# How does a transformer work?

david90
how does a transformer work? Transformers could make 9v from a battery into say 250v but isn't that getting something for nothing? what about conconservation of energy?

Does a capacitor with 9v potential discharge energy faster than a 9v battery? Can capacitors be replace with transformers since transformers can also output a high voltage from a low voltage source?

Staff Emeritus
Gold Member

Originally posted by david90
how does a transformer work?

A transformer relies on properties of Electo Magnetism. Essentially a changing EM field produces a current. So a AC current is applied to the primary windings, this current produces a changing EM field, which induces a current in the secondary windings. The Voltage and current produced in the secondary is determined by the ratio of windings between the primary and secondary coils.
Transformers could make 9v from a battery into say 250v but isn't that getting something for nothing? what about conservation of energy?
No, transformers require AC, they do not work as a transformer when a DC voltage is applied. When AC is applied the ratio of Current to voltage remains the same. So a Transformer which steps up voltage, steps down current. Likewise a Voltage step down transformer steps up current. The only change in power out is losses due to the fact that the materials are real.

Does a capacitor with 9v potential discharge energy faster than a 9v battery? Can capacitors be replace with transformers since transformers can also output a high voltage from a low voltage source?
The discharge rate of a capacitor depends on the load just as the current from a battery does, it is not an easy thing to compare.
Capacitors and transformers are complementary electronics components they each perform unique tasks and definitely cannot be exchanged.

scientist0
yep

the voltage is increased or decreased, depending on the number turns in the secondary coil.
but Power remains constant ... so energy conserved...

scientist0
power(watt) = voltage(V) x current(A)
(P=VI)

the power input = power output
in a transformer

so V and I change themselves keeping the power P constant

& since Energy = Power x time

Energy is conserved

david90
if I attach a 9v bat to a transformer that converts it to 1000v, would it shock me if I touch the transformer terminals?

One time I connect a 9v to the output end of a transformer and touched the two prongs that go into the wall socket and it shocked me. why?

Staff Emeritus
Gold Member
Originally posted by david90
if I attach a 9v bat to a transformer that converts it to 1000v, would it shock me if I touch the transformer terminals?

Read my first post, no transformer will convert 9VDC to anything.
One time I connect a 9v to the output end of a transformer and touched the two prongs that go into the wall socket and it shocked me. why?

It is really hard to say. Since I was not there to see what you were doing, likely it would have shocked you with or without the battery.

david90
Will turning the current to a solenoid or coil on and off rapidly shock u if u touch the circuit? If so why?

S = k log w
Because it changes the potential over time. Actually placing a battery would cause a change on the other side, if the battery were to run down, depending on the resistance of the circuit. It might be too small to measure. A constant VA power supply, would not.

Mentor
Originally posted by david90
Will turning the current to a solenoid or coil on and off rapidly shock u if u touch the circuit? If so why?
Touching any energized circuit could shock you. These are not experiments you should be performing unless you are competing for a Darwin Award.

Homework Helper
Originally posted by david90
Will turning the current to a solenoid or coil on and off rapidly shock u if u touch the circuit? If so why?
Of course it could. You didn't specify the amount of current or the threshold of electrification that qualifies as "shock." If you've got current, then you can get shocked with or without the solenoid.

Braumin
A transformer is two inductors a certain distance apart, but not touching. An insulated Iron core is put through both inductors, and is used to carry the magnetic field. Like it has been said, they only work with AC current. If you have 10,000,000 Volts at 1 Amp, and step it down to 110 Volts, you get about 90,000 Amps. It takes heavy gauge wire to move a lot of amps, so high voltage is used instead so that smaller wire can be used. Remember Power(watts)=Volts*Amps

As for the Capacitor question, if you have a Cap charged to 9V, and you wanted to discharge it, it would discharge at a rate proportional to the resistance in the discharge circuit. After about 5 time constants, the cap would be fully discharged. If you have a 1microFarad Cap in series with a 1kOhm resistor, the time constant would be 1E-6*1E3=1millisecond. It would therefore take 5ms to fully discharge the cap.

zoobyshoe
Originally posted by david90
Will turning the current to a solenoid or coil on and off rapidly shock u if u touch the circuit? If so why?
If you imput pulsed DC to the primary of a transformer you will get pulsed DC out of the secondary. This could give you a painful shock. From my own experimentation the shocks start to be painful around 20-25 volts if you hold one or both of the terminals loosly. Gripping them tightly may only cause a tingle, but if there is enough of a gap for a spark to jump it hurts.

