Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How does a transformer work?

  1. Nov 21, 2003 #1
    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?
  2. jcsd
  3. Nov 21, 2003 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: transformer

    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.
    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.

    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.
  4. Nov 22, 2003 #3

    the voltage is increased or decreased, depending on the number turns in the secondary coil.
    but Power remains constant .... so energy conserved...
  5. Nov 22, 2003 #4
    power(watt) = voltage(V) x current(A)

    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
  6. Nov 23, 2003 #5
    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?
  7. Nov 23, 2003 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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 with out the battery.
  8. Nov 23, 2003 #7
    Will turning the current to a solenoid or coil on and off rapidly shock u if u touch the circuit? If so why?
  9. Nov 23, 2003 #8
    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.
  10. Nov 23, 2003 #9


    User Avatar

    Staff: Mentor

    Touching any energized circuit could shock you. These are not experiments you should be performing unless you are competing for a Darwin Award.
  11. Nov 24, 2003 #10


    User Avatar
    Homework Helper

    Of course it could. You didn't specify the amount of current or the threshhold of electrification that qualifies as "shock." If you've got current, then you can get shocked with or without the solenoid.
  12. Nov 27, 2003 #11
    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.
  13. Nov 28, 2003 #12
    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 occured 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.
  14. Nov 28, 2003 #13


    User Avatar
    Science Advisor

    Re: transformer

    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.
  15. Nov 28, 2003 #14
    Re: Re: transformer

    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
  16. Nov 30, 2003 #15


    User Avatar
    Homework Helper

    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.
  17. Nov 30, 2003 #16
    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?
  18. Nov 30, 2003 #17
    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.
  19. Dec 1, 2003 #18
    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?
    Last edited: Dec 1, 2003
  20. Dec 2, 2003 #19
  21. Dec 2, 2003 #20
    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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook