I have messages which has to be cleared
1. whether A.C voltage is effective or D.C voltage is effective. what
is the D. C equivalent A. C voltage
2. when current is passed into human body what is minimum d.c and a.c
voltages
3. when telephone is ringing what is the a.c voltage and d.c voltage
are produced. and explain how
1. DC voltage generally does more "work" than the same AC voltage -
this is because the AC voltage is constantly changing polarity and so
can't spend all it's time at peak. The equivalent voltage for AC to do
the same work as DC is called "Root Mean Squared" - you may have seen
RMS referred to a lot. When a voltmeter measures AC, it will most
likely give you RMS, but because the voltage is changing, it has to
peak much higher than this to cover up for the short falls in the
times when it is doing very little work. Consequently if you look with
an oscilloscope at 240V line voltage, you see that it actually peaks
at about 340V.
I can't remember the equation for RMS (google it), but if you remember
the magic number 0.7071, this is the ratio of DC to AC for equivalent
work done, so 1:0.7071 will give the work done by the same peak AC.
240v / 0.7071 gives 339V - so the RMS is calculated by multiplying the
peak by the ratio above.
2. the question doesn't give enough info... what are you expecting the
human body to do? Are you looking for typical resistance of body
tissues (and thus current at specific voltages?)
3. A telephone line has about 50VDC sat on it all the time. When a
ringing signal is passed to it, the 50VDC is momentarily replaced with
70-100VAC - the 50V you won't feel across your fingers but ring signal
stings a bit. The AC signal used to be generated from a cam on a motor
rotating at a set speed. If the cam has two bumps (UK BRRRR BRRRR ring
as opposed to US BRRRRRRRRRRR) on it, as it rotates, the bumps cause a
set of contacts to close and the 70VAC signal is fed to the line.
Often there were several cams to reduce the current requirements on
the ring generator - these would be out of phase with each other so as
to share the load. Modern exchanges generate the ring tone
electronically and do not use a motor with cams anymore.