Blog posts of '2014' 'June'

Electronic ignitions explained

Q. Cars now have electronic ignitions. What is the difference between an electronic ignition and the old kind?

A. The old kind used a set of switch contacts in series with the primary of the ignition coil. The contacts, called “points,” were opened and closed by a cam linked to the distributor shaft. When the points closed, current flowed through the primary of the ignition coil, building up a magnetic field in its core.

Then the contacts were opened so that the magnetic field began to collapse quickly. This induced a high voltage in the secondary of the ignition coil, producing the spark to fire each cylinder at the right time. A capacitor, which mechanics usually call a “condenser,” was connected in parallel with the points. Its purpose was to resonate with the inductance of the primary of the ignition coil when the points opened. This made the spark voltage an oscillating Waveform of decreasing amplitude. Instead of being one brief pulse, the spark voltage continued for a short time, helping to ignite the fuel / air mixture more effectively.

In an electronic ignition, the points are replaced by a transistor that is operated as a switch. It still sends on/ off pulses to the ignition coil’s primary in the same way as the points had. In some cars, there is a separate spark coil and transistor for each pair of cylinders. 

Various methods are used to control the timing of the transistor`s pulses, most of them being either magnetic or optical devices operated by a mechanism connected to the crankshaft. The points in the old kind of ignition would gradually burn up as they operated, so that they had to be replaced periodically. The mechanism which operated them would also wear a bit, so that periodic adjustments were necessary. This was the purpose of the “tune-up” The capacitor was usually replaced in a tune-up, too.

Since the electronic ignition has a transistor instead of points, the problem of burned points has been eliminated. The transistor can also deliver a more energetic spark, making ignition more efficient. For these reasons, modern automobiles do not require this kind of a tune-up. Usually, all that needs to be done in a tune-up now is to make minor adjustments to the ignition timing, and sometimes replacement of the spark plugs.

Colorful Language

Q. The lessons refer to the “video” signal in a TV receiver. Just what is the video signal, and why is it separate from the color signals?

A. When the standards for color TV were established, there were already many blackand-white receivers in use. To make the new signal for color compatible with these existing receivers, a black-and-white signal was included inthe color system.

It is derived from the three primary colors by mixing them in speciñc amounts: 11% blue, 30% red, and 59% green. This is properly called the “luminance” signal, since it is the brightness content of the picture. The author of the TV lessons usually calls it the “video” signal.

The color content of the picture is contained in the “chroma” signal, from which the three color difference signals are derived in the receiver. When these are mixed with the luminance signal, the original three color signals, red, blue, and green, are produced. These are then used to drive the cathodes of the three electron guns in the CRT.

A standard black-and-white receiver uses only the luminance signal in the broadcast. A color receiver basically takes the luminance signal and adds color to it.

Scope the Situation

Regular oscilloscope versus digital oscilloscope

Q. I have seen advertisements for digital oscilloscopes. What is the difference between a regular oscilloscope and a digital one?

A. In a conventional oscilloscope, the signal to be viewed is simply amplified and applied to the deflection plates in the CRT. A digital scope has a very fast electronic switch that is connected in series with the input. The switch is turned on for very short periods of time to “sample” the input signal. While the switch is on, a very fast analog-to-digital converter takes the sample and produces a digital signal.

The digital signal is a binary number whose value corresponds to the voltage of the sample. Successive samples are taken, so that the waveform of the signal is converted into a stream of binary numbers. Digital circuits in the CRT scanning electronics are used to light up individual dots on the screen called “pixels” Each pixel corresponds to a sample of the waveform. The value of the binary number for each sample determines the height of the pixel that represents it. The pixels are displayed in succession across the screen of the CRT. Together, the pixels forrn a line which represents the waveform of the signal.

There are also oscilloscopes which have electronic switches that break the signal up into analog samples that are instead simply amplified and applied to the deflection plates in a conventional scope CRT. These are called “sampling” scopes. They are used for high frequency signals.