J
John Larkin
Guest
On Sat, 20 Nov 2004 18:04:31 -0000, "john jardine"
<john@jjdesigns.fsnet.co.uk> wrote:
describe both the amplitude and the phase angle of a sine wave.
Vectors aren't bad to visualize with a little practice, just think in
terms of "how far" and "which direction"; any time you deal with
locations on a 2-d surface, vectors become pretty obvious.
The three impedances add up: Resistance, capacitance, inductance are
in series. The resistance vector is R long and, at zero degrees,
points due east. The inductor is Xl long (Xl=2*pi*f*L) and points 90
degrees, north. The cap is Xc long and points -90, south. Draw the
vectors end-to-end on a map and you get the total impedance, Z
The current is Vsource/Z, just another vector. Dump that current into
the resistor and you have the voltage across the resistor, as a vector
(voltage and angle.)
This is fairly instinctive. And then if you express the vectors as x-y
coordinates (rectangular notation, instead of polar) the math is a
little easier. The real axis is just east-west, and the imaginary, j,
axis is just north-south. It's just like pacing off distances with a
compass.
John
<john@jjdesigns.fsnet.co.uk> wrote:
No, it's really not bad, but you do have to accept vectors as a way to"Steve Evans" <smevans@jif-lemon.co.mars> wrote in message
news:rtiup090nmm9d3mg5ak7knnmpnvun6i529@4ax.com...
On 19 Nov 2004 14:38:54 GMT, cbarn24050@aol.com (CBarn24050) wrote:
For a series circuit it's best to use the current as the reference as the
current is the same throughout the cicuit. The voltage accros the
resistor will
allways be in phase with the current,
This is at odds to what I believe others here are saying. Perhpas I
havent explained it clearly what I'm getting at. I'll restate the
question:
Say an AC signal leaves a reactive chain of components with a phase
difference between its voltage and its current. The signal is fed over
an arbitrary length of PCB tracking, then through a (non-inductive)
carbon resistor and down to ground.
Now, there is clearly a phase difference across the last reactive
component in the chain. What I need to know - is this phase difference
*still* present:
a) along the PCB track?
b) across the end resistor?
(ignoring paracitic effects and treating all parts as 'ideal')
the volage accros the capacitor will lag
by 90deg for any non electrolitic cap
What\s the difference with an electrolytic?? I've never heard of this
before.
--
Fat, sugar, salt, beer: the four essentials for a healthy diet.
Steve. Your queries are perfectly logical. I know cos I've been down the
same path and never got a straightforward answer.
Problem is that VI phasing is a circuit aspect that's quite awkward to
mentally grasp or visually model what's going on in the first place, even
less to describe it in words.
The resistor doesn't care about any phase difference that exists elsewhere.
The resistor will develop a voltage across itself in sympathy with the
current through it. I.e Its volts and amps are in phase, measured -at- the
resistor.
But ... to complicate things, adding that resistor changes all the other VI
phasing angles throughout the whole of the rest of the network
Electrolytics are not perfect and electrically look like a cap and (small)
resistor in series. Here though, it just confuses the issue.
Getting ones head round the VI phase displacements in even a very simple
reactive network is nigh on impossible. -Everything- depends on everything
else in the circuit. Mentally you have forget Ohms law and try to step
rotate your imagery, through parts of a circle as each component is looked
at in turn. Sinewaves are an appalling shape to mentally deal with.
With massive effort it is sometimes possible to get a grip.
Most humans drop out at this point and either give up the electronics
subject completely, start to draw phasor diagrams or turn to the maths such
as J notation, Laplace etc. Which although giving little understanding does
at least give answers.
Add just a couple more reactive components and even the most hair shirted
mathematicians start to run for their Spice programmes.
regards
john
describe both the amplitude and the phase angle of a sine wave.
Vectors aren't bad to visualize with a little practice, just think in
terms of "how far" and "which direction"; any time you deal with
locations on a 2-d surface, vectors become pretty obvious.
The three impedances add up: Resistance, capacitance, inductance are
in series. The resistance vector is R long and, at zero degrees,
points due east. The inductor is Xl long (Xl=2*pi*f*L) and points 90
degrees, north. The cap is Xc long and points -90, south. Draw the
vectors end-to-end on a map and you get the total impedance, Z
The current is Vsource/Z, just another vector. Dump that current into
the resistor and you have the voltage across the resistor, as a vector
(voltage and angle.)
This is fairly instinctive. And then if you express the vectors as x-y
coordinates (rectangular notation, instead of polar) the math is a
little easier. The real axis is just east-west, and the imaginary, j,
axis is just north-south. It's just like pacing off distances with a
compass.
John