# Possible mistake in AC current page

#### bluskies

Joined Feb 16, 2012
2
In the "What is alternating current (AC)?" page (http://www.allaboutcircuits.com/vol_2/chpt_1/1.html), I believe there may be a mistake in the analogy between mechanical geartrains and transformers.

The way the article is written portrays high speed/low torque as analogous to high current/low voltage. I believe it should be the other way around, since voltage is more related to speed than torque.

For instance, a taser does not kill even though its voltage is massive - because the current (amps) is very low. In other words, the speed is very high and the torque is very low. But a standard housefold current of "only" [compared to a taser] 240V will kill because the current is relatively massive.

I think the images should be reordered on that page.
Cheers!

#### Wendy

Joined Mar 24, 2008
22,547

The fundamental significance of a transformer is its ability to step voltage up or down from the powered coil to the unpowered coil. The AC voltage induced in the unpowered (“secondary”) coil is equal to the AC voltage across the powered (“primary”) coil multiplied by the ratio of secondary coil turns to primary coil turns. If the secondary coil is powering a load, the current through the secondary coil is just the opposite: primary coil current multiplied by the ratio of primary to secondary turns. This relationship has a very close mechanical analogy, using torque and speed to represent voltage and current, respectively: Figure below

Speed multiplication gear train steps torque down and speed up. Step-down transformer steps voltage down and current up.

If the winding ratio is reversed so that the primary coil has less turns than the secondary coil, the transformer “steps up” the voltage from the source level to a higher level at the load: Figure below

Speed reduction gear train steps torque up and speed down. Step-up transformer steps voltage up and current down.
OK, looking at it for a while I see what you are talking about. In the scheme of things I don't think it matters though. The point is one goes up the other goes down. Unlike hydraulics, the two are not directly analogous to each other.

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#### bluskies

Joined Feb 16, 2012
2
Okay, just thought I'd run it by people in the forum in case someone was interested (or I was mistaken lol).

#### Wendy

Joined Mar 24, 2008
22,547
We always like input. I tend to be a go between for the editor, and mine is not the last word.

Thanks for taking the time.