Sort of OT - transformers

[Paul Marossy]

Maybe this is a dumb question, I just have never really thought about this until now...

OK, here we go: Does it matter to a transformer if it sees AC or DC? In other words, if you were to pull something like a 120VAC/18VAC transformer out of something and you wanted to use it as an DC step-up transformer instead, would the transformer care? Or are transformers designed specifically for use with AC or DC?

[Tubebass]

Transformers only work on AC.

[smoguzbenjamin]

Wouldn't DC kinda screw up the tranny? :?

[The Tone God]

It wouldn't screw it up unless you went past the transformer's maxium ratings. A transformer to DC is  just an inductor.

Andrew

[smoguzbenjamin]

Dang technology teachers :x The lie, all of them!

[The Tone God]

Dang technology teachers :x The lie, all of them!

Stick around here for awhile then start to correcting them.

Andrew

[niftydog]

A transformer to DC is just an inductor.

and to DC, an inductor may as well be a piece of wire.

[R.G.]

It wouldn't screw it up unless you went past the transformer's maxium ratings. A transformer to DC is just an inductor.

That's a good answer, but it deserves elaboration.

If you must apply DC to a transformer, pick a winding to use and apply a **current limited** source to the winding; in that case you will not burn up the transformer if you have the current limit set so that the current is below the rated current for that winding. It acts just like an inductor. There is exactly zero volts across the other windings because there is no voltage  change or current change to transform.

Note that the current is limited by only the resistance of the winding, so a 50 milliom winding connected to a 6 volt battery will try to pull 6V/0.05 = 120A of current and will try to dissipate 120*120*0.05 = 720W and you have just invented the toaster.

Just like an inductor, if you interrupt the current, the inductor seeks to keep the current flowing by inverting the voltage across the winding and trying to suck the pre-existing current out of the source that was supplying it. The voltage is limited only by the self capacitances of the coil and the breakdown voltages of the stuff around it. The other windings, which are still coupled to the energy in the core through the magnetic field also see a change in voltage and display a sudden voltage, in proportion to their number of turns. This is how a car ignition coil worked in the days of non-electronic ignition.

If you do not limit the current in the winding, the heating burns up the wire.

Transformers work on **changing** voltage. The cannot transform DC at all.

A mixture of AC and DC is interesting. If you impose a little DC on a power transformer working on 120Vac, the peaks of the voltage are usually pushing the core as close to saturation as it can stand without the DC. The DC causes a sizeable offset to either positive or negative, the peaks saturate the core, and the available 120Vac peaks are not limited by the transformer core inductance, so **large** currents flow. You're back into toaster territory.

If you're working the transformer under its rating, say like a 220Vac transformer on 120Vac, you still can't add 120V of DC because the DC current is limited only by the resistance of the winding. You can, however, offset the core to one side or another with a current limited DC. There's hardly ever a need for this knowledge, however.

[Paul Marossy]

Gee, thanks for all the replies.  

I suspected that you could only use them with AC power. (Sorry, I don't know that much about transformers, yet)

But, now I see inductors in a new light. They always seemed mysterious to me until now.

OK, so next dumb question, then. These wall wart transformers, how do they convert AC to DC? Is there just a 4-diode full wave rectifier inside of them, and no filtering of any sort? That would seem to explain why some of them hum like a son of gun...

[niftydog]

sometimes not even FOUR diodes, just two!

The better quality ones have full wave bridge rectifier, filter caps and regulation.

The crap ones have two (or even one!) diode, and rely on the circuit it's feeding to provide the filtering.  This is especially so in wallwarts designed as battery chargers.

A general rule of thumb; the good ones are SCREWED together, not "plastic welded".  ie; good ones can be opened up and repaired without destroying the case.

[Paul Marossy]

Half wave rectification is even worse! No wonder those things hum like they do. Come to think of it, I have noticed that the good quality wall warts are screwed, together that is.  :wink:

[niftydog]

JINX!!