# What is the importance of phase shift???

Discussion in 'Electronic Basics' started by sf1, Oct 11, 2005.

1. ### sf1Guest

i study RLC circuits in school but i just dont understand why i stud
it for..
that mean and why is it important to study it

2. ### Lord GarthGuest

Good old ELI the ICE man! It's just there to weed out those that aren't
serious

;-)

3. ### John PopelishGuest

Right now, you are just learning what the components do (how the
voltage across them relates to the current through them. Once you get
into amplifiers with feedback networks, one important effect of such
phase shifts is that it can turn negative feedback into positive
feedback and turn an amplifier into an oscillator. Very useful when
you need it, very bad when you don't want it. You will find that any
component with a sense of time (resistors don't remember anything
except a severe overload) involves phase shift between its voltage and
current. This is part of its memory of what has been going on,
recently. The present value of the relationship is a combination of
the present signal and the memory of earlier signals.

4. ### Kitchen ManGuest

On Mon, 10 Oct 2005 19:35:54 -0500,
You study it in order to get a diploma that pays well for a lifetime.

5. ### BobGuest

First, try to really understand what a resistor does. It's pretty easy,
because the relationship between the voltage across a resistor and the
current through that resistor is always fixed -- at all points in time, and
for all types of applied signals. The current through a resistor is ALWAYS
in phase with the voltage across the resistor regardless of the applied
waveshape of the voltage (or current).

Also, understand that resistors do NOT store electrical energy, they only
radiate the energy as heat. Disconnect them from a circuit and you will not
be able to recover any of the energy that has been radiated by them.

Inductors and capacitors ARE electrical energy storage devices. If an
un-energized L or C is somehow enticed to store some electrical energy then
it takes time to change that stored energy from one level to another
(regardless of whether you're increasing or decreasing its energy level).

Here's an easy way to understand a capacitor:

If you start with an un-energized C (i.e., its initial voltage = 0V), and
then connect that C to a constant current source, then the voltage across
the capacitor will increase linearly with time. The voltage is "lagging" the
current because it takes time for the voltage (and energy) to build up.

If you start with an un-energized C, and then connect that C to a current
source that fluctuates with a sinusoidal characteristic, then the voltage
will also have a sinusoidal characteristic, but it will have a phase shift
of 90degrees with respect to the current through that C. This 90degree
relationship is ONLY true for a sinusoidal stimulus. The voltage is said to
"lag" the current by 90degrees (for a sin stimulus).

A similar thing happens with an inductor. If you connect that L to a
constant voltage source then the current will increase linearly with time.
If you apply a sinusoidal voltage then the current will have a phase shift
of 90degrees with respect to the voltage across that L. This 90degree
by 90degrees (for a sin stimulus).

Think and experiment.

Bob

6. ### PeteSGuest

Further to John's post

RC networks have certain characteristics (phase shift amongst them).
These are found everywhere (even when you don't expect it) and you need
to understand what is happening - to understand that, you need to
understand the basics, of which RC networks is merely a part.

Apart from that, we deliberately exploit the effects of RC networks, so
a thorough understanding of what is *going* to happen is required.

As some food for thought (where you can have a capacitor without
knowing it) - capacitance exists between any two points of differing
potential (voltage).

Cheers

PeteS

7. ### redbellyGuest

Bob,

I do hope the OP appreciates the time you have spent on a thoughtful
post. My own feelings on the matter are either
1. He's a troll
or
2. He is severely lacking the math prequisites (i.e., familiarity with
the sine and cosine functions) needed to understand AC circuits.

If he were a home hobbyist trying to teach himself electronics, it
might be different. But he is taking a class in school, surrounded by

Regards,

Mark

8. ### BobGuest

Mark,

You may be right. There's always that chance that he's a troll, but I didn't
sense that.

These concepts are difficult, at first. There may be nobody around that can
explain them in such a way that they "click". When I was in school, I had
the luxury of working with some experienced engineers. They tought me most
of the fundamentals. The books and teachers just confirmed what they were
telling me.

Bob

9. ### Bill BowdenGuest

i study RLC circuits in school but i just dont understand why i study
Well, think about current charging a capacitor. If you decrease the
charge current, the capacitor still charges and the voltage continues
to rise because current is still flowing into the capacitor. The only
way to stop the voltage from rising is to reduce the current to zero,
or go negative.

That means the maximum voltage will be reached when the current is
zero, and visa versa. Thus, you get a 90 degree phase difference
between the capacitor voltage and current.

-Bill

10. ### redbellyGuest

Now I believe you're right, a troll would have been more active with
follow up responses in the thread. In that case, I wish him luck in
his studies.
Getting things to click is the key. If somebody asked me in person
about phases, I could sketch a graph of two sine waves with a phase
offset, and say "Look ... this wave leads the other wave".
Good one!

11. ### Rich GriseGuest

The "click" came for me when the teacher said, "A capacitor opposes
a change in voltage, and an inductor opposes a change in current."

Cheers!
Rich

12. ### ehsjrGuest

Studying electronics without studying RLC circuits would be like
studying to be an MD without studying the skeleton. To be effective
at designing or working on electronic circuits, you need to
understand what each (resistance, inductance, capacitance) does
in the circuit.

Ed

13. ### BobGGuest

A big click of the lightbulb up above the head came when someone said
the cap in an RC lo pass filter was just a 'frequency dependent
resistor', and the filter had an R in series and this magic resistor to
ground, just like a voltage divider or a pot. At hi frequencies, the
bottom R is real small, and the voltage divider is turned all the way
down. AT DC, the bottom R is real big, and the voltage divider is all
the way up. Great intuitive seat of the pants explanation that I never
heard in 5 years of engineering school and maxwells equations and a
bunch of other stuff. To the guy wondering whats important: I get A LOT
of mileage out of ohm's law.... like use it every day. Learn it.
Memorize it.

14. ### YDGuest

It didn't "click" for me until I realized you could force the
_current_ and measure the voltage. Until then I had only thought in
terms of applying a voltage and measuring the current.

- YD.