AC...DC Waveform

[Alan Douglas]

The AC component on a DC offset is by far the easiest way to understand this.

[pwdrive]

What would be the DC offset of a 5 volt pulse sitting on a common rail of zero volts.

Lawrence.

[Mark Hennessy]

What would be the DC offset of a 5 volt pulse sitting on a common rail of zero volts.

Sneaky question

The answer depends on how long you look at it. If you consider what went on for 24 hours before, and 24 hours afterwards, the DC offset would essentially be 0V. But if you looked immediately either side of the pulse, then perhaps it would be 2.5V

Things do become tricky with non-repetitive events - you need to think about Fourier and similar - an infinitely narrow impulse having infinite bandwidth, for example. Probably best not to go there

In the not so recent past, when the BBC electronic engineering "A Course" was about 2 months, an entire week would be spent talking about "DC Transients". With a test on Friday!

[pwdrive]

Yes, infinite, Fourier etc I always thought that a fourier transform revitalized a slow horse...

Getting back to the 5 volt pulse sitting on a zero rail, if there was a series of these pulses sitting on the same common rail of zero volts and of the same 5 volt amplitude how do we rectify the negative half.

Lawrence.

[Mark Hennessy]

Getting back to the 5 volt pulse sitting on a zero rail, if there was a series of these pulses sitting on the same common rail of zero volts and of the same 5 volt amplitude how do we rectify the negative half.

Put the signal via a capacitor first, which will block the DC offset and give you the AC component

[pwdrive]

I was thinking more of directly, not via a capacitor.

Lawrence.

[Alan Douglas]

No need, you just average them.

[Mark Hennessy]

I was thinking more of directly, not via a capacitor.

Why do you ask - practical reason, or understanding?

[pwdrive]

Just trying to understand how an intermittant direct current can be an alternating current, disregarding offsets, capacitors, inductors etc, which is what my original post was trying to resolve.

Lawrence.

[Kalee20]

Its something I've puzzled about for years, if you have a bar magnet and revolve it in front of a coil as a simple dynamo, as the N pole approaches the coil a voltage will be induced, say a +V, then as it revolves further this V would reduce to zero and as the S pole approaches the V would go to -V, this would be an AC voltage, but what if instead of revolving 360° the magnet reverses direction after 90° back to its original starting point? The induced voltage will have risen from 0V to +V and back to 0V, if this was repeated wouldn't this just be a stream of DC pulses

Sorry, no it won't.

As the N pole approaches the coil you could well get a +ve voltage. As the magnet revolves further the voltage may well fall to zero. If you then reverse direction, the voltage won't rise again. You will instead get a negative voltage.

That's because the voltage depends not on the magnet's position, but on its velocity. Moving backwards reverses the voltage.

It's not possible to move a magnet cyclically and get a unidirectional voltage - you'll always get as much negative as positive.