electric coils (ex: transformers)

[Red Squirrel]

Something I've been wondering about since I was a kid... which is where I learned the dangers of short circuits by trying it at home and playing with the mains power. (I still do, but now I'm more careful. )

A coil is technically a short circuit as the wire just goes from negative and goes back to positive, even though it is very long because it's forming a coil, the resistance is not really enough to loose all the power once at the other end of the circuit. And especially with 120VAC it would take a crazy ammount of turns to make enough resistance so that there's 0 volts left at the other end so that it does not actually cause a short circuit but instead acts as a load (light bulb etc).

My question is, how do they make transformers and such so that it does not blow up when turned on? I had the thought before that it may be in a vacuum, but that would not do anything since it would just "blow up" at the switch. Even if you just let it blow up and keep it on, it would get insanely hot and melt down.

I understand that AC + and - changes every 60 seconds, but it still makes short circuits the same as DC.

I noticed with low voltages it does not do much, as I've built various coils with 17VAC but the adapter did get very hot so I assume leaving it plugged in for long would make it blow. (ex: if I built a door magnet or something)

So how do they do coils so it does not short out like that?

 

[Geniere]

Indeed a coil is a short circuit to DC, but NOT instantaneously. As DC begins to flow through the coil, a magnetic field develops. This magnetic field acts in opposition to the DC voltage source and lessons the amount of current. As time progresses the current will increase at a lower rate and the opposing magnetic field grows a lower rate. Eventually the current will only be limited by the diameter of the wire forming the coil. The limitation of the DC by the wire size is called resistance and its unit is the ohm. The instantaneous limitation of DC by the induced opposing magnetic field is called impedance. Its unit is also called an ohm. If the power source produces AC, the current is caused to flow in opposite directions 60 times per second. Each reversal is accompanied by a magnetic field opposing the source voltage. The end result is that to an AC voltage source, the coil is not a short circuit. This is a very basic answer to your question neglecting some other factors.

 

[MrThermistor]

High resistance.

Have you ever tried installing a transformer backwards?

 

[CTho9305]

Originally posted by: MrThermistor
High resistance.

Have you ever tried installing a transformer backwards?

I would expect that a transformer would have almost zero DC impedance.

 

[Heisenberg]

Transformers utilize (among other things) a property called inductance. Basically, inductance is a property that resists changes in the current direction and magnitude. For low frequencies like the 60Hz that US power is, transformers are designed so that if no load is present on the secondary winding, very little current will flow in the primary windings. This is how a transformer doesn't just act like a short circuit when there's nothing connected to the other side. Also, transformers are non-Ohmic, meaning V=IR doesn't work for them, so you can't assume that all the voltage has to be dropped across the transformer.

 

[Mday]

typically there is a resistance in the primary coil which is established by having a really long wire. in terms of simulation, any spice program will yell at you for not putting a resistor in series with the primary coil.

 

[RossGr]

Originally posted by: MrThermistor
High resistance.

Have you ever tried installing a transformer backwards?

Yep, and it didn't make any difference. Transformers are not polarity sensitive.


The Resistance of a transformer depends on diameter, length and material of the wire, they all have a significant and measureable resistance. The impedence is determined by the number of coils and frequency of the applied voltage, the more turns of wire the higher the higher the impedence, the higher the frequency the higher the impedence. This is one reason why one must be careful useing American made electronics in Europe, their AC is 50Hz so transformers have a lower impedence.(~220VAC is another factor).

 

[Mark R]

As has been said before, transformers (or indeed any coil of wire, or in fact any length of wire) has a property called 'inductance'. When you turn on a switch in a circuit, the current does not instantly start to flow, instead the current 'ramps up' to the steady state value.

In a simple circuit, with a battery, switch and a light - the inductance in the circuit is extremely low, and the current will ramp up to its maximum level in a few microseconds.

In an AC circuit, the voltage is constantly changing and more importantly constantly changing direction - it flows one way and then the other. Inductance acts like intertia - it takes time for the current to change with the voltage. That means that in an AC circuit an inductor limits current - the current only gets to a certain point before the voltage changes direction and starts pulling the current in the other direction. This means that a higher frequency of AC will push a lower amount of current through an inductor. If you put a diode in series with the input to a transformer (thereby converting the AC to DC) it will promptly catch fire or blow fuses because the inductance will not limit a DC current - the DC current will be limited by the low DC resistance of the winding.


