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2:1 ratio step down transformer question from newbie

Discussion in 'General Electronics Discussion' started by wozzer, May 12, 2012.

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  1. wozzer

    wozzer

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    May 12, 2012
    Hi,
    I am new to this forum, I restore pinball machines and whilst I am fairly competent at repetitious board repairs when it comes to designing i'm not very comfortable.
    Basically i'm repairing a pin that should have a 50vac motor which drives a swing target, the motor is missing and is very difficult to find 50vac 25rpm pinball motors. I have a 24vac motor from a different manufacturer and want to basically step the voltage down.
    I have been told that the best way to do his is to use a 2:1 step down transformer. The motor I have is 24vac 3watt, so i'm thinking the current rating will be 125mA. I don't want to spend a fortune on this and there are some cheap 2:1 transformers on Ebay but they are labelled as pulse transformers. Can I use these?
     
  2. duke37

    duke37

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    Jan 9, 2011
    Pulse transformers are no good at all in your application.

    The cheapest way may be to use a 24-0-24V (marginal) or 30-0-30V mains transformer without the primary connected.
    Your power is very low and such transformers for powering amplifiers are much bigger than you require. It will depend on the space you have available.

    Another way would be to use a pair of similar wall wart transformers with the primaries connected in parallel and the secondaries connected in series (I think!).
     
  3. BobK

    BobK

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    Jan 5, 2010
    Could you explain further how this would work? Are you talking about using the two secondaries as an autotransformer?

    Bob
     
  4. duke37

    duke37

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    Jan 9, 2011
    Yes, that is the idea. By connecting the primaries in parallel, the secondary voltages must be similar. Connecting them in the correct phase, will give the same effect as an auto transformer but with lower efficiency since the power has to go out of one transformer and back into the other.
    In the true autotransformer case, only half the power goes through the transformer.

    It may be possible to wind an autotransformer. Wall warts have very fine wire but in the case of a 50V transformer, the wires will have 5 Times the area and 5 times fewer turns than a 250V transformer.

    Wozzer has not said where he is located so we do not know what voltages and frequencies he has in the mains.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If you can find a 50V (or thereabouts) centre tapped transformer then you could wire it up as an autotransformer (ignoring the primary -- insulate the wires!!)
     
  6. wozzer

    wozzer

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    May 12, 2012
    Hi,
    Thanks for your answers guys, i'm in the UK and on 240v 50Hz. We don't have Wall marts over here, just RS and Farnell and to be honest they don't tend to be very cheap once you add vat and minimum shipping. If anyone could give me a url or link to a transformer that would be ok just so that I could look at the specifications that would be great.
     
  7. wozzer

    wozzer

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    May 12, 2012
  8. gorgon

    gorgon

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    Jun 6, 2011
    If the transformer is like the ones in the picture, you hav (0 - 24V) - (0 - 24V) on four terminals. You connect the middle two, resulting in a 'three' terminal, 0(A) - (24V+0)(B) - 24V(C). The 50V output goes to the outer, A and C. The motor between A and B. (or B and C). The two are equal.
     
  9. wozzer

    wozzer

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    May 12, 2012
    Thanks very much, just to double check, do I ignore the primary side, i.e. don't connect or join together? I assume I don't connect but just thought i'd check.
     
  10. wozzer

    wozzer

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    May 12, 2012
  11. duke37

    duke37

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    Jan 9, 2011
    I would think either transformer would do the job.
    If you have a Maplins close, they should have one and you would save postage.
    The primary is not used and should not be connected to anything BUT it will bite.

    Have you thought of just putting a resistor in to drop the voltage, some experimentation would be required.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Actually, I'm feeling rather less comfortable with this proposed solution.

    It turns the transformer into a current transformer, and without load the voltage on the transformers other winding will go extremely high.
     
  13. duke37

    duke37

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    Jan 9, 2011
    *Steve*
    "It turns the transformer into a current transformer, and without load the voltage on the transformers other winding will go extremely high."

    I do not understand this, with a 240V to 50V transformer, 50V on the secondary will produce somewhat less than 240V on the primary. No load will be required.
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Looks perfect for the job. It has 2 separate 24 V windings 8-9 and 13-14.
    At 16VA (Volt*Ampere) you can drive up to 5 of your 3 W motors by one transformer.

    Harald
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Dule37, I prepared a detailed response about current transformers prior to leaving home today. Clearly I omitted to post it :D

    My concern is that any non-reactive current that you force through the secondary winding could induce a current in the primary. Without a load, that will induce a very high voltage (even greater than the turns ratio would suggest).

    My experience with current transformers is nil, but I know enough to beware of the newbie dangers. I am cautioning that some slightly unexpected behaviour might be observed.

    OK, maybe it *won't* operate like a current transformer, but to the extent that it does, interesting things might happen.
     
  16. CocaCola

    CocaCola

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    Apr 7, 2012
    A bit overkill and more costly upfront (but nearly foolproof)

    Install a new 24 volt supply into the machine, use the existing 50VAC to trigger a relay that switches this new 24volt power line to the motor? Doing this also provides you with a 24 volt rail that can be used down the line for other component replacements that need the 24 volts...

    As I said more costly, not as pretty but it does have it's own set of advantages...
     
  17. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Duke37, *steve*,
    methinks your' talking along different lines:

    If you apply a voltage (duke) to one of the transformer's windings, another voltage will show up on the other windings and the voltage(s) are defined by the turns ratio(s) of the respective windings to the coil where you apply the voltage.
    This method is used for measuring voltages, adapting voltage levels in audio applications, power supplies etc.

    If you apply a current (steve) to a transformer's winding, also a current will be induced in the other windings. This application normallly uses only 2 coils. Then the equation is I1*n1= I2*n2 where I,n are the current and number of windings in the respective coils. The secondary winding acts as a current source. If you leave the secondary open (no load), then this current will be forced to flow through an "infinite" load resistance and by U=I*R the voltage rises until the current finds a way to limit the voltage (e.g. by arcing, breakdown of the coil's insulation etc.).
    This method is used mainly for measuring currents and often safety measures are incorporated to prevent the high voltages, e.g. varistors, short circuit contacts in plug-in connectors (where a short circuit is created within the connector when the plug is pulled and therefore no load is present) etc.


    The difference is that in the first case the current follows from a defined voltage (AC voltage source), in the second case the voltage follows from a defined current (AC current source).

    Harald

    P.S.: I'm assuming more or less sinusoidal voltages amd current in my discussion.
     
  18. wozzer

    wozzer

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    May 12, 2012
    I should just clarify here that the way this motor works is, there is a 43-48vac which powers this motor, this goes through a relay first with one line from a lamp driver board which I assume switches the relay and therefore the motor on and off. When you press start on the pinball machine the geared motor starts turning until you lose a ball, it then starts over again with another ball being dealt. Is this going to be a problem using the 24v-0-24v tranformer because I don't want anyone getting a nasty shock off the primary although I will insulate it well?
     
  19. duke37

    duke37

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    Jan 9, 2011
    The old primary will need insulating properly and you should hide the transformer inside the machine so that no-one can get at it.

    An advantage of a special wound auto transformer is that there will be no high voltages unless the common connection fails when it becomes a current transformer - see *Steve*'s post.

    As I asked earlier have you thought about a simple resistor? The power is very low.

    How far are you from Derby?
     
  20. wozzer

    wozzer

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    May 12, 2012
    Burton on Trent! So am very close to Derby, what value resistor could I use do you think?
     
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