Testing circuits with Frank

Following from his previous introduction to electrical basics, Frank describes voltage and current

Published:  03 May, 2013

By Frank Massey

I was fortunate that back then there were no specialist tools and even serial communication was still several years away. I decided the best way to begin and understand vehicle electronics was to understand how sensors and actuators worked and how to measure them. I also took what turned out to be a wise step, adopting the same tools used by the electronics industry. I was criticised by most, if not all diagnostic tool manufacturers of the time, for going 'over the top' by using oscilloscopes for component testing. How attitudes have changed! The point I'm making is that it is systems and component knowledge accompanied by correct measurement technique that ensures successful repair and that's exactly what our training programme promises, as well as 45 years' experience.

Let's begin with the 'invisible enemies', namely voltage and current - well actually they are our friends and I intend to simplify their assessment. Traditionally they were treated separately. A simple yet often difficult challenge is to prove the integrity of a circuit. A circuit is a set of components and for it to function it must be intact and have 'voltage potential', (more at one end than the other), or carry data. This can be analogue (smooth) or digital (on/off).

Let's take a look at the simplest of components, the starter motor. At one end is the voltage, the electrical unit of pressure or potential, then there's the battery which stores chemical energy to be converted into electrical energy when the circuit is closed (or to put it another way, when a ground path exists.) This is achieved through the starter motor windings. In a good circuit, the load should be taken only by the component. This implies corrosion-free joints and a path robust enough to take the load, which is current, the electrical unit of flow. To establish a good circuit we must understand four things - voltage supply, current consumption and circuit resistance, under load. The fourth and often overlooked parameter on our starter motor is rotation speed; this is calculated from two compression events from the current flow Vs time.

Now let's consider a sensor circuit, nominal voltage supply is 5V and the ground path is usually through the Powertrain Control Module (PCM), it may be shared by other components. The output is the prime function of the sensor and is the measurement of an event. Initially, it should be evaluated serially with plug in diagnostic equipment by comparing specified, actual values where possible.


The real bonus is the ability to prove the circuit's ability to take load. The load is adjustable between 2A and 65A with overload protection. We use a 2A load to check integrity of sensor circuits - isolate the circuit in question, connect the redundant ground to one end, select 2A current load probe the other end and depress the power button. A load of 2A will be applied to a circuit, more than enough to prove integrity without damage.

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