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Thread: Transformer usage

  1. #1 Transformer usage 
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    Can you reverse a transformer? As in, could you take a 120v to 12v and input the 12v to the secondary to get 120v? Essentially turn a downstep into an upstep transformer.

    Are transformers based only on a ratio? Could I take a 12v to 120v transformer and input 1000v in order to get 10,000v?

    What happens if you exceed the amperage limit?


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  3. #2  
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    Yes, they are reversible. But if you are talking about a 120v to 12v ac adapter they also include a rectifier to go from a-c to d-c so you can't feed backwards from a 12 volt battery. For that you need an inverter.

    If you exceed the voltage limit, the insulation will break down and the wires will short out. If you exceed the rated current, you will generate a lot of heat and melt the wires or the insulation or both.


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    Transformers are actually quite complicated because the windings have resistance depending on the thickness of the wire in the primary and secondary. The high voltage one will use thinner wire than the low voltage one. There will also be stray capacitance and also loss in the core which will increase the effective resistance of the windings and other effects such as core saturation, flux leakage and hysteresis which make a real transformer behave quite differently to the simple 'ideal' step-up/step-down one governed by the turns ratio.

    A typical 120 to 12 V transformer will produce 16 - 18 Volts with no load. You will get 12 Volts when you load it at its stated rating ..usually in VA or kVA.. Volt-Amps and not Watts... VA ratings are used when the V and A are not in phase (in step).

    Say it was a 120 to 12 V, 12 VA transformer. It would deliver 12 V with a 1 Amp current flowing. The turns ratio will be that which would give say 16 V (light or no load) output so would be more like 7.5:1 rather than 10:1.

    You shouldn't exceed the power, voltage or current ratings as the last poster has said.

    The equivalent circuit of a real transformer is far more complicated that an ideal transformer with just two coupled windings....

    Scroll down 'Equivalent Circuit'...

    http://en.wikipedia.org/wiki/Transformer
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    Everything that Pumblechook says is true but still the short answer is yes, it can be done. In fact, it is routinely done at power plants to backfeed from the main transformer to the unit station service transformers when the main generator is out of service. It does require changes to the tap settings on the USST to adjust the voltage.

    If you have a home generator and do not have the proper disconnect switch, then during a power outage your emergency generator system could backfeed though your pole transformer and possibly kill a lineman working on the supposedly dead power lines.
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  6. #5  
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    In an audio forum that I was in a while ago people posted their amplifier schematics (they built them). In them the transformer did not have any additional components, besides the bridge rectifier, drainage resistor, and filtering capacitors. Maybe the extra components are not necessary with all electronics?

    Is it possible to ice the transformer (with it insulated with plastic rap) in order to push it higher?

    I need around 10k volts if not more for an experiment that I want to perform. What is the best way to get there? I can't find a transformer (non industrial...as in the ones that cost $10,000 +) that can bring 12v (with an oscillator), or 120v that high. The power output will be around .0001 amps.
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  7. #6  
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    I found these: http://search.ebay.com/search/search...mer&category0=

    Particularly this: http://cgi.ebay.com/High-Voltage-Tra...QQcmdZViewItem

    You can undervolt the transformer's primary, right? If its possible I guess I could use a variable resistor to drop the voltage down a little to get to 10kv, and leave it at full to create really large sparks :-D

    When dealing with 10kv what sort of safety precautions should I take? 120kv?

    Can I plug something like this directly into a house electrical outlet? Is there anything I could add, like a fuse, between the wall and the transformer to make it completely safe?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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    That is a one Watt output.

    I would run it off batteries (sealed lead-acid or Ni-MH).

    You need a 15.625 kHz (UK) or 15.750 (US) (although exact frequency is not important) oscillator and the line output transformer (flyback..not to be confused with other sorts of 'line output') + rectifier from an old TV.


    http://en.wikipedia.org/wiki/Flyback_transformer
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    ahhh, neon transformers. Do you need to input a high frequency to them, or will they run 60hz wall? I'm assuming the output is also at a high frequency; do you need a special type of diode to rectify high frequencies?

    Say I wanted to use batteries as a source for portability, where can i get/how can I make a 60hz oscillator? I have seen a few schematics......but am unsure whether they can power a transformer like that.

