Eye On Electronics

Motor, Sep 2004 by Dale, Mike

Conventional oscilloscope tests are certainly valuable. But with the addition of specialized probes, scope diagnosis expands well beyond voltage vs. time measurements.

As good and useful a tool as the oscilloscope is, it does have some limitations. The scope itself is really useful only to display the relationship between voltage and time. Multiple-trace scopes can be used to compare waveforms occurring in different parts of tlie circuit at the same time, but its still only a voltage vs. time waveform. Even this cannot be done without some way to connect tiie oscilloscope to the circuit being measured. Getting the best information out of your scope depends on how you connect it to the vehicle.

The probes are where eveiy oscilloscope measurement begins. Some probes are simple; they don't amplify or otherwise change the signal. Some are designed to measure current while others reduce high-voltage signals so they too, can be measured on the scope. Some probes contain active electronic circuitry to increase the high-frequency and high-sensitivity function of the scope.

The basic probe that comes with every scope accomplishes three diings: It provides a means of connecting the scope to the circuit to be tested, the probe matches the input impedance of the scope and the probe cable shields the measurement signal from noise radiated by sources such as fluorescent lights and motors.

Since most scopes have a limited input voltage range (±200 volts typically, less on some models), the probe is often used to attenuate, or reduce, the incoming signal to allow high voltages to be measured. If you want to look at the primary waveform of an ignition coil, for example, you'll need a 10:1 probe to divide down the incoming voltage. On some scopes, the 10:1 probe sends a signal to the scope advising that it's in place. The scope can then respond by changing the vertical reading to show that you're reading ±500 volts.

The most common probe is the 10:1 probe. If you're looking at signal voltages in the 1-volt peakto-peak range or less, the 1:1 probe may be more appropriate. Switchable probes that can handle either ratio can be useful. These are really two probes in the same handle, and not only are the signal reduction factors different, but other probe characteristics-such as bandwidth, rise time and impedance (R and C)-may also be different. You may see more than just amplitude changes in the waveform when you switch between the two settings.

The maximum possible input voltage your scope can stand is listed in the owners manual. Be sure to use the 100:1 probe when you think you may be looking at voltages above the scope's capability. If you want to look at the output of a coil, you'll need special high-voltage probes or capacitive-couple scope probes.

You expect scope probes to last forever. In a laboratory situation, scope probes get pulled on, dropped and run over by the wheels on chairs, with the general result that they get used up and need to be replaced. The wear circumstances are even worse in a service bay. When you do replace them, make sure you match the impedance (50 ohms or 1 megohm, usually the latter) and that you properly adjust them for the scope being used. While generic brand probes can be used, you're often better off with the one that was designed for the scope you own.

To expand the capability of the oscilloscope beyond voltage vs. time, there are a variety of scope probes and accessories called transducers. Transducers are devices that convert power from one form into another. The loudspeakers in a car audio system are a type of transducer in that they convert amplified electrical power into sound. Other transducers, such as MAP sensors, include devices that take pressure and convert it into electrical values. Not all sensors are transducers, though. Oxygen sensors work on an oxygen diffusion principle more like a battery; crank position sensors generate signals rather than transform them.

The most common accessoiy probe is the current probe. Current flow through a conductor causes an electromagnetic flux field to form around the conductor Current probes are designed to sense the strength of these fields and convert them into a corresponding signal the oscilloscope can read. With a current probe its possible to measure tilings like inrush current to a starter motor, phase angles of motors and the commutator switching in a motor. Motors with bad brushes or open commutator segments can be seen easily on a scope with a current probe.

There are basically two types of current probes for oscilloscopes-AC current probes, which are usually passive, and AC/DC probes, which are usually active. When sensing AC current, both types of probes use the transformer principle. The AC causes a flux field to rise and fall according to the amplitude and direction of current flow. If a coil from a current probe is placed into this field, it will have a voltage induced into it proportional to the current flowing through the main wire. The scope can display this as a measurement of the current that is flowing, its frequency and, if need be, its phase angle.