by tt010101 on October 22nd, 2006

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Why is it inadvisable to use DC current through a transformer?

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by Anonymous on October 22nd, 2006
  It's not a matter of “inadvisable”. Transformers just don't work with DC.

  Transformers only work on AC.  AC going through one coil produces an alternating magnetic field which induces a current in the other coil.

  If you put DC into one coil of a transformer, it will produce only a steady magnetic field, which will have no effect on the other coil.
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- http://home.howstuffworks.com/stun-gun3.htm
  I rest my case unless you want to insist that they actually use a ac power cord and the DC batteries are just for show.
  
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  by scubabob on November 22nd, 2007
- Not a very good description at that URL of how the stun gun actually works, but take note that that article mentions that the device contains an oscillator. The purpose of the oscillator is to convert the DC from the battery into AC, so that the transformer can work with it.
  
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  by The NeXT Guy on November 25th, 2007
- Wrong! Scubabob. The DC from the batteries is "chopped up" into a square-wave A/C before it enters the transformer. High efficiency power supplies have been using this method for power conversion for decades but the batteries DO NOT connect directly to the transoformer. You have been saying to several people and they keep tell you the truth. DC into a transformer does nothing but mess everything up.
  
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  by douglasmkilmer on October 20th, 2009
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by Stableboy on October 22nd, 2006
To tag on to Bob Blaylock: the transformer won't work with DC. The reason its inadvisable is that (a) you can damage the transformer with enough current (b) if you have a DC component in addition to the AC, the transformer will "saturate" at a lower level of AC than otherwise. This reduces the transformer's overall current capacity as long as the DC component is present.
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- http://home.howstuffworks.com/stun-gun3.htm
  
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  by scubabob on November 22nd, 2007
- Yes, a handy tool! Anything you wanna say about that? :)
  
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  by Stableboy on November 22nd, 2007
- Just that they run on batteries ( dc current) and use transformers.
  
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  by scubabob on November 22nd, 2007
- Yes, but they have to turn the DC into AC before it goes into the transformer (that's what the oscillator is for). Actually in the cheaper circuits of this type, the oscillator is actually a mechanical "vibrator" which uses a magnet and a set of contacts -- every time the magnet pulses, the contacts open, interrupt current to the magnet, which causes them to close, re-energize the magnet, etc.
  
  The same principle is work in older cars: the ignition coil is a transformer fed by pulsating DC (i.e. AC), where the pulses are produced by opening and closing contacts -- in the distributor.
  
  If you feed plain old dc into a transformer, it just sits there looking dumb.
  
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  by Stableboy on November 23rd, 2007
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by scubabob on October 22nd, 2006
Some transformers run on DC, so maybe you are talking about some particular application?
Then Blaylock, perhaps you can explain why this step down transformer I'm holding, currently hooked up to a 1.5 volt AA battery, is doing it's job. Last time I checked, AA batteries are DC. The question just asked why it can't be used. I pointed out it was possible to use it and get a result.
Edit: Ok...Blaylock, this is for you. http://home.howstuffworks.com/stun-gun3.htm
There are a lot of other instances where a dc current is passed through a transformer.
I'm amazed at the lack of knowledge displayed here and the people that insist that the above wasn't true. I've seen and experienced it with my own eyes, so...unless you can come up with a viable explanation as to why that 1.5 volts jumped up in voltage after going through a transformer, you can stuff it.
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- Incorrect. Transformers only work with AC. There is no such thing as a transformer that will do anything useful with DC. It's physically impossible.
  
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  by Anonymous on October 22nd, 2006
- If a battery or other DC source is the source of current going into the transformer, then there's an oscillator or other circuit between them that is converting the DC to AC before it goes into the transformer.
  
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  by Anonymous on October 22nd, 2006
- If the transformer is being used to charge a battery or power a DC device, then there's a rectifier between them, converting the AC output of the transformer to DC.
  
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  by Anonymous on October 22nd, 2006
- In no case is the transformer itself working with DC. That would be physically impossible.
  
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  by Anonymous on October 22nd, 2006
- Bollocks. Go grab one off an old fashioned school bell.
  
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  by scubabob on October 23rd, 2006
- Is it possible that you are using the word “transformer” to refer to an entirely different device than what I understand that word to mean? I think, perhaps, you are describing an electromagnet or a solenoid; not a transformer.
  
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  by Anonymous on October 23rd, 2006
- http://en.wikipedia.org/wiki/Transformer#Direct_current
  
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  by Anonymous on October 23rd, 2006
- Exactally what job do you think the transformer is doing with the 1.5 VDC? It certainly isn't "stepping down" or "stepping up" the voltage. It is quite possible that a transformer in a circuit could be connected to one side of a battery; however, if the + side of the battery is connected to one side of the primary (or secondary) coil and the - side of the battery is connected to the other side of the same coil, you most likely have a dead short across the battery. In that case your will either burn out the coil, the battery will burn out, it will get very hot or maybe even explode. If it doesn't, the coil is probably open.
  
