Was watching a Mike Holt DVD.. Have questions.

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jason

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I was watching a Mike Holt DVD, Basics Of Electricity DVD 1, and I have a question or 2. At some point in the video he takes 2 (500 or 1000') rolls of single conductor (#10 or #12) and does something. This is where it's vague. He puts one of the spools on top of the other. He also has a light bulb and a screwdriver. He says he put 120v on something somewhere and places the light bulb and screwdriver on top of the top spool of wire. The light bulb comes on.

Now the video isn't exactly clear on what he was doing or how. I watched it 4 times and couldn't figure it out. It looked like he picked up the top spool of wire and it wasn't connected to the bottom spool in any way.

Ahhh, hopefully someone saw the video I'm talking about and can tell me what he was doing and how it was working. Thanks.
 
I have not seen that video. But to me it is obvious that he is creating a transformer. It will be a crude one, at best, and it would not surprise me if he actually says so in the video.

What is happening is that he is applying a source of power, a 120 volt source, to the two leads at the two ends of one of the coils. He is then connecting a light bulb to the two leads at the two ends of the other coil. When current flows in the "primary" coil, it creates a magnetic field around the coil. The other coil, the "secondary" coil, will sense that magnetic field, and that field will establish a current in the secondary. That current will illuminate the light bulb.

How the screwdriver comes into play I am not certain. My guess is that it is intended to give the magnetic field of the first coil a stronger influence on the second coil.

Go back and watch the video again, and see if this is what is happening.
 
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charlie b said:
I have not seen that video. But to me it is obvious that he is creating a transformer. It will be a crude one, at best, and it would not surprise me if he actually says so in the video.

What is happening is that he is applying a source of power, a 120 volt source, to the two leads at the two ends of one of the coils. He is then connecting a light bulb to the two leads at the two ends of the other coil. When current flows in the "primary" coil, it creates a magnetic field around the coil. The other coil, the "secondary" coil, will sense that magnetic field, and that field will establish a current in the secondary. That current will illuminate the light bulb.

How the screwdriver comes into play I am not certain. My guess is that it is intended to give the magnetic field of the first coil a stronger influence on the second coil.

Go back and watch the video again, and see if this is what is happening.
Click to expand...

Charlie, I haven't seen the video in question, but I have done what sounds like the same demonstration in my apprenticeship classes.

The screwdriver is indeed used to simulate a crude iron core for the rudimentary transformer.

Chris
 
Thanks Charlie. That sounds like what he was doing. Even though he was doing it to show something else, electromagnetic fields? Yeah at one point he forgot to put the screwdriver through the center hole in the spool of wire (on top) and was reminded to do so by one of his guests.

So if I wanted to do this demonstration for myself, I'd hook the 120v to each end of the spool on bottom and the light bulb to each end of the spool on top? And of course stick a screwdriver through the hole in the top spool.

Thanks.
 
Two suggestions:

First and foremost, plan for safety. Find a way to avoid making any connections while power is applied. Connect the light bulb first, and the power last. I would also find a way to avoid turning this thing on by inserting a standard plug into a standard receptacle. I just put away my Christmas lights, and there was a switched cord that would work well in your experiment. You plug a standard cord into this thing, and plug this thing into the wall, and you have a switch located at the end of a 6 foot cord, so that you hand is not near the receptacle when you turn on the switch.

Secondly, presuming the two coils are the same size (I mean in terms of the number of wraps of wire that create the coil, not so much the AWG sizes of the wires), you need to make sure that the light bulb is rated for 120 volts.

You might find that the light bulb still illuminates, though perhaps not as brightly, even without the screwdriver inserted. But to be fully effective, the screwdriver has to be long enough to reach through both coils. Also, the fatter it is the better it will work. Let us know how it turns out.
 
Charlie, actually it seems like the bulb was working on about 50 volts ???? when they did what they did. Or maybe they said watts?? Also I remember the amperage being around 35 or so on the bottom coil and around 5 or 6 on the top coil. So I may have to fuse the wire at 40 amps for the experiment.

It didn't seem like Mike had any problem being close to any of it but I understand your point. Safety is always a major concern of mine. Thanks for the help.
 
The key parameter is the number of wraps of wire (also called ?the number of turns?) on each spool. Let?s assume they are the same wire size, and that the ?primary? coil (to which the 120 volts is applied) is a spool of 1000 feet of wire, and that the ?secondary? coil (to which the light bulb is connected) is a spool of 500 feet of wire. This gives you what is called a ?transformer turns ratio? of ?2 to 1.? That will cause the voltage on the secondary to be half that of the primary, and will cause the current on the secondary to be twice that of the primary.

So the design process might go like this:

1. You have a 120 volt source, and a turns ratio of 2:1. Therefore, the secondary voltage will be 60 volts.
2. You must therefore select a light bulb that is rated for at least 60 volts.
3. Go get the bulb, and see what it has for a wattage rating. Let?s (randomly) guess that you find a light bulb rated for 120 watts.
4. You divide the watts by the volts: 120 watts divided by 60 volts, gives you a current of 2 amps.
5. That means your primary current will be 1 amp. So a standard receptacle outlet should be able to handle this load.

I do not see how you are going to get a current in the order of 35 amps in this situation. If that is the primary current, then a standard receptacle will not handle that load. But 35 amps times 120 volts is over 4000 watts, and I doubt you will find a light bulb with that high a rating.
 
090207-1923 EST

That one coil of wire is 1000 ft and the other is 500 ft does not mean that one coil has half the number of turns. This is a function of spool diameter and wire size. Further unless all primary flux lines encompass the secondary, the secondary voltage is not based on the turns ratio.

The screw driver increases the flux linkage, but nothing compared to a closed high permeability core coupling the two coils.

An important consideration is the inductance of the primary because this will be the principle limiter of primary current. This is a function of the coil shape, number of turns, and the magnetic path.

The type of transformer described has a large amount of leakage inductance and thus the secondary source impedance is high meaning very poor voltage regulation with a varying load.

.
 
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