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The electricity in your car is DC (Direct Current) but your alternator produces AC (Alternating Current.) Isn’t that kind of weird? It’s perfectly easy to get a device that takes rotational energy and makes DC but that isn’t the case today. Why is that? A quick phone call to the service guys at Thompson Automotive in Baltimore, MD, gave us the whole story.

What’s DC mean
First, let’s clarify what we mean by AC and DC electricity. DC or Direct Current is the type of electricity produced by batteries. It flows in one direction. You can picture it being like a garden hose; it flows from one end of the hose to the other. It’s also the type of electricity produced by a generator, which was used in automobiles from the early 1900s up until the 1960s.

What’s AC mean
The other sort of current is AC or Alternating Current. It’s called AC because it quickly reverses direction every second. Unlike the DC water hose we discussed above, AC goes one direction, then the opposite direction -back and forth. AC electricity is the sort of electricity accessed via the wall outlets in your home (120VAC). The reason that we find AC electricity in our homes is that AC electricity is easier to transmit long distances. You know, on those huge transmission towers with drooping wires you see beside the highways. Those wires are carrying high-voltage AC electricity.

What do all my electronics use?
Most electronic devices (including most everything in your car) use DC current internally. You may have to connect an adapter to your wall outlet to change the AC to DC but inside they use DC.

So why use alternators that generate AC?
The answer is pretty much just that alternators are just better at turning rotational motion into electricity. Alternators are cheaper to make than generators, they last longer, and they produce more current across the speed range. They’re just so much of an improvement and it’s just no big deal to convert their AC to DC—well, at least it hasn’t been a big deal since the development of the cheap, reliable, diode rectifiers.

When did alternators start being used?
The first car to come standard with an alternator was, believe it or not, the 1960 Plymouth Valiant. Some of the more immediately apparent advantages of this was that at low speed, the alternator still produced enough current to keep the battery charged—something most generators did not do.

So, let’s just recap
All the methods of making electricity by spinning something (like your engine does) must be turned into DC to use in a car. The old way was to use a generator which handles this mechanically with a commutator and brushes. An alternator produces AC but is this is filtered down to DC.

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