    If I end up getting a 100kv transformer, would it be a bad idea to make a voltage multiplier to bring it to 1Mv? (Does Mv stand for million or mili?)
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

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    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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    The neon sign transformer just works off normal house current, 120 or 240 vac, 60 hz.

    The device you want to convert 12 vdc to 120 vac is called an inverter. You can buy a UPS for your computer that will generated 120 vac from battery power. I guess one of those would run your jacob's ladder for a while.
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  12. #11  
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    ok......I will order the parts soon. 10Kv should be good to start.

    I'll try to post a vid on youtube if the results are particularly favorable.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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    Could you make a variable transformer (they are kind of expensive) by putting one variable resistor on either end of the transformer set at say, 10 ohms. By turning down the first one to zero at the V+ end you would eliminate the 1/2 voltage drop at that end and allow full original voltage through until the second resistor which would create the second voltage drop. Turning the second resistor all of the way down down while the first one is at full would turn the voltage down to zero. Turning both down would result in the same outcome as the first one, allowing full voltage through. Would this work? Wouldn't using a 10ohm resistor at the primary end limit the maximum output current at the secondary end accordingly? I would use smaller resistors, although then I would not have full limiting capabilities since the resistance of the transformer itself would play too large of a role.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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    I'm not sure I understand your idea. You can drop the output voltage with a resistor, but you will dissipate heat, and the voltage drop will depend on the load current. So it could work if you had a consistent load current, and didn't care about wasting the power.

    What would be the point of an adjustable resistor set at zero? It would be equivalent to no resistor at all, wouldn't it?
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  15. #14  
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    I could take it out, but that would be laborious. If I only had a variable resistor at the negative end I could not adjust the voltage. The resistor would take the vast majority of the voltage drop at any level; it would have to be extremely precise, although I have never used one that is that precise.

    I used either on or off to demonstrate the main idea, but I would vary either one to acquire the needed voltage.

    I cannot find a variable transformer under $90, and those do not really do want I would like them to do. I found a DIY a while ago about how to make a variable HV power supply. They used a variable transformer that brought 120v to 6,000v and then resorted to a voltage multiplier to bring it to around 50,000v. I cannot find one like this, and it would likely cost way too much anyways.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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  16. #15  
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    It turns out that I also need a 1:1 transformer for a project I am working on. It will need to take the full 10-15,000 volts that I plan on outputting to it. Does a transformer like this exist that does not cost hundreds of dollars? If not, how should I make one?

    http://www.allaboutcircuits.com/vol_6/chpt_4/3.html

    How many turns are necessary? If possible I want to do as little as possible for obvious reasons. What will make it 15,000v compatible without melting itself? I will only run around 20ma through it.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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    If I were to only do a couple of winds, if not one, would the transformer work at all? What is the purpose of using hundreds/thousands of winds?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

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    Quote Originally Posted by Cold Fusion
    If I were to only do a couple of winds, if not one, would the transformer work at all? What is the purpose of using hundreds/thousands of winds?
    The power is transferred from the primary to the secondary through the magnetic core of the transformer. With fewer windings, it requires more flux density to trnsfer the power, which will saturate the core. There is a minimum number of primary turn based on the core size and the power requirements.
    Google "transformer design" and you will find more details on how to design transformers.
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    If you wish to more volts in, you need to raise the frequency but you will quickly run into limits.

    If you compare the transformer in a switched mode powersupply (which will run at around 50,000Hz) with that in an old fashioned 'linear power supply' (50 0r 60 Hz) you will see the former (excuse the pun) is a great deal smaller and has a much greater 'power density'
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  20. #19 Transformer Usage 
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    As an alternative suggestion for 15 kV or more why not wreck an old TV and take its HV tube transformer out. The old B/W sets used a ferrite rectangle core with input windings and an insulated 15 kV output winding (rectified by a vacuum diode with a 1 turn heater winding)

    These operate around 20 kHz and you can easily rig up a 2N3055 power transistor as a self oscillating converter. The transformer runs at 1 turn / volt so you can add 12 turns or wire to the core and connect this to the +12 battery and the transistor collector. The base feedback winding is another 2 turns. Provide simple 2 -resistor bias and you will easily get 15 kV (makes very attractive arcs up to several cm!)

