Re-sampling

robert bristow-johnson wrote:

   ...

> Jerry pointed out to me that this assumption is *not* reliable for AC
> power signals (which was my previous thought).  in that case, zero-
> crossing is not legitimate since there are possibly more than two zero
> crossings per period (something i did not expect).

If electrical garbage bred germs, most of us would be terminally ill. 
There's a tremendous amount of garbage on power lines, and I don't mean 
just sneakers hanging by their laces. You can hear some of the older 
thyristor lamp dimmers on an AM radio. I can hear leaky pole insulators 
on the AM radio in my car.

Years ago, I made a lamp dimmer for an inside room that consists of a 
two-on, center off switch that closes the circuit directly when up and 
through a diode when down. Consider the poor pole transformer. They say 
it can't pass DC, but pulsating DC is what the dimmed lamp runs on. So 
where does the DC come from?

Discussion on request. :-)

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:
(snip)

> If electrical garbage bred germs, most of us would be terminally ill. 
> There's a tremendous amount of garbage on power lines, and I don't mean 
> just sneakers hanging by their laces. You can hear some of the older 
> thyristor lamp dimmers on an AM radio. I can hear leaky pole insulators 
> on the AM radio in my car.

Seattle has an electric bus system (a fraction of the whole system) that
seem to run on DC with a converter inside the bus.  The converter runs
at somewhat higher than 60Hz (or maybe three phase at 60Hz), that you
can easily hear on an AM radio while under or near the bus wires when
a bus starts up.

> Years ago, I made a lamp dimmer for an inside room that consists of a 
> two-on, center off switch that closes the circuit directly when up and 
> through a diode when down. Consider the poor pole transformer. They say 
> it can't pass DC, but pulsating DC is what the dimmed lamp runs on. So 
> where does the DC come from?

If there are other (resistive) devices on the line, some of it might
goes through them.  When the diode turns off, there will be an LdI/dt
across other devices on the line.  That might make up some of the
difference.  Otherwise, there would be a DC current through the
secondary lost as heat.  Presumably in cases where the load is a
large fraction of the transformer rated load, this loss must be
considered.  I presume this is one reason for the popularity of
other types of rectifiers.  (Though simpler filtering is probably
a more important reason.)

----------------------------------

My favorite rectifier circuit is the one used on most computer
power supplies that conveniently allows for switchable 120/240
operation.  On 240V it is a bridge rectifier into two capacitors
in series, resulting in about 300VDC.  For 120V close a switch
between the capacitor center tap and one side of the AC line,
it becomes a voltage doubler again supplying about 300VDC.

The 300VDC then gets converted to about 20kHz and into a ferrite
core transformer.

-- glen

Jerry Avins wrote:

> glen herrmannsfeldt wrote:
(snip)

>> Yes, I wasn't too serious about that one.  At the time I was wondering
>> what half cycle meant when the current and voltage are not in phase.
>> You then have to say which (current or voltage) you use to measure
>> cycles.  The OP explained the problem in more detail today.

> I haven't read today's message yet. A half cycle needn't start and end 
> at zero crossings. In fact, it can't do both in the presence of second 
> harmonic. A half cycle can start at arbitrary phase and continue for 180 
> degrees.

Some time ago I suggested a PLL locked to 60Hz.  That would follow slow
changes in frequency, but (hopefully) ignore other changes, such as
noise caused zero crossing.  That also allows easy production of a
multiple of the line frequency for synchronous sampling.

Thinking about it in terms of a second harmonic, that may be why
you want to measure it on a half cycle.  You can then determine
the second harmonic content.  One might just as well compute all
the significant harmonics, though.

-- glen