Ac/dc?

Discussion in 'Electrical Forum discussion & Blog' started by Bassman, Sep 28, 2007.

  1. Bassman

    Bassman DIY Member

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    Does the transformer on a low voltage lighting system, such as a monorail, put out AC or DC. Inquiring minds want to know. Thanks.
  2. jwelectric

    jwelectric Electrical Contractor/Instructor

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    The transformer will state the voltage, wattage and hertz
  3. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

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    Usually AC, but what Mike (jw) said is better.
  4. BrianJohn

    BrianJohn DIY Senior Member

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    A Standard transformer coil to coil (primary to secondary) no rectifier will only transformer AC voltage. With the addition of a rectifier it can produce DC.
  5. Bassman

    Bassman DIY Member

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    Location:
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    Thanks, I have a reasonable understanding of what transformers do, but I wanted to know whether to buy a DC or AC for the monorail light system. I'm purchasing online and the sites carry both. I'll be using two pin xenon lamps in the fixtures.
  6. jwelectric

    jwelectric Electrical Contractor/Instructor

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    They should be able to advise you on which one you need when you make your order.
  7. jimbo

    jimbo Plumber

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    A transformer can only take and AC input, and output an AC voltage. A power supply, which would include a transformer and some type of rectifier ( half wave, full wave, bridge) and would output DC votage.

    Most light bulbs are rated for AC or DC, but not both, although with the proper circuit, you could possible use one either way. Basically, you should find out the voltage , number of volts, and AC or DC, for the bulbs you will use, then get the appropriate power.
  8. hj

    hj Moderator & Master Plumber Staff Member

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    ac/dc

    Basically, for the same load DC requires larger wiring, (which is just one of the reasons the country does not run on DC), although for small loads the difference may be acedemic.
  9. Furd

    Furd Engineer

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    Do you have a source for that, HJ? I've actually studied this a bit and while I can not find any specific source for D.C. ampacities it might be just the opposite of what you state because A.C. has a tendency to travel on the surface of the conductor. This is known as the "skin effect" and is why increasing the size of a conductor does not necessarily give a proportional increase in the ampacity.
  10. Bob NH

    Bob NH In the Trades

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    The skin effect is negligible at 60 Hz. The resistance is substantially the same for AC and DC.

    One disadvantage of DC is that it is harder to extinguish the arc in switches because the voltage and current don't transition through zero. Therefore, most switching devices have lower ratings for DC than for AC.

    Also, many (most) dimmers will not work with DC.
  11. availability of the bulbs,
    cost of the bulbs, and
    lifespan of the bulbs. - Some DC voltage systems are known to blow their bulbs a lot.

    Those are three factors that would make me order one system over another.

    Also, if I wanted to mix it with another type of bulb, I would have to stick with 110 V AC bulbs. No choice.


    The manufacturer will say what the output of their transformer is. It keeps AC input as AC, and DC input as DC.

    Another word for rectifier is inverter. A transformer combined with one of these is now a more complex device. Can be called a power supply. Cannot be called a transformer because it alters the type of output waveform, from AC to DC. or from DC to AC.


    David
  12. Bob NH

    Bob NH In the Trades

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    A DC to DC device is a converter; not a transformer.

    A transformer connected to AC is a short circuit, or very nearly so.

    A transformer requires the inductance together with the alternating nature of the current to provide enough impedance to prevent the circuit from darwing too much current.
  13. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Didn't you mean to say DC instead of AC in the statement above?
  14. DC bulbs' lifespan may be shortened by the DC power source ("supply") even though the bulbs themselves may have an official lifespan in the thousands of hours. I have heard of that happening.

    Someone else signed in here may know more and tell you more; I'd like to know too.

    Bassman, this is to stay on topic and tell you things that relate to your question.

    As for the guys wh want to discuss short circuits, impedance, inductance, converters and all that, I think I'll wait a bit and let Electrical Engineers add definitions and a little bit of direction. I am sure someone knows how to explain it all in a few words.

    Besides, Bassman, you deserve an answer to your first question. After that, if people want to keep on discussing other things closely related to your question, I won't speak up to disagree.


    David
  15. Bob NH

    Bob NH In the Trades

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    Yes, DC will make it approach a short circuit; not AC.
  16. Bassman

    Bassman DIY Member

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    I figure I'll get an answer to my question eventually. :) In the meantime I'm learning some things! I think that for the layman end user the stores call all the above "transformers" even though technically some may be converters, etc. I'm sure when I place an order they'll be happy to sell me the correct thing.
  17. jwelectric

    jwelectric Electrical Contractor/Instructor

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    And here-in lies your answer as was given to you here
  18. jimbo

    jimbo Plumber

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    And herein lies the reason for his confusion, and he still doesn't have an answer, but maybe has been given enough information to figure out what he needs:

  19. Bassman

    Bassman DIY Member

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    Location:
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    Correct you are. I got an email from the vendor who states that their DC "transformers" are only used, and not often, in very long runs where voltage drop is a real problem. 99.999999999% of the time, AC is called for. That was the answer I was looking for.
  20. tjbaudio

    tjbaudio Sound and Light Suppervisor for a School District

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    Wisconsin
    Wrong! A transformer will convert Voltage and impedance for AC. If you feed a transformer DC it will block it. Depending on the transformer, circut voltage and power capasity some thing may get damaged. In other cases this blocking propterty is used to advantage.

    An inverter does the oposit of a rectier. A rectifire takes AC and converts it to DC with ripple, filters in a power supply reduce this ripple to sute the application. An inverter takes DC and converts it to AC. This is a complex process.

    A power supply in this case is a unit that takes the main house current and converts it to soem usable form for the system. It will contain a transformer. If it outputs AC that may be it. If it outputs DC there will be a rectifier also. None of this talks about a switch mode supply which is a compleatly different animal and beyond the scope of this conversation other than it can also supply DC current for your system. Recently the cost of complexity of a switch mode unit is offset by the cost of raw materials for a transformer.

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