A classic example of this is the little generators for bicycle lights. The lights come on at low speed, but don't burnout even if you are going 5x the speed. The generators are in-fact alternators, and as the generator speeds up, so the voltage and frequency produced increases - but because the generator is designed to have a high inductance, at the higher frequency the current is restricted by the inductance preventing the bulb from blowing.

 

[Red Squirrel]

Interesting, so because it takes some time for the electrons to get from (-) to (+) terminal of the source, it does not short out, as with AC it usually never actually gets to the (+)? So If I make a big enough coil and plug it in, it should not blow up, correct? Or is there more to it then that? Let's assume I'm using copper wire or something which has low resistance, so I'd obviously have to make quite allot of turns. But is that all there is to it? I always thought electricity did not really have a "speed", where no mater how long a conductor is, the electrons are still at the end instantly.

 

[RossGr]

It is all about E&M fields. An inductor is and interaction of electron current and electric fields. As has been said already there is an AC equivelent of resistance called impedence. This impedence is due to the CHANGING electro-magnetic fields created by the AC current. The size and number of turns on the inductor as well as the frequency of the applied voltage determine determine this impedence.

The only connection between the primary coils and the secondary coils of a transfromer is Electro-Magnetic coupling of the fields. This is why you will sometimes hear of something called an isolation transformer. There is no direct connection from one part of a circiut to the next, only electromagnetic coupling through the transformer.

 

[PowerEngineer]

Originally posted by: RedSquirrel
Interesting, so because it takes some time for the electrons to get from (-) to (+) terminal of the source, it does not short out, as with AC it usually never actually gets to the (+)? So If I make a big enough coil and plug it in, it should not blow up, correct? Or is there more to it then that? Let's assume I'm using copper wire or something which has low resistance, so I'd obviously have to make quite allot of turns. But is that all there is to it? I always thought electricity did not really have a "speed", where no mater how long a conductor is, the electrons are still at the end instantly.

As people have already said, the reason that a coil is not just a short circuit in an AC circuit is that it has an inductive nature that opposes changes in current. For AC circuits, V=IZ where Z is impedance, and Z=R+jX where X is the reactive impedance (and the j means that the X component is in quadrature to the R, making this a vector sum). For a coil, X is directly proportional to frequency and to the inductance of the coil. The inductance of the coil is not affected by the conductive material of the coil (i.e. the size or metal used in the wire). It is affected by the number of turns in coil, the geometry of the coil, and the material around the coil through which the magnetic field flows. If you want to enhance the inductance of a coil, you want to wrap it around a "field friendly" material like iron. In most 50-60 Hz electrical systems, the reactive impedance components of equipment (e.g. transformers, generators, lines,etc.) are so much larger than their resistances, that you can ignore the resistances completely and still get a reasonably accurate answer for currents. It's the reactive impedance that really counts!

Before you launch into "trying this at home", I strongly recommend you explore the theory behind AC systems more thoroughly. Calculating the reactive impedance of a coil is not trivial; you could find yourself with another virtual short circuit if you make a mistake. If you insist on the "hands on" approach, then I strongly advise you to find a 5V AC adapter and carefully experiment at that voltage level. You should also put some resistance in series with the coil to limit the current to the rated amount (and make sure that wattage ratings on the resistors are adequate).

P.S. -- Electricity is electromagnetic radiation just like light, and (like light) travels at the speed of light.

 

[Dctr]

I think there's a bit of confusion going on here so I'll try to clear up what I can.

1). Don't think of a short circuit as having 0 resistance. All non-super conducting material has some resistance, so the curent passing through will be finite.

2). Any voltage source will also have inherent limitations on the amount of current it can produce further limiting the current through a short

3). Too much current = too much heat. The amount of current a wire can take is proportion to the resistance of the wire. Small wire = high resistance (for a wire), that's why very high current applications use busbar.

