Re: 90 degree phase shift for capacitor heating
- From: Industrial Electrician <invalid@xxxxxxxxxxxxxxx>
- Date: Sat, 15 Apr 2006 18:47:59 +0000
John Popelish <jpopelish@xxxxxxxx> wrote:
Will this give me a 7VAC sine wave with close to a 90 degree phase shift
between 220V and Amplifier input?
Putting the capacitor to the hot line will give you a nearly 90 degree
phase lead, but will also differentiate any noise spikes on the line
and pass them to the resistor with little attenuation. Connecting the
capacitor at the neutral side will integrate the line voltage, causing
an almost 90 degree phase lag, but will low pass filter any noise,
giving you a much cleaner sine wave signal.
I only had time for a quick test before we closed down Friday, but
I did have time to try it both ways, and the noise is much, much
lower doing it the way you suggested. Thanks! My oscilloscope trigger
circuit thanks you as well. ;=)
A 10 uF capacitor has about 265 ohms impedance at 60 Hz. As long as
the series resistor has a much higher resistance than that, the phase
shift (across the resistor) will approach 90 degrees.
That makes sense. Just to see if I understand, the above relates to
what I read about the -3DB point because the bigger resistor moves
that frequency down, and if I then lowered the signal frequency I
would not only moving closer to the -3DB point but also lowering the
impedance of the capacitor. Am I getting all of this right?
In order to have about 10 volts appear across the capacitor impedance,
you need about .038 amperes to pass through it. A resistor of about
6000 ohms rated for at least 10 watts would provide about that current.
I see how that works. Just to see that I understand, that also means that
the resistor/capacitor phase shift has a voltage loss, and that someone
who needs the phase shift at the original voltage would need to use another
technique such as an operational amplifier based circuit. Am I getting
this right?
Then you could connect a 100k pot across the capacitor, and use the
wiper to adjust the output voltage to exactly whatever value you
wanted that any fraction of what is across the capacitor. That high
resistance load will alter the phase shift almost not at all, but will
provide a low enough impedance to drive the 1 meg input impedance with
little sag (measure the final voltage while connected to the input, or
substitute a 1 meg load across the pot to set the voltage).
That's a great idea. All of my high wattage pots are much smaller in
resistance but using a parallel pot like you suggest beans that I can
use a regular pot. That's very helpful.
It would be a very good idea to have an isolation transformer between
the line and this circuit, so the output low side has no voltage with
respect to the amplifier common.
This is a temporary setup on a wooden bench, and I have separate
3 foot lengths of 8 AWG wire connecting chassis, line neutral, and
amplifier common to an earth ground that directly connects to a 12
foot ground rod in moist soil, which in turn connects through a
short length of grounding braid to the rebar in the concrete
foundation and connects through another short length of grounding
braid to the building water pipes. I have tested this earth ground
with 250 amps. I seriously doubt that there will be a noticeable
voltage at anything that I have connected to earth ground.
Isolation transformers are very useful if you are poking around inside
a TV, but when working with high current and high voltage systems, you
are far safer by enclosing everything in an earth-grounded case, having
neutral connected to earth ground instead of floating, and never
energizing the circuit without having everything closed up inside earth-
grounded finger-proof grounded conduits and boxes. If you or "Phil
Allison" disagree, please describe, in detail, the exact path that you
think current will take through the human body from a system using the
industry-standard grounding system I am describing. I am open to being
shown that I am wrong, but not without a good explanation showing why I
and everyone else in my industry is wrong.
.
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- Re: 90 degree phase shift for capacitor heating
- From: John Popelish
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