If you got a shock from imputting non-pulsed DC into a transformer it came either from the initial change in the magnetic field, or the change that occurred from breaking the circuit. The magnetic field has to be changing to induce current in the secondary. Steady DC, once it has come up to full strength, will not produce current or voltage in the secondary. There will, however, be a surge when you first close the circuit, and then another when you open it i.e. when the magnetic field is in a state of change. Repeatedly opening and closing the circuit to the primary with DC will, indeed, produce eqivalently repeated voltage and current in the secondary. With a nine volt imput your transformer would only have to do a little more than double the voltage for it to hurt.

Originally posted by david90
how does a transformer work? Transformers could make 9v from a battery into say 250v but isn't that getting something for nothing? what about conconservation of energy?

Does a capacitor with 9v potential discharge energy faster than a 9v battery? Can capacitors be replace with transformers since transformers can also output a high voltage from a low voltage source?

As stated by many responders, you cannot transform a low DC voltage to high unless you make it AC first. One way this is done is by a multivibrator. (Look up info with Google.) These are used in electronic flashes. A low voltage, typically 3 or 6 volts, is "transformed" by first going to AC of a usually audible frequency (that high-pitched noise you hear as the flash is charging), rectified and used to charge up a capacitor to around 300 volts. When the flash is triggered, the capacitor is discharged into the flash tube.

zoobyshoe

Originally posted by krab
As stated by many responders, you cannot transform a low DC voltage to high unless you make it AC first.
The transformer, per se, is designed to run on AC.

You don't, however, need to actually convert the DC to AC to transform the voltage. What is necessary is to keep the strength of the DC always varying. DC from chemical cells was being converted from low to high voltages (long before anyone knew how to generate AC) by means of induction coils which pulsed the DC by means of vibrating switches that turned the current off and on several times a second. Here is a link:

Induction Coils

Homework Helper
Originally posted by zoobyshoe
From my own experimentation the shocks start to be painful around 20-25 volts if you hold one or both of the terminals loosly. Gripping them tightly may only cause a tingle, but if there is enough of a gap for a spark to jump it hurts.
This is the same principle on which the spark plug works. The air has a breakdown field, above which the molecules ionize and pass current. If you grip tightly, the potential doesn't have to get as high as it does to achieve breakdown across the gap between the terminal and the finger. If you grip loosely, you increase the potential difference required for breakdown, thus increasing the amount of energy being stored before the zap.

philipc
since the subject is about transformer and DC power, figured I might ask?
What is the best way to drive a transformer with DC? The 555 chip? I want to build some back up fluorescent lights using a 12 volt battery and a 6 volt transformer wired backwards, am I thinking stupid here?
Philip

zoobyshoe
If you have a 12 volt battery I'm thinking the easiest way would be just to get a power inverter. Decide how many lights you want to run and get an inverter than can put out the wattage you'll need. I bought a 100 watt inverter second hand once, pulled off the plug that goes into the cigarette lighter and soldered clips on to clip it right onto the battery terminals.

david90
since dc won't work with a transformer, what if u charge a capacitor with dc circuit then discharge it to a transformer, would the voltage be amplified? why?

When u first connect a dc power to a transformer, doesn't the voltage gets amplified then drops to zero? change in magnetic field right?

also, I'm hearing people say that amp kills not voltage. So if u cut yourself and stick a 9v bat to the cut, it would kill u? Would a 1.5v bat kill if the resistance of ur body is low enough to get high amp?

The van graf machine makes 10k volt so why is safe to touch it? Would u feel a shock if ur barefooted and touched the dome?

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Originally posted by philipc
What is the best way to drive a transformer with DC?
Depends on the application.

The 555 chip?
Simple, inexpensive, and lends itself to easy manipulation of pulse width.

I want to build some back up fluorescent lights using a 12 volt battery and a 6 volt transformer wired backwards, am I thinking stupid here?
Not at all, it’s quite doable, though the 555 would be best used to drive the input of a power transistor which in turn pulses the DC through the transformer (in order to handle a heavier current). Still, without the power transistor you could readily drive a neon lamp if you so desired. This is an extremely easy ciruit that can be built in less than an hour.

Good luck, and try not to get shocked.

philipc
Terry,
Thanks for the input, question about the transformer(since this is a transformer thread )I was thinking about getting a regular step down 120 to 6 or maybe 12 volt transformer, and wiring it backwards. I only plan driving a 4 watt bulb with it, pretty sure it won't over do it? Would you happen to have any other ideas for a transformer for this case?
Thanks
Philip

Braumin
"also, I'm hearing people say that amp kills not voltage. So if u cut yourself and stick a 9v bat to the cut, it would kill u? Would a 1.5v bat kill if the resistance of ur body is low enough to get high amp?"