4). Wires never blow up, they melt. Same thing with current produced by a short. But shorts can overdrive voltages which can cause other components blow up (drum capacitors are about the most spectacular)

5). Coils have inductance w/ acts as a LPF (Low pass filter) So they pass DC current and Block AC (essentially). You can cook a coil w/ enough dc current. (This also means that a coil will be rated for different voltages at different frequencies)

6). Transformers are a bit more complicated than coils. Not to go too deep into the theory but if you wrap a wire (Primary - p) around ferrite (say N turns) and pass ac you create flux. This flux is propotional to N. Now if you wrap another wire (Secondary - s) around the same ferrite (say M turns) it has the same amount of flux passing through it. Bunch of math later you get Vp/Vs=N/M.

Current is a bit more complicated as the one side of the tx has to drive the other side, but it works similar to bycycle gears. Step voltage up and you need much more current on the primary side to drive the secondary. Step down and current is easier to generate on the secondary.

If you flip the primary and secondary sides on a a tx you invert what happens. A 12:1 step down that takes 120Vrms to 10Vrms flipped would generate 1440Vrms and would probably fry whatever was on the other side.

Reversing the polarity of a side can also have an impact on the circuit, depending on the nature of the tx. Some coils have taps on the secondary coils to generate fractional voltages. (This is the way you get 110 in your house, the network provides you 220 that is center tapped to provide 3 wires (N +110 -110) ) So if you flip the polarity on a secondary with a nonsymetrical tap you can fubar a circuit.

(reversing the primary won't do anything as long as the secondary is isolated from the primary as it is only a 180 shift in phase)

7). MOST IMPORTANTLY - fool around w/ low voltage electronics as much as you want, worst you can do is ruin whatever you are working with. But unless you are an expert DO NOT FOOL AROUND WITH TRANSFORMERS, THEY ARE NOT TOYS!, especially ones in microwaves, tv's etc.. These are dangerous even when off and unplugged becasue the can have large capacitors capable of storing imense amounts of charge at imense voltages which can discharge almost instantaneosly. They can and will KILL you if you screw up.

 

[Red Squirrel]

Yeah transformers I understand how they work. I reversed a 17VAC adapter once, it wasn't pretty. I charged a capatitor, then hooked it up and touched the other end and it "stepped up" the capacitor to around 100volts.

I think I understand more why coils don't do shorts. I probably won't try it though, too risky if I misscalculate something, and with a 30% math average it's something I might do. (though this is simple math in this case).

I'm a electric dare devil, I've been known to blow fuses and get things to turn black. Either I'm good with the safety part, or I'm just plain lucky.

 

[DrPizza]

Originally posted by: RedSquirrel
Interesting, so because it takes some time for the electrons to get from (-) to (+) terminal of the source, it does not short out, as with AC it usually never actually gets to the (+)? So If I make a big enough coil and plug it in, it should not blow up, correct? Or is there more to it then that? Let's assume I'm using copper wire or something which has low resistance, so I'd obviously have to make quite allot of turns. But is that all there is to it? I always thought electricity did not really have a "speed", where no mater how long a conductor is, the electrons are still at the end instantly.

And, wanting to clear up one major misconception in electronics, I think I'm about to add just slightly to the complexity. If you're thinking of current as the flow of electrons, that's just fine. However, the electrons are not moving at the speed of light. (In fact, they're barely moving at all) An oversimplified analogy is this:

Imagine a miniature choo-choo train on the wire, stretching the length of the wire. Except the locomotive is at the rear of the train pushing it. When you start the train (close the circuit), the locomotive pushes against the next car, which in turn pushes against the next car, etc. (electrons pushing on each other) When the train starts, the caboose (at the other end of the train) moves almost instantly... this push travels along the wire at [edit: 1/3 of] the speed of light, but the cars (electrons) don't travel that fast.

 

[rjain]

I think the typical rule of thumb is that the signal propagates at 1/3 of the speed of light

 

[DrPizza]

Thanks, rjain, I had just thought about it and realized I did that. You're correct, about 1/3 the speed of light, not the speed of light.

 

[FrankSchwab]

(sigh) How about 2/3 the speed of light?

At least in Coax or Twisted pair wiring.

Having done my share of coding for time-domain reflectometers, I'd be willing to bet a beer on it.

For example, the nominal velocity of propagation for
this Cat5e Cable
is 70% of c.

Never mind, that's a session-based URL. You can search the Belden site for Technical info on their Cat5e UTP cables to find the info, though.

/frank