Beginner Questions Related to Current Reduction

Discussion in 'General Electronics Chat' started by quicksilver, Jun 20, 2008.

Mar 6, 2008
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Transformer theory is (to me) quite complex. I want to understand from a very practical standpoint what happens with a transformer that is designed for a particular current level & frequency.

How could HV transformers such as Micro Wave Oven transformers (MOT's) designed for mains's frequency (60Hz) have their output current reduced while maintaining their voltage level?
While I recognize Ohm's Law; what I am looking for are methods of further reducing the current in the output of a MOT & this leads me to other questions of transformer design that I am having trouble finding concise answers for.

If such a transformer (a MOT) is fed AC with a starting current lower than main's level, would it still function to the same degree of voltage enhancement? Say, instead of using main's voltage a series of resistance devices (or a dimmer, etc) was introduced to the energy before the Xformer, could the device still function? If the device is designed for 60Hz what takes place if a differing frequency (but the same voltage) introduced into it?

EXAMPLE: A MOT is fed 120Vac but @ 10ma (or whatever; but LOW current), does the Xformer still create a voltage increase but in ratio to the current or does that operate independently of the amperage? AND if the current were knocked down AFTER the fact (say from 300ma to 30ma) what would happen to the arc, the magnetic field, etc? Would it simply diminish in proportion or would it (the transformer) stop to function at a certain point ???

I hope the basic elements of my questions are clear; I've had a tough time finding answers to them - Thank you very much for any answers

2. beenthere Retired Moderator

Apr 20, 2004
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Transformers work a bit differently than you imagine. The output of the secondary is the result of the turns ratio (giving the voltage step up or down) and the structure of the core, which governs the amount of energy in the form of magnetic flux that can be handled. The flux produces current in the secondary. The product of the current and voltage is the power the transformer can handle.

Transformer cores are somewhat resonant structures. Changing the frequency will result in decreased power handling ability.

Reducing the current out of the secondary without affecting voltage will require an external device to limit current. Otherwise, the voltage will simply reduce to whatever level will supply the lowered current through the load.

If you limit current in the primary, you may be able to see the full voltage in the secondary, but only after a critical current flows in the primary. A load in the secondary will require a proportional increase in the primary in order to maintain the secondary voltage.

Mar 6, 2008
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Excellent stuff!
I didn't think of transformers being resonant structures: this is very significant & logical. Your last comment just made sense of a formula that had me stumped. If you limit current in the primary then you MAY be able to see full voltage but only after a critical point....

You see, superficially, I had wondered why Variacs and adjustable devices of that sort were used when I had thought of just beginning with a powerful transformer and using some adjustable resistor. I imagine that would be possible to some limited degree but the resonance factor escaped me!
THIS HAS REALLY BEEN VALUABLE TO ME!

Getting back to your comment on the external current management; this has opened a whole area that could be very valuable!
On an "EXTERNAL LEVEL" of current alteration does a formula exist as related to BOTH resistance and wattage? My interest would be the above style example: "If a MOT has (standard mains) 120Vac going in (A) with (X) amps - and 2400Vac (B) coming out with (Y) 300 milliamps what would the relationship of wattage have to resistance to achieve certain results"? Where would "wattage" be in such an equation?

If current was to be limited to 30 ma from the original 300 ma, let's say that a 450 ohm resistor fits the standard equation of current reduct. How do you determine what watt level that resistor needs to be?

We are going from 300ma to 30ma; does that mean that the original primary current does not matter?

-={I apologize for asking so may questions but I find text books that don't use examples that fit the questions. In this case the resonance factors were found only AFTER you alluded to the construction of the CORE.}=-

Last edited: Jun 21, 2008