Transformers - Frequency of AC?

.... Phil

Yes, and it depends on the circuit.

Yes, but it's a bad idea for several reasons. What are you trying to do?

Mark L. Fergerson

I was trying to increase the DC voltage of a source, and I figured that switching it on/off rapidly would kind of emulate AC (at least in the physics sense - that's my background - the current is changing and do the transformer works).

Why is it a bad idea? I can only guess that the current change it too rapid, and so you get huge voltage spikes?

It is often understood that transformers DO NOT work with DC due to the effects of overheating resulting from huge hysteresis losses. There has got to be a reversal of flux and that reversal must take place BELOW the X axis . DC in any form is DC if the values remain above the x axis. I have had transformers exploding and burning out when my students carried out these experiments. We are aware that emf in the secondary is based upon mutual induction....right??? Regards and hope I am right! Elstan

I am aware that the magnetic field due to the primary induces a current in the secondary... with the turns ratio providing step-up/ down.

Not really... you can put DC currents through a transformer and while, sure, it doesn't show up at the secondary directly, it doesn't necessarily cause overheating or significant hysteresis loss either if you've designed the transformer properly. (Plenty of designs make use of a "standing current" like this either to change the core's effective permeability or just because it's convenient from a circuit design point of view.)

You might want to get them some current-limited/current-controlled sources to play with. Have them demonstrate how changing the DC bias on a core changes the resonant frequency in an LC tank or somesuch.

Unfortunately there's a lot of misinformation out there about how magnetics "work" based on people always thinking of single-phase line-voltage input/low-voltage output power transformers when they try to explain things, when such a transformer is one of the most boring/unimaginative use of magnetics possible. Much better to start with inductors with multiple windings and not even call it a transformer for awhile, I think. (Work your way up to circulators by the end of the day. :-) )

---Joel

"Joel Koltner"

....... Phil

What's not sensible about it? I didn't say such transformers weren't incredibly useful -- they are. Just that if you want to really understand magnetics, using a closed core with tightly coupled windings and fixed-frequency AC excitation is an incredibly tiny part of the "all things magnetic" universe and thus a bit limiting to many people.

One of the most useful things someone learning can do with such a transformer is to take it completely apart. :-)

In my mind an inductor with multiple windings performs the exact same function as what we'd conventionally term "a transformer," but opens up a lot more variables to play with (coupling, turns ratio, materials, air gaps, etc.) Is it different to you?

Isn't that the purpose of Usenet? :-)

It's primarily a semantic difference, similar to saying that resistors and potentiometers are different things: The way they work is fundamentally identical, one just has a few more "warts" on it than the other.

---Joel

Well, give us your definition then. In general I'd say a transformer is meant to transfer power from one winding to another whereas inductors generally only have one winding, but this gets muddy when you have things like "tapped inductors" (where the tap provides feedback so is really working more like an auto-transformer), "flyback transformers" (that are really more like inductors in that the power is not transferred from one winding to the other continuously), and so on. Oh, and are "transmission line transformers" still transformers? Since they don't have DC isolation?

---Joel

It's not necessarily a bad idea, and is often a very good one.

If all you need is a solution, consider buying a DC-DC converter module, a neat little package that chops up your DC and reassembles it for you at another voltage, often without a transformer.

How much voltage do you have and how much do you need? How much power? What's the application?

-- Joe