I guess in theory a 9V battery could kill you, but it can't for a couple reasons. First off, the resistance in your body is pretty high to get much current with 9 volts. Second, the battery can usually only put out a number in the milliamps anyways. As far as the 1.5 volt, sure if your body had low enough resistance you could get enough amps to kill you, however, your body doesn not have the .00000000001 ohm resistance anyways.

"The van graf machine makes 10k volt so why is safe to touch it? Would u feel a shock if ur barefooted and touched the dome?"

You answered your own question, it is the amps that kills not the volts. Even though it is 10k volts (actually some are way over that in the 500k-700kV range) the amperage is very small.

Originally posted by philipc
Terry,
Thanks for the input, question about the transformer(since this is a transformer thread )I was thinking about getting a regular step down 120 to 6 or maybe 12 volt transformer, and wiring it backwards. I only plan driving a 4 watt bulb with it, pretty sure it won't over do it? Would you happen to have any other ideas for a transformer for this case?
Thanks
Philip, were you speaking to me?
(Terry is the person whose quote I am using).
I don't know what you have available in your parts box (assuming you have a parts box), but using a 120 to 6 volt transformer will work fine with your 4-watt bulb, just get one that isn't too wimpy on the power output side of the equation.
(250 to 300 milliamp unit will work).

pallidin
Originally posted by Integral
Read my first post, no transformer will convert 9VDC to anything.

It is really hard to say. Since I was not there to see what you were doing, likely it would have shocked you with or without the battery.

Actually, upon first contact with the conductors of a transformer, a DC voltage will impart a temporary AC transformer effect, given that it is momentarily and rapidly rising from 0 to x volts DC. Nothing is instantaneous, is it?! After this brief rise in voltage the constant current flow creates no effect with respect to this issue.

TillEulenspiegel
The 555 with a power mosfet works great, I've used it. You can also use a tip120 ,cascaded power darlington or a to-3 2n3055. Two things get the duty cycle as close to 50% as possable and don't try to overload the output side of the transformer a few watts is no problem anything higher and you have start to worry about wave shaping on the input side. Remember the output of the transistor is square wave which is essentally pulsed DC and transformers don't like dc! =).

actually if you vary the pulse width you can use the circuit not only to drive the light but you can use it as a dimmer ! =)

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philipc
I'm working on my little project to drive a fluorescent bulb with a 12 volt dc supply, and trying to wind my own transformer for kicks and grinds :)
My question is, does it matter if the primary is on the outside or inside? And if not, why not, because in my Physics class when we found the magnetic field inside a solenoid using Ampere’s law, we took the path outside the solenoid to be approximately zero, but if I wrap a coil around the same solenoid I would see an induced EMF? So if there is approximately zero magnetic field outside a solenoid, how does wrapping a wire around it create an induced emf?
Thanks
Philip

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Gold Member
The field inside of a solenoid is not zero. The field outside is small but not exactly zero unless you have a toroid or an infinitely long solenoid.

It does not matter where you wrap the secondary, inside or outside as long as the secondary encloses the interior of the solenoid.

It is the CHANGE IN FLUX inside of a loop which induces current. When the current changes in the solenoid the magnetic flux inside the solenoid changes this change in flux is what induces the current in the secondary. This works as long as the change in magnetic flux is inside the loop. It is not the current in the wires you are enclosing it is the magnetic field inside the solenoid.

This trouble you are having hits pretty close to home. We were doing the Roland Ring experiment, that is a small secondary warped around the OUTSIDE of a toroid coil. Class room work had taught me that there was ZERO field OUTSIDE of the ring, how could this possibly work. But it did! I took my questions to my Prof, he stood me in front of the black board and had me write out Maxwell's equations. I had to explain the meaning of each term until if finally sunk in. Thank you Dr. David Burch, When I finally understood, the Earth shook beneath my feet as some long held mental models came crashing to the ground.

Learning is the ability to overcome preconceptions.

philipc
Thanks for the input,
Ok I see that the equation is based of current enclosed, so let's say I took a 1" solenoid and wrap a 10 foot loop around it, the solenoid is enclosed, even though B will be proportional to 1/r, I should still see a slight EMF right? Let's say I want to run some experiments with a large solenoid, maybe around 2 ft large, would you have any suggestions on what to use for a core? I would like to be able to calculate the impedance, that way I don't blow another circuit breaker Now all my experiments have a 2 amp fuse with a variac along with current shunt I've built.
Philip

Staff Emeritus
Gold Member
Originally posted by philipc
Thanks for the input,
Ok I see that the equation is based of current enclosed, so let's say I took a 1" solenoid and wrap a 10 foot loop around it, the solenoid is enclosed, even though B will be proportional to 1/r, I should still see a slight EMF right?

Not necessarily, you have left out an important factor, you must CHANGE the current to see and induced emf in the outer loop.

