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

Generator and Zener questions

  1. May 30, 2009 #1
    Hey guys, i'm in need of some help.

    I know that the formula for a generator is V = N(turns) * A(m2) * B(tesla) * RPM

    1. Is that valid for both AC and DC generators ?
    2. Is the formula for a generator with only one magnet and one coil ?
    If so, then adding more coils changes the RPM value ? In other words, are 2 coils regarded as a doubling of rotor frequency (RPM) ?

    The zener diode produces a voltage drop, but how much current does it allow to pass in reverse ?
    Is it like this :
    15v - 5.6v = 9.4v, and the amperage is proportional to the passed voltage, or does the zener diode introduce more resistance and limits the current ?

    Thank you
  2. jcsd
  3. May 30, 2009 #2


    User Avatar
    Science Advisor

    The Zener diode conducts very little until the Zener voltage is reached.
    After that the resistance starts to drop rapidly with each slight increase in voltage and the current increases accordingly.

    The effect of this is that the Zener voltage is affected only slightly by the current that is allowed to pass through it.
    So, allowing 10 mA to flow through it, by limiting the current with a resistor, will produce only a slightly greater Zener voltage than allowing 1 mA to flow through it.

    If the load is varying, it is better to have 10 mA flowing in the Zener off load than 1 mA because when the load increases, some of the 10 mA can go to the load and the Zener will still keep regulating the voltage. The load would be a varying resistance across the Zener.

    If the Zener current drops to too low a value, the correct Zener voltage will not be achieved.

    If the Zener current is allowed to be too large, the Zener power dissipation can get too large and the device can be destroyed.
  4. May 30, 2009 #3
    A zener diode is usually used as a voltage reference when it is reverse biased. In the forward direction it acts very much like a normal diode with forward voltage drops ranging from around 500mv to 1.5v. They are rated by their breakdown voltage (reverse bias voltage) which can range from around 1.8v to 200v, and their power dissipation which can range from around 100mw to 50w.
    Once the voltage across the zener surpasses it's breakdown voltage there is no limit on current. So if there is not enough series resistance in the circuit the zener power rating will be exceeded and it will be destroyed.
  5. May 30, 2009 #4


    User Avatar
    Science Advisor

    When you use a Zener as a simple reference, you have to work out a value for the series resistor. You can just take the voltage across the series resistor as the supply voltage minus the nominal Zener voltage.

    You divide this by some optional value of current, say 20 mA to get the resistance.

    So, if it was a 6 volt Zener and a 14 volt power supply and you allowed 20 mA current, the resistor would be
    (14 - 6) / 0.02 = 400 ohms

    See attached diagram.
    The area of interest is the extreme left of the diagram where avalanche current flows.

    Many Silicon NPN transistors have an excellent Zener diode built into the base emitter junction. It is typically about 7 volts. It is usually better and cheaper than an actual Zener you might buy.
    You just leave the collector lead unconnected.

    Attached Files:

  6. May 30, 2009 #5


    User Avatar
    Science Advisor
    Gold Member

    Yes. You have a magnetic field B moving in and out of one turn of area A at a rate of RPM.

    No. The RPM comes from your driving device. When you double the RPM, you double V. Or you can hold the RPM constant and double the turns to double V.
  7. May 31, 2009 #6
    I didn't know that. Thanks for the info. Now another question.
    How do you saturate a transistor, by increasing the Base voltage or the Base current ?

    So then increasing the number of coils has no effect on the output ?
  8. May 31, 2009 #7


    User Avatar
    Science Advisor

    How do you saturate a transistor, by increasing the Base voltage or the Base current ?

    A transistor saturates when it is unable to increase its collector current with increasing base current.
    The usual reason this happens is that there is a limit on the available collector current.

    Suppose you put a 1000 ohm resistor in series with the collector to the power supply.
    If the power supply is 15 volts, the maximum current that can flow into the collector is 15 mA.

    So, any base current that causes the transistor to draw 15 mA will cause the transistor to saturate. There is a small voltage across the transistor when this happens but most of the supply voltage appears across the series resistor.

    Is it base current or base voltage that causes this? It is either. The base voltage causes base current to flow or the base current causes a base voltage to develop.

    Base current is more useful though, because the collector current is roughly proportional to the base current and you can calculate base current if you know the collector current you want to flow and the gain Hfe of the transistor.
  9. May 31, 2009 #8


    User Avatar
    Science Advisor

    If so, then adding more coils changes the RPM value ? In other words, are 2 coils regarded as a doubling of rotor frequency (RPM) ?

