# Re: A few questions...

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> 1) What is RMS? I have a vague idea that the letters stand for Root Mean
> squared but what does this mean and what are the applications for it?

rms == root mean squared as you say... What is it's significance?  Think
about the instantaneous power being carried or dissipated... It is
proportional to the square of the voltage (or current).  So, if you average
the instantaneous power, you get the average power.. Now, to get the
corresponding voltage, you take the square root.. SO.. If you have a source
with a certain RMS voltage, it will dissipate the same power in the load,
regardless of what the actual peak voltage is, or what the average voltage
is, or whatever....

You can also think of the RMS voltage as the DC voltage that would
dissipate the same power in the load.  Likewise, if you are running a
motor, or getting some sort of useful work out, then RMS voltages are what
you want to work with, because the Average power is proportional to the RMS
Voltage.

Take some examples:

DC   - Easy one, RMS voltage = DC voltage...
50% duty cycle square wave on/off = Power when on = V^2, Power when off =
0, so average power = .5 * V^2. so RMS voltage (=sqrt(power)) = .707 Vpeak

1% duty cycle pulse on off = power when on = V^2. Poff = 0, Pave = .01 V^2,
Vrms = .1 Vpeak

50% duty cycle bipolar square wave alternating between +V, -V, P+ = V^2. P-
= V^2, Pave = V^2, Vrms = Vpeak.

Now it gets tricky...

A sine wave: V = Vpeak * sin(omega * t)... Power = Vpeak^2*sin(omega t)^2 =
.5*Vpeak^2*(1 + sin(omega *2 * t)). Pave = .5 * Vpeak^2, Vrms= .707 *
Vpeak....

You can see that the ratio of Vrms to Vpeak varies quite a bit depending on
the waveform.  Vrms is never > Vpeak, by the way (at DC it is the same).
Most cheap AC meters just measure the peak voltage, then multiply that by
.707 to get the RMS voltage.  A "true rms" meter actually measures the RMS
voltage, either by sampling quickly and doing the calculations on the fly
(typical of most RMS meters), or by measuring the heating of a calibrated
resistor (typical of laboratory standards equipment). That little battery
tester on the side of the battery is actually a true RMS meter....

>
> 2) I've seen the number 555 several times, what does this mean?

Refers to a very common timing IC with a multitude of uses.  If you look it
up, you'll want to look for NE555 or SE555.

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