Lets say I want to run some experiments with a large solenoid, maybe around 2 ft large, would you have any suggestions on what to use for a core? I would like to be able to calculate the impedance, that way I don't blow another circuit breaker Now all my experiments have a 2 amp fuse with a variac along with current shunt I've built.
Philip

2' is not a very helpful dimension for this. We need length, diameter and number of windings and the frequency of the current to compute the impedance.

philipc
Does the core also play role on impedance? Sorry I should of mentioned the dimensions in more detail, I was thinking 2' for the length, and the diameter will be based of the core material I can find. Number of turns, well I'm not sure yet, but maybe something like 200? Guess I just want to copy some experiments I've seen done, and play with secondary loops to see it for myself. I just haven't had much luck building transformers, need more theory :)
Philip

Staff Emeritus
Gold Member
If you really want to learn something useful you need to find a copy of a Physics text such as Fundamentals of Physics by Halliday and Resnick. To get the most out of it you will need to understand basic calculus. Study the chapters on Electro Magnetism.

My apologies but I cannot reproduce the necessary equations from memory, and do not have time to do the reserach for you.

TillEulenspiegel
Originally posted by philipc
Does the core also play role on impedance? Sorry I should of mentioned the dimensions in more detail, I was thinking 2' for the length, and the diameter will be based of the core material I can find. Number of turns, well I'm not sure yet, but maybe something like 200? Guess I just want to copy some experiments I've seen done, and play with secondary loops to see it for myself. I just haven't had much luck building transformers, need more theory :)
Philip
Yes the core plays a role , that's why in tuned circuits ( if your familiar at all with them ) use inductors with slugs that go thru the center of the coil. The slugs are made of ferrite or powered iron, as you insert more of the slug into the core the inductance, hence the Fq response of the circuit changes.That's why power transformers are so heavy more core means easier saturation and more efficient transfer of energy from s1 to s2 .If you understand field collapse and the subsequent induction of current, I wouldn't worry too much about the physics rather look up transformer construction on the net. The concepts are fairly simple. Windings are usually stated in ratios I.E 1:1 equals the same amount of windings on both sides, 2:1 equals 2 windings on s1 for every 1 winding on s2, ect. The only equally important factor is that as you half voltage you double current ( the inverse is true also) so if you have a 2:1 transformer with a supply of 100v 1@ s2 output will be 50V 2@.. If you use a low V wall transformer you can play all day and not worry about frying anything

edit to add: Inductance constraints are important in tuned circuit and in output stage transformers not usually a BIG worry in power x-frmers ( just remember Ohms law, wall current usually doesn't like 2 ohm loads =) )

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philipc
The only thing is I don't know where to find a 2 foot ferrite rod, I have steel pipe, would this be ok to use? Or do I need something solid? I'm only looking to run some experiments on Ampere’s law so efficiency is not of matter. Right now my main concern is building a solenoid that won't blow a fuse
Philip

TillEulenspiegel
I'm not quite sure what your attempting but in power X-formers and solonoids solid is best Steel is great ( in a solonoid the core is hollow and when energiezed "pulls in" the solid slug.. I only mentioned ferrite in example of tuned circuits. ( those components are small and very low current. ) My first x-former was bell wire and a 10 penny nail . If your building a solonoid it is one winding around a plastic or some other non-ferrous meterial The two are different , one (x-former ) uses saturation and field collapse to induce a current on phyically parallel winding, the other ( solonoid ) uses a coil to generate a magnetic field in order to pull a solid slug into it's core. Is this a school project or are you experimenting? Radio shack used to have books on basic electronics when I was a kid ...cheap and not too drenched in theory.

A 2Ft. Solonoid..?? You trying to build a rail gun? If you are the electronics are advanced, expensive and LEATHAL if you don't know what your doing.

philipc
No rail guns here, I just finished up Physics II and my wanting to do this experiment comes from the question I ask a few post earlier,

"My question is, does it matter if the primary is on the outside or inside? And if not, why not, because in my Physics class when we found the magnetic field inside a solenoid using Ampere’s law, we took the path outside the solenoid to be approximately zero, but if I wrap a coil around the same solenoid I would see an induced EMF? So if there is approximately zero magnetic field outside a solenoid, how does wrapping a wire around it create an induced emf?"

So the truth is I'm trying to prove Ampere's law to myself, unfortunately we did not go very deep into transformer theory, that will be in the future classes, but on my Christmas break I wanted to play around with a few of the things I've learned in Physics II. I'm having a hard time building a transformer that doesn't blow my 1 amp fuse, much less even think about building a rail gun. So my idea was to build a 2 ft solenoid for I could run various experiments, but have no clue on how to build one the proper way.
Philip

Staff Emeritus