    No. The RPM comes from your driving device. When you double the RPM, you double V. Or you can hold the RPM constant and double the turns to double V.

    So then increasing the number of coils has no effect on the output ?

    The RPM is shaft RPM. How often does the generator actually rotate?
    It has an effect on the output voltage and frequency and so does the number of field windings and field turns as well as armature windings and turns.
    The field windings are usually referred to as pole pairs. So, you might have one north and a south field or you could have two of each or more.

    To produce a useful output at a stable standard frequency, the generator must be held to strict rotational speeds depending on the number of poles.

    I borrowed the following from Wikipedia:
    Poles .........RPM at 50 Hz ....RPM at 60 Hz
    2 ...............3000 .........3600
    4 ..............1500 .........1800
    6 ..............1000 .........1200
    8 ................750 ..........900
    10 ................600 ..........720
    12 ................500 ..........600
    14 ................428.6...........514.3
    16 ................375 ...........450
    18 ................333.3 ...........400
    20 ................300 ...........360

    There are several kinds of AC generator and the output may come from the rotating part or the stationary part.
  10. Jun 6, 2009 #9
    Okay, but if frequency increases, then the period decreases, and
    According to that, the voltage will increase. It is hard for me to think that alternators have more than two coils just for the reason of increasing frequency.

    One more question: if a generator has no load on it, then there will be no resistance and the windings will fry if it spins too fast and there's too much current. However, if i add a resistor on the line, the resistance will not only be visible to an eventual load i'll add, but also to the windings, thus lowering the amps traveling through them. True ?
  11. Jun 6, 2009 #10


    User Avatar
    Science Advisor

    Increasing the number of poles increases the efficiency of the generator. If you picture the armature rotating in magnetic fields from a single pair of poles (say in the top and bottom of the rotation), there must be areas at the sides of the rotation where the generator armature is rotating through a relatively weak magnetic field. Addding extra poles at the East - West position helps to maintain the magnetic field.

    It may also be convenient to run the driving motor at lower RPM and still get the same frequency out. Nobody wants a gasoline motor running at 3600 RPM when it could be running at 1800 rpm.

    There is no reason a shunt wound generator would fry if it had no current flowing in the armature. Usually it would be supplying current to its own field windings, so it should be stable.

    Actual voltages produced are part of the design of the generator, so you may well have more turns on the armature and field if you had lower shaft RPM.
  12. Jun 6, 2009 #11
    Thank you very much, but i have one more question about transistors. I understood how the current is controlled, but i don't know how the voltage is affected. Does the Emitter Voltage vary depending on the Base voltage or current ?
  13. Jun 6, 2009 #12


    User Avatar
    Science Advisor

    The emitter doesn't really have a voltage. You measure voltage from somewhere to somewhere else. Depends what you mean.

    You can have a voltage from base to emitter. This is just like a diode, though and it doesn't conduct until the voltage is about 0.6 volts. When it conducts, it suddenly becomes a very low resistance and you have to be careful ot it will draw too much current. Normally, you put a resistor in series with the base to stop too much current flowing.

    The collector current is related to this base current and it is typically 50 to 500 times as great as the base current and this ratio stays fairly predictable for the one transistor.
    So, if the base current is 1mA and the transistor has a current gain of 50, the collector current will be about 50 mA. This changes with the collector voltage, but it is a good rule to remember.
  14. Jun 6, 2009 #13
    I'm sorry, i meant Collector-Emitter voltage.
  15. Jun 6, 2009 #14


    User Avatar
    Science Advisor

    Suppose we continue from the example above.

    The base current is 1 mA so the collector current will be about 50 mA

    If there is no load resistor in the collector circuit, the collector voltage will be the same as the supply voltage. 50 mA will just flow from the supply.

    Now suppose there is a 100 ohm resistor there.
    The voltage across this with 50 mA flowing is what?
    V= I * R = .05 amps * 100 ohms = 5 volts

    So, if there was a 12 volt supply, 5 volts appear across the resistor which leaves 7 volts across the Collector - Emitter.
    A question for you. What would happen if there were 2 mA of Base current?

    This breaks down if you had a much higher value resistor like 1000 ohms in there. It can only pass 12 mA from a 12 volt supply, so 50 mA cannot flow. In this case the Colector emitter voltage drops to almost zero and the transistor is said to be saturated.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook