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Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
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Posts: 119
I am trying to get my thinking straight here, am I correct in assuming that they are always talking RMS values when a line voltage (like household) is stated...like say 120 VAC here in the US or 240VAC overseas., or the voltage of a step down transformer's output is stated...like 12.6 vac or an NST at 4kV? Is RMS the universal way of stating AC pretty much?
My second question concerns a step-down transformer with a center tapped secondary, am I correct to assume that you get the same full sinewave waveform, just half the amplitude in voltage from the center tap?
Thanks a million, I'm just thinking about alot at once here now and my mind is getting muddy as I'm trying to design some circuitry LOL.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Yes 120Vac or 240Vac means volts rms, as does the output voltage rating from a transformer.
Yes the center tap is as you describe but USUALLY (not always) the center tap is considered as 0 volts and the two ends are the rated voltage, but opposite in phase. If you connect the center tap to ground an ac voltmeter would give the same reading for both ends but if you look at the voltage with an oscilloscope you will see that the voltages are a mirror image of each other, while one end is going positive the other end is going negative. Center tapped transformers usually connect the center tap to 0V/ground and use diodes/rectifiers from both ends of the windings to produce dc. This is why the secondaries are usually written as e.g. 12 - 0 - 12 instead of 0 - 12 - 24
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2 Quick general AC Questions