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  by Elsabioanciano on October 23rd, 2007
- Bob Blaylock is correct here.
  
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  by H on October 23rd, 2007
- Bob has stated that a transformer doesn't work on DC. Some will. I'm not going to bother with this further.
  
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  by scubabob on October 23rd, 2007
- For a transformer to work, you need a flux change.
  
  The only way you can achieve the flux change is have a flux move relative to a stationary transformer, or have a moving transformer relative to a stationary flux.
  
  Do you want the proof in integral form or in derivative form?
  
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  by H on October 23rd, 2007
- Give me proof in reality form.
  Equations prove NOTHING. Give me a DC Power Supply, a transformer, and a volt-meter.
  Before you try to jump my chops, I know all the equations. But it's these little electrical engineers coming out of school that blow up more crap than I've ever seen (I know cuz I used to be one).
  At some point, you have to put down the pencil and calculator and start testing equipment to see what it'll do.
  
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  by Bonfireking on October 25th, 2007
- Hrm.
  
  Equations prove alot, actually, that's the whole point of maths.
  
  I can tell you EXACTLY what will happen if you hook up a DC Power supply to a transformer and voltmeter.
  
  Oh, and, I'm not an electrical engineer either, I'm a physicist.
  
  Hook up the voltmeter and the transformer.
  Apply the DC current.
  Your voltmeter will pick up a signal for the first couple of seconds, as a B-field is induced across the transformer. Once the B-field achieves maximum strength, as per, it will stop inducing voltage in the voltmeter.
  
  If this didn't hold true, conservation of energy, etc, would all go out the window. Not to mention we'd be able to create greater-than-unity transformers and solenoids/
  
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  by H on October 26th, 2007
- I like physics, but obviously not as much as henpc! As usual, I didn't make my claim clearly for lack of time/patience. That has led you to an invalid assumption... my fault.
  
  At no point in my comment did I say you or the equations were wrong. I KNOW you're right, but I know you're right because I know the equations AND I know from experience.
  
  My point was that no one was putting forth any real EVIDENCE, only real HYPOTHESES. It is my opinion that an argument is strongest when augmented by actual experience.
  
  Lastly, equations don't prove anything... experience proves equations as valid or invalid.
  
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  by Bonfireking on November 1st, 2007
- I've played around with transformers a little bit, and also current inductance, the first thing they did to demonstrate to us how current inductance works was getting a magnet and a voltmeter and playing around with that, seeing when and how much we could induce:)
  
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  by H on November 2nd, 2007
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by H on October 23rd, 2007
The voltage induced is given by - d Phi / d T, where phi is the flux lines. d/dT is the derivative, e.g. the rate of change of it.
Essentially, what this equation tells us, is that we need flux line change to induce voltage. Either the current has to alternate (thus causing flux change) OR the transformer has to move in a steady current.
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by baaap on October 23rd, 2007
mutual induction is the principle of transformer. on
applying dc on the primary wng there ll b no variation in
the flux linked with the secondary n primary windngs. ie no
emf induced in the secondary winding except at the time of
switching and so no output u get...v know that the wndg
resistance is low.. in normal case although we r applying a
high voltage, due to the self induced emf in primary wndg,
which opposes the applied voltage, the primary current is
limitted to a low value.. if the applied voltage is dc no
self inuced emf to oppose and hence leads to a heavy
primary current that causes burning out of primary windg
transformer works depending upon flux linkage changing.it
needs some change of flux linkage.otherwise it will not
step up or step down the input ,which is the basic
principle.for dc supply is constant,no question of flux
linkage change arises.
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by Radiography Instruct on October 22nd, 2007
A STEADY/CONSTANT DC current will not work b/c it would produce a steady magnetic field. Therefore there would be no "reason" for the electrons in the secondary coil to continually move as in a current. (The idea of interia doesn't work except in superconductors but that is a whole different story) AC both increases/decreases and reverses constantly causing the electrons in the secondary coil to constantly adjust to the constantly changing magnetic field, ergo AC current. What I cannot get answered is why a fluctuating DC current would not work. When dealing with transformers most discussions assume or state that any DC current discussed is steady. Theoretically it makes sense that a primary current of varialble DC current, e.g., full-wave rectified, would produce the necessary continuously changing magnetic field that would then induce an AC in the secondary coil. My guess is that using AC is both easier and more efficient but DC would work in theory.
Anyone?
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- http://home.howstuffworks.com/stun-gun3.htm
  Not theory...fact and I'm the one that everyone gave a negative rate to.
  
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  by scubabob on November 22nd, 2007
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by alexgeps on April 8th, 2011
I do not think it is just that. If you run dc current through a transformer, you may just burn it, because there is no e/mag induction and no counter emf is being produced that would reduce the current. thus your only impedance is the coil resistance and your current may be excessive.
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