explanation of parasitic draw and a solid diagnostic procedure watch the YouTube video below.

Which Parasitic Draw Test Method is REALLY the Best? | FordTechMakuloco | Duration 38:08

As discussed in the “Which Parasitic Draw Test Method is REALLY the Best?” YouTube video, measuring voltage drop across a fuse and determining current flow is not accurate. In my video below, I make the same point. If you are trying to quickly identify if a fused circuit has any draw on it, it is a fast test. Then grab an amp clamp and a fuse loop for an accurate measurement.

Find parasitic draws fast by measuring voltage drop across fuses