Wednesday, September 2, 2009

Playing with Transformers

I'm on vacation now. Naturally, this means that I no longer have to program, or do circuits labs, or build cardboard bridges. Equally naturally, I'm hoping to get some programming for iGEM done and maybe start a woodworking project and play with electrical stuff...

It was while playing with electrical stuff recently that I found out just how little I actually recalled from my class in Electricity, Magnetism, and Optics (EMO). I mean really, how hard can it be to hook up a fluorescent light? (Answer: if you don't have the ballast, good luck with that.) Even making LED circuits involves a surprising amount of calculation which, engineering student that I am, I immediately delegated to an online designer.

My dad also keeps a box full of random parts salvaged from various radios and computers. This is pretty cool. The box had a few transformers in it, so we decided to test one by hooking a battery up to one end and a multimeter to the output wires to see what we obtained as output voltage. At this point, the electrical engineers reading this (and who have seen the image at the top of this post) can probably guess what happened.

Nothing happened. At least not at first: we had connected the battery, then I connected the multimeter probes, holding them to the output wires of the transformer with my fingers. My dad then removed the battery, and I jumped out of my skin. Fortunately, even with the transformer, the shock from the AA batery was far from deadly.

So what happened here? Why, and this is becoming something of a theme on this blog, am I such an idiot?

The thing is, transformers are meant to be used with alternating current (AC) and not direct current. The reason for this is pretty simple: the transformer consists of two coils of wires which are, as wikipedia says, inductively coupled. And, as I should have known, the induced voltage in an inductor is proportional to the rate of change of the magnetic flux through it. With the battery attached, there was no change in current, therefore no change in magnetic flux and no electrocution. Remove the battery, however, and things do change.

There's nothing quite like a light shocking to act as a remedial science lesson.

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