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Illegal RFI? Who says? |
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The Federal Communications Commission (FCC) is the government agency which tells people--meaning everyone!--just how much RFI they can legally emit. In fact, just recently the FCC confiscated a large number of personal computers from various manufacturers who had not complied with FCC regulations regarding RFI emissions. And we'll bet you've never heard of a computer fouling up someone's TV set!
In fact, if you ever get a chance to peek inside the computer or even look at the back data panel on the computer used at your jobbing store (if you use computers), you will probably see a small tag which says something like, "Complies with FCC Class B requirements."
All devices which have the capability of emitting RFI must comply. If they don't, fines--at the very least--can be assessed. And the offending hardware may be confiscated. This includes offending automotive products.
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How RFI is produced: the down-and-dirty technical details. |
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The movement of electrons in a wire produces a magnetic field. This is called "induced" magnetism.
The reverse of this is also true. If the movement of electrons can cause magnetic force to be induced in a wire, then the movement of a magnet near a wire can induce an electrical flow.
The critical word in all of this is movement. It is the movement of the magnet near the wire--or the movement of the wire near the magnet--which causes the magnetic lines of force surrounding the magnet to excite the electrons in the wire to move.
Or, it is the movement of electrons within a wire which excites a magnetic field with lines of force.
Remember that if there is no electron movement, there will be no RFI. However, movement of electrons will create moving lines of force which create RFI.
Note: Here we have to put in a few words for EMI again. Very weak RFI or EMI signals can cause many problems. It takes much power (relatively speaking) to cause radio static. But it takes very little power to induce stray voltages in a computer system. And we've got many computers on cars, nowadays! Plus, we have all sorts of pulsating DC voltages on cars, too.
Thus, we need to pay attention to all kinds of new problems with EMI. The type of wire used in an application--and particularly the routing of the wires on the car--are critical to eliminating or at least minimizing computer problems related to induced EMI "signals" to the various computers on today's cars.
| The principles: |
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Even if you don't understand all of the above science data, please understand this one concept:
RFI, or static, can be caused whenever a direct current is started or stopped, or whenever an alternating current flows in a wire. Both alternating current and pulsating direct current can cause RFI.
| RFI in the car. |
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We said direct current (DC) was used to give power to the various devices used in the car. Indeed, the battery is the basic electron source for everything the car powers with electricity. However, there are four major ways in which RFI or EMI is created in the car, even though the source is 12 volts DC:
1) When a switch of any type is turned on or off, or when the DC flow in any wire is turned on or off by any control device.
2) When a pulsating DC current flows--such as when a starter motor or other motor is turned, varying in speed as it works against cylinder compression.
3) When the alternator--which is a generator of AC current--is operated.
4) When the coil sends its high-powered electron charge down the spark plug wires to the plugs. When this is done, there are actually three electrical components to the flow: a) a DC component; b) a low-frequency AC component; and c) a high-frequency AC component.
This last alternating current component--the high-frequency AC component--is one which has enough power behind it to cause significant RFI. It is caused by a phenomenon called "circuit resonance," and can be (and by law, must be) made ineffective.
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Pulsating DC? When did that ever cause problems? |
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Pulsating DC, from starter motors, in particular, have caused major problems in some cars. A large amount of electrical energy is used to rotate the starter. As the DC current pulsates, the electromagnetic field generated around the starter cable and the field inside the starter itself, pulsates. This pulsation of the lines of force is enough to cause the emission of an RFI pulse.
In fact, the pickup coil for the ignition system in some GM cars has been designed "directionally"--with color coded pigtails to prevent the starter RFI from disabling the ignition system. The combination of improper wire routing and the RFI from the starter would actually defeat the ignition and prevent the cranking car from starting.
Strange business? Yes, it is. As a counterman, you'd better become very aware of these problem areas as well as the quality of parts you sell for underhood work. Parts which meet or exceed OE physical characteristics (in other words, look-alike parts) may or may not meet performance characteristics. Your electrical parts supplier should have done the testing required to make sure the parts perform to OE or better.
Circuit resonance in DC circuits. |
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A wire can be thought of as a container for electrons. The metal molecules have "free electrons" on them. These electrons have a like charge, and, therefore, tend to repel each other. As such, when not flowing, they tend to be relatively evenly distributed throughout any wire in which they are found.
Electrons also have a certain amount of inertia, meaning they tend to stay at rest when left alone, and tend to keep moving once they have been set in motion.
The critical question: |
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Ask yourself, "What happens if electrons are flowing freely through a wire when suddenly the circuit breaks?" Because the electrons have inertia, they tend to keep flowing. The tend to "stack up" at the end of the wire where the circuit has been broken. Then, because they repel each other, they tend to move toward the other end of the wire where they stack up again. In fact, they keep moving back and forth until their energy runs out due to internal resistance in the wire. (See Fig. 5.)
Fig.5
Engineers say the electrons "oscillate," meaning they go back and forth within the wire. This oscillation within the wire is called "circuit resonance." The electrons resonate back and forth in the wire, even though no electricity is actually flowing through the wire in a complete source-to-ground circuit. And, as the electrons resonate, they cause the movement of an electromagnetic field around the wire, therefore creating RFI or EMI.
This resonance happens very quickly at what is called a "high frequency." High-frequency electromagnetic waves require very little power to drive them long distances. This is why we say the high-frequency AC radiation is what causes the problems in the spark plug wires.
How bad can the RFI get? |
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The worst possible case is caused by unsurpassed solid metal conductors, such as stranded copper or tinned copper used as spark plug wires. These wires were very common conductors in the 1950s (pre-RFI control days). The RFI could be so intense that it was easy to induce a spark plug firing voltage in a wire lying next to another spark plug wire. This is called "crossfire" because the spark power in one wire crosses to another and fires that adjacent wire as well. When crossfiring, the engine will run horribly "off time" as the wrong spark plugs fire.
The RFI from a system such as this could be powerful enough to mess up a radio or TV signal on a receiver hundreds of feet away, giving you many angry neighbors.
Today, of course, that level of RFI would interfere with any computer on the car or any type of sensitive electronic pickup device.
Recap. |
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Radio frequency interference (RFI) or electromagnetic interference (EMI) is produced when electromagnetic radio waves with enough power driving them escape from a wire or other conductor. In order to produce an RFI wave, the electric current reverses direction--as it does in an alternating current--or it pulsates in a direct current conductor.
RFI can produce "static" on radio speakers or "snow" on TV screens.
The major RFI source in the ignition system is the high-frequency AC component of the spark current. A second source of known problems is the pulsating DC in the starter circuit. However, other wires in the automotive electrical system can also generate RFI and EMI, usually at low power levels.
The major problem today is that RFI and EMI interfere with computer circuits.
