Critical facts

This month, Frank Massey looks at current measurement, tools and process

Published:  12 June, 2014

By Frank Massey

Voltage is electrical pressure, or potential difference. Voltage differential cannot exist without a complete circuit and load. The unit of measurement is the volt and current is the measurement of flow. Current cannot exist without a consumer, in a conventional ground-on circuit, when voltage falls an increase in current will start to occur at the same time. The unit of measurement is ampere. The load in a circuit should only be consumed by the component, the unit of load is the watt. It should therefore be possible to predict the response in a circuit by applying Ohm's law - voltage divided by amps and multiplied by resistance but in complex vehicle control circuits it's not that straight forward. However, the tools and process do not change.

Vehicle components vary in nature but they do however share one common fact - they apply a resistance in a circuit and sometimes it is so small, or switched so quickly, that it cannot be measured accurately without special high performance tools. When load is applied in a circuit the voltage will change very quickly and this depends how the load is switched, the current takes longer to accumulate. This phenomenon depends on the available pressure, flow rate and load.

Most vehicle components discussed here fall into the category of inductors. These are essentially coils, the nature of which apply considerable loads when introduced into a circuit. They are usually switched and ground-on but this can be reversed. The onset of complex control motors in vehicle systems demands a more responsive means of switching whilst reducing where possible the current through the PCM (Pulse Code Modulation). This method is referred to as Pulse Width Modulation, PWM. By pulsing a circuit the duty cycle controls the current ramping. It also provides accurate response and feedback.

What is the critical criteria when evaluating actuators, what tools actually work and where can I get them? Correct current flow is very important but it is only one of three critical components, we also need rise time and induction.

Rise time relates to the resistive value and the available pressure - ever tried to get a golf ball down a drinking straw? The unit of induction is the henry, H. This applies especially when testing the rate of response within an injector, against the control on signal. It takes into account voltage, current flow, current rise time, speed of current interruption and the electromagnetic field effect the pintle movement has to the current path, temperature also has a part to play.

One thing is for certain, as I recently observed to the contrary in a technical journal, the voltage and current flow must share a common response.

So, how do we measure current and why current first? Current flow is equal throughout the entire circuit, so the opportunity is much easier. The control fuse or power relay for example. Using an inductive current clamp (it's actually a Hall Device) ensures a non-intrusive means of measurement. The sensitivity and rise time of the clamp is vital. It not only provides measurement of flow but the effectiveness of the current interruption, this of course is imperative for good induction. This is the responsibility of the power transistor within the PCM, or ignition coil as with the latest direct ignition systems.

Figure 1: A simple ground on injector

01772 201 597.

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