Hobbies And Interests
Direct Current
Electricity is the flow of electrons. DC electricity is when the electrons always move in the same direction. This is the kind of electricity that comes from a battery -- smooth and continuous. Sometimes a DC voltage may vary slightly or ripple. The electrons still move in the same direction, but the number of electrons that pass a fixed point varies. If this number varies a lot and in a periodic manner, the DC looks a lot like AC, although the electrons always move in the same direction. The large variation is between very little movement and a lot of movement -- but always in the same direction.
Alternating Current
Alternating current is the kind of electricity that comes from a generator. Electrons start moving in one direction and smoothly increase to a maximum amount and then gradually fall of to zero. Then electrons start moving in the opposite direction and increase to the same maximum current flow in the opposite direction and then slowly decrease to zero. This pattern continues endlessly; If you have the same pattern but everything is shifted up so that the midpoint is between zero to +20 volts, the cycle that once went from -120 to +120 now goes from -100 to +140 and the electron flow reaches a larger maximum in one direction than in the other direction.
DC Offset
When AC and DC mix, it shifts the AC waveform up or down. The electron flow is greater in one direction than the other -- but still periodic. If the DC is large enough, the electronic flow will always be in the same direction -- but still periodic. A typical result is the headroom problems when the AC signal is voice or music. An audio signal that varies from -5 volts to +5 volts might go to a meter that has a range of -6 volts to +6 volts. With a couple of volts of DC offset, the meter will reach the extremes and add clicks or snaps to the audio signal.
Causes of DC Offset
DC offset is almost always caused by a minor flaw that is not serious enough to show up as anything more than DC current appearing someplace where it does not belong. Typical examples are shorts in a few coils of a transformer or cracks in a circuit board. High end electronics use "isolation capacitors" judiciously to isolate modules from each other because capacitors pass AC but not DC and effectively remove the unwanted DC that is causing the DC offset.