Newsgroups: sci.electronics.misc
Subject: Re: Voltage Divider - help with simple AC circuit
References: 
Beside Myself  writes:

> As you'll gather quickly enough, I'm a novice, and I would appreciate
> the guidance of someone experienced in electronic circuit design.
> 
> I would like to build a simple Voltage divider circuit. I want to take
> the single phase 120VAC, 15A, 60Hz out of my wall and step down the
> voltage/current to around 2 - 4VAC & 1A - 1.4A. 

You are talking about pretty high power voltage divider.. 
1.4A from output of voltage divider would indicate that your
voltage divider would need to take at least this amount of current, 
practically usually considerably more from 120V AC. 

We are talking here ower 160W power losses here!

Not for any small resistors.

> My goal is to feed
> this signal into my audio converters so I can see the constituent
> harmonic energies and other noise in the mains AC. I would like to
> read from 1Hz to 20kHz.

Where do you need here that high current ? 
Audio converters have typically quite high impedance inputs.

> I'm guessing the easiest way to do this would be to use a transformer,

That's usually the easiest and safest. The transformer typically gives 
you safety isolation from the mains voltage.. With resistor divider 
you measuring circuit is directly connected to mains wires. 

> but as I understand it, transformers don't handle frequencies equally.

This is usually more or less the case. Some transformers do this 
better, some worse. 

> So I'm thinking a resistor-based circuit might do it. Do they make
> resistors that could dissipate that much energy safely? (i.e., 1800W
> less approx. 5.6W). 

There are high power resistors that can dissipate lots of power 
when properly cooded. Usually the cooling is made my using 
a large heatsink or make the resistor heat some liquid (water, oil). 
High power resistors are used in heaters, ovens, washing machines 
(to heat the water in it), tea water heaters, coffee makers... 
You can easily see 1 kW resistors in those heating applications. 

> Is it that easy...? (My suspicion is that nothing
> ever is) I would also like to build in a fuse so that a transient in
> the mains doesn't blow my converters!!!

A fuse in the circuit powered from mains is practically a must 
for safety reasons. It will burn if your circuit accidentally 
starts to take too much power (for example short circuit). 
A properly sized fuse will blow before some part of your circuit 
will start burning. 

Fuses do not help much to solve mains transient problems. 
Fuses are too slow protection components to protect 
against transients. Against transients need othet kind of 
protectors. 

> There is also the possibility of a zener diode-based circuit, but I'm
> unsure whether the diode would introduce unwanted changes to the
> frequency spectrum, as in the case of transformers. 
> Your help is very much appreciated.

Zener diodes are used in many protection circuits. A properly 
sized zener diode should not have too much changes in measurement 
results. There are lots of measuring electronics that use zener 
diodes and other protection circuits in their inputs to protect them. 
The key in selecting those components is select them in such way 
that they do not do nasty things on the normally used measuring 
voltage range. 


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/