    Color TV's also use such transformers but at higher HV (and use cascaded silicon diodes if you want DC). A simple self oscillating DC-20kHz inverter can be wired in a jiffy. A small heat sink is needed as it will draw several amps (36 Watts in, perhaps 18 watts out)

    Don't touch the HV wires though - they may not electrocute you due to the high operating frequency but they will burn an extremely deep hole in your skin, to a depth through muscle tissue that could take months or years to heal!!!
    __________________________________________________ ___________________

    From a previous post, quote "What will make it 15,000v compatible without melting itself? I will only run around 20ma through it."

    15 kV @ 20 mA is fairly high power e.g. Pout = 300 Watts! A TV line output transformer should produce up to 60 Watts (put 5 in parallel) but core saturation will be excessive if you run their magnetizing currents too high (the core will get hot).

    If you need 300 Watts then I suggest microwave oven transformers followed by diode voltage multipliers. Very approximately, say 20 mA flows through a capacitor of 0.1 uF at f=50 Hz - its AC reactance will be -j 32 kOhms, so 20 mA would result in a voltage drop ~ 640 V!

    Obviously you will need at least 1 uF capacitors with adequate voltage rating.

    If you rip some old long fluorescent lamp housings apart you will find a "ballast" inductor and a polycarbonate "phase angle compensation" capacitor that may be suitable. If you can find some dumping ground for these units then you may get the HV for free (the transformer in a dud microwave oven will probably be OK - the magnetron may be fried!) Similar capacitors are used in capacitor start motors - maybe some old washing machine units could be deconstructed?

    If you want even more serious power, multiple microwave oven transformers (650 watts) would be worth investigating with their outputs connected in series and the inputs in parallel. However this combination will be lethal.
    "The sky cannot speak of the ocean, the ocean cannot speak of the land, the land cannot speak of the stars, the stars cannot speak of the sky"
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    HAHAHAHAHAHA!!!!! I received the 10kv transformer today and immediately hooked it up to a 100 million ohm ohmite resistor, rated at 16w and 80kv. I turned it on, attempting to get a nice arc across the 1inch gap, but, all that happened was that two of the alligator clamps arced to the pavement. Even when I moved the copper wires for the arc 2 feet away, they still went to the ground. Apparently my concrete pavement is conductive, and was more favorable for the electricity to go through than the resistor. Who would have known? I mean, the two leads that arced to the ground were around 3 feet apart! Unfortunately though, the resistance must have decreased while I must still trying to figure out what the problem was, and shorted the neon transformer. I took it apart, but the components are completely encased in some kind of white plaster. Any ideas on how to reset the circuit breaker? I contacted the seller, but every time before he took a week to respond, and I need to get this running as soon as possible. Besides resetting it, should I have known that the pavement could conduct electricity? Very, very strange.... :?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

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    Use your computing strength for science!
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    Is it ok to pass Hv through water? We have all been told, repeatedly to the point of our ears bleeding that "water and electricity do not mix" but I do not see why not if I am only passing current through it. What about other liquids like alcohol? (I am sure it will catch on fire, but that would be the point)
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

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    Use your computing strength for science!
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    Yes you can but the transformer must be the isolation type.
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  24. #23  
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    Quote Originally Posted by Cold Fusion
    ahhh, neon transformers. Do you need to input a high frequency to them, or will they run 60hz wall? I'm assuming the output is also at a high frequency; do you need a special type of diode to rectify high frequencies?

    Say I wanted to use batteries as a source for portability, where can i get/how can I make a 60hz oscillator? I have seen a few schematics......but am unsure whether they can power a transformer like that.

    If I end up getting a 100kv transformer, would it be a bad idea to make a voltage multiplier to bring it to 1Mv? (Does Mv stand for million or mili?)
    If you are fooling around with neon sign transformers, diodes and capacitors, just be careful. I have seen some of the rectified DC output, start to connect to things through the seam in formica. That power is going to go somewhere.


    Sincerely,

    William McCormick
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