ECN Forum Forums General Topics of the Trade General Discussion Area Flamed and need reassurance

right i understand that, but what i'm saying is, i know 120v can kill you, so why is it that we did this experiment in one of my classes where the teacher used a transformer to give us a jolt (using dc...collapsing field) with insanely high voltages and none of us died or were even burned?

I admittedly, will have to let one of my buddies here answer that one...

(Read: got me?)

But I will guess that the current was limited in some way, either by resistance or by time.

Where's Pauluk or Scott35 when ya need 'em?

thanks sparky, but i wont be sticking around...i feel stupid missing hte simplicity of ohm's law It just confused the crap out of me how people can be hit with large voltages with transformers involved and live (and often done in demonstrations relating to transformers) yet you can be killed by 120v. Also i know how in power transmission they increase voltage which decreases current so they can use smaller wires...then when it's back at the distrobution transformers, they do the opposite, so why couldnt they just keep decreasing voltage (which would thereby increase current, following that same logic) and have a large amount of current? Confusing confusing (even as you mentioned, it had you confused for a minute too =)

It is limited by the cost of the larger wires required to carry the current.

It is always a trade-off.

Higher voltages are limited to dielectric strength of the insulators, high current to the limit of cost of large conductors.

Voltage Drop would also cause problems, especially with the heating due to resistance which causes the resistance to rise and creates a viscious cycle.

I bid farewell to you Argile, stop back in sometime.




[This message has been edited by sparky66wv (edited 09-15-2002).]

Actually in hospitals, O.R line isolation monitors are factory calibrated to go into full alarm at 2ma because any thing over 8ma can send a heart into defib depending on the voltage and resistance right?

[This message has been edited by frank (edited 09-15-2002).]

[This message has been edited by frank (edited 09-15-2002).]

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:

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.... )

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).]

Another angle....

there is a strong Telsa following, with many pages on the 'net.

they like to fark with higher voltages....

Apparently one can jack up the frequency so as to have 99% 'skin effect' resulting in a flow around more than through conductables

I wish i had bookmarked the particular individual whom ,operative to the latter, had posted himself with lightning bolts appearing from his pointed finger....

Sparky,
That was always Tesla's secret. He used very high frequencies to perform his "tricks". When he lit the lights every mile for 16 miles with no wires in between, it was also with high freq.. I must have a dozen Tesla books, including a book of his notes, and while he thought 60 hz was good end user stuff, he wanted to deliver at much higher, which proved very impractical.

THAT was how he invented the radio, not that other guy who's name I forget (according to the 1943 decision of the Supreme Court anyway), by playing with this stuff.

'nuff said for Sunday morning.

Sparky66wv, MAN my head hurts from being slammed around so much.

We have a TV program in England called "Science Shack." A few weeks ago they did a show on lightning, so I made a point of watching.

At one point they took the presenter into a research lab, suited him up, and hoisted him into the air toward a high-voltage source charged at several hundred kV. He was drawing sparks from his fingertips like something out of a 1930s sci-fi picture!

The other impressive demonstration was putting him inside a Faraday Cage and hitting it with a 1 megavolt discharge. The slow-motion pics clearly show the arc tracking around the cage, leaving him perfectly unharmed inside.

It's simple OHM's law

I=E/R

120v/10,000 ohms (roughly your body inpedance) = 12ma

It all has to do with the electrical impulses from the heart. If 10ma is flowing = and opposite to the electrical impulse from the heart it can stop it from beating.

If you are unfortunate enough to come in contact with 18,000volts....See YA...

18,000volts/10,000ohms = 1.8amps now we are cooking flesh.

[This message has been edited by tsolanto (edited 09-15-2002).]