This is a dumb newby question, which ought to have been posted on sci.electronics.basics.
alternating current and direct current can both be effective in many applications. In some you need to use one or the other. When it comes to heating up a resistive load, the heating effect of the sinusodal alternating current has to be integrated over one full cycle of the sine wave, and people talk about the root means square voltage averaged over the full cycle, which turns out to be 2/pi of the peak voltage.
The "230V" 50Hz main voltage has the same heating effect on a resistor as a 230V DC voltage, but the alternating current cycles between peak values of +361V and -361V.
50Hz AC is much more effective at damaging human beings than direct current.
Voltages less than 24V - in the range +24V to -24V - are legally accepted as not dangerous.
Telephone ring tone is an interrrupted direct voltage, nominally
50V, though it can be quite a lot lower at the end of a long connection to the local exchange.
It used to be produced by 48V stacks of lead acid batteries. I don't know what is typically used today.
Not quite right here - telephone line voltage is generally -50VDC - the ring voltage is usually 70-100 VAC
The line voltage drops to about 11V when the receiver is lifted, this is down to the voltage drop of the telephone circuit and the "relay coil" in the exchange (used to be a relay but now simulated for compatibility by the electronics)
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.
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?)
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.
"Start me up"! "Come on, Baby, light my fire"! "Ring of fire"! (cue Johhny Cash) "I saw the light"! (cue Hank Williams) "Great balls of fire"! (cue Jerry Lee Lewis)
;-)
--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
I got a tour of the #5 ESS installation in our local phone exchange (I'm pretty sure they are STILL running this system) and they had an ANCIENT looking, like 1930's rotary converter with cam-operated switches to generate the
90 V 20 Hz ring signal, as well as the 600 + 1000 Hz dial tone signal. I was astonished that they were using such an antique, but it apparently was the accepted way to do it. It had oil cups that had to be filled every couple of days to keep the Babbit bearings from burning up. I think it had a centrifugal governor roughly of the fly-ball sort that switched contacts on the DC motor field circuit. It ran from the 48 V main system batteries, just like everything else, including some of the building lights. They had a Solar gas turbine generator set on the roof for extended power outages.
you are absolutely right Jon. I was in Skyport Exchange (at Heathrow) a couple of months ago and while they don't actually uses this generator in daily use, they still have two as standby. They are both kept running (as they have done for years & years). the reason behind the thinking is that in times of national uproar (worst case scenario a nuking - remember this stuff has been there since the 50's when it all looked like it was gonna go off with the USSR) a motor and contacts will not be affected by EMP. If they pull the preference jumpers (and the phone system becomes specific to government and the emergency services) they can resort ot an electrical system based around relays and stuff which is rad-hard.
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