# AC sine wave: What does increasing the frequency do?

Discussion in 'Electronic Basics' started by Commander Dave, Nov 26, 2004.

1. ### John LarkinGuest

At the intuitive level, a Fourier series answers the question "how
much does this waveform look like a 60 Hz sine wave? How much like a
120 Hz sine wave?...". The answers are of the form "2 volts, 45
degrees" and such, one answer for DC and one for each harmonic. The
Fourier transform is a math operation that gives these answers. It
produces the same results you could get using a bandpass filter bank
at f, 2f, etc (plus the DC term, the zero frequency Fourier term,
which you'd get using a lowpass filter).

You can do an eyeball Fourier by printing the waveform on a piece of
paper. Suppose some waveform has a basic frequency of 60 Hz. Now plot
a 60 Hz sinewave on another piece of paper and hold it next to the
original waveform. Slide it horizontally until you see the best match,
so that the input waveform "helps" the sinewave template, pushing it
up and pulling it down in the best places. Now make a rough estimate
of how much it helps (amplitude), and how far you shifted the papers
time. Fourier!!

The SCR phase control waveform at 50% on has power on the load in the
last half of each half-cycle. That shifts the center-of-gravity of the
waveform later in time from the line voltage wave, so the fundamental
component, the 60 Hz Fourier line, lags. By something like 32 degrees,
some people have calculated in other posts. That does look inductive.

John

2. ### John LarkinGuest

Bright. Won't last long.

John

3. ### peterkenGuest

lasts like forever if the diode is selected correctly
(say 1N4007 or so for standard domestic bulbs)

5. ### peterkenGuest

"standard domestic bulbs" is any type from say 5W upto 250W
for these types power rating of 1N4007 diode is ok

6. ### John FieldsGuest

---
Perhaps you missed this:

"Oh, this is nothing compared to the thread I instigated a couple of
years ago about running a 120V bulb off 240V mains by just putting a
diode in series!"

Notice that it's not a 120V lamp running on 1/2 wave rectified 120V
mains, it's a 120V lamp running on 1/2 wave rectified 240V mains.

That being the case, how much power would a 120V 100W lamp dissipate
if it were being run on 1/2 wave rectified 240V?

Don't forget to consider the tempco and thermal time constant of the
filament...

Not bright.

John

Uh-oh.