Somebody call me?
That danged 5-hour time difference means I get to miss all the excitement again!
Argile,
Allow me to quote from one of your posts:
If a transformer is used to increase the voltage, the current decreases. Also the same goes if you just flip the transformer around (so that the input windings are now the output windings)...your voltage is going to decrease and your current is going to increase. Therefore if you have a transformer that can lower the 120v in the wall to 10mV, that's going to create a ***** load of current...there's no way you'd be grabbing ahold of that and just "feeling a tingle". In fact, you probably wouldnt feel anything, because you'd be dead.
Your basic theory that if you decrease the voltage you simultaneously increase the current holds true for comparing the primary and secondary of a transformer delivering a specified amount of power to a load.
Example: Suppose you had a 60W 6V bulb run from a 120V outlet via a step-down xfmr. The secondary current in the bulb would be:
I = P / E = 60 / 6 = 10A.
The primary current on the 120V side (ignoring xfmr losses) would be:
I = P / E = 60 / 120 = 0.5A.
Would I put my fingers directly across the 6V secondary terminals, even though the current flowing there is 10 amps? Yes, without hesitation. Would I do the same on the primary side? No, not even if the primary xfmr current were only 1mA.
The 10A of secondary current is flowing due to the low resistance of the bulb. A 60W 6V bulb would have a filament resistance of only 0.6 ohm.
If I put one hand on each terminal of the xfmr secondary, that doesn't mean that 10A would flow through my body. I would be creating a parallel path. The bulb would still be drawing 10A, but the current going through me would be limited by my body resistance. If my hand-to-hand resistance were, say, 10K ohms, the current through me would be just 0.6mA. The fact that 10A is flowing in the rest of the circuit is irrelevant -- Ohm's Law will dictate the current through my body based on the applied voltage and my body resistance.
If I were to touch the 120V primary terminals with the same 10K body resistance, the current would be much higher: 120/10K = 12mA.
Taking your example of stepping down the voltage to 10mV, yes, if it were to provide any reasonable amount of power the current would be massive. If you could have a 60W 10mV bulb it would draw 6000 amps! (Boy, that would be some transformer!
)
But if I touched the 10mV secondary terminals I wouldn't feel a thing. With the same 10K body resistance, the current through me would be just 1uA (that's 1 microamp, or 0.000001A), even though there might be 6000A flowing through the circuit.
Virgil,
The collapsing field experiment has two things to consider. First, disconnecting DC from the primary causes a pulse of secondary current of very short duration. (Think automotive ignition coil.) Second, it's quite likely that the coil used had a very high turns ratio and the secondary winding would have an appreciable amount of resistance to limit the current.
The electric fence controller is a good example. The
open-circuit voltage of each pulse may well reach a couple of thousand volts or more. But it's fed from a relatively high-impedance source which will severely limit the available current. When you grab hold of the wire, your body resistance to ground forms a potential divider with that source impedance so that the voltage to ground actually appearing on the wire (and across you!) is then much, much lower.
Another example of a high-voltage with a high-Z source is the final anode supply to the tube in your TV (or computer monitor). Even on a small tube, the final anode can run on 10kV or more, and for a big-screen color set it's easily in excess of 30kV. However, the source (in all modern sets) is a winding on the line-output xfmr and a voltage multiplier chain. The overall source impedance is very high, in the order of many megohms, so the current is severely limited, e.g. 15kV with source Z of 10 MEG = max. current of 1.5mA. You can draw sparks from the HT lead, and get fantastic blue corona displays around the tube connector in a darkened room, but there are many TV techs who have touched it and lived to tell the tale. (Probably a little cursing was heard....
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In old tube-type TVs a far more dangerous jolt could be had from the line sweep boost rectifier, typically 500 to 800V with an available current of several tens of milliamps.
[This message has been edited by pauluk (edited 09-15-2002).]