Voltage Divider - help with simple AC circuit

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.

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

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.

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

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.

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.

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/

Simple answer: this cannot be done.

If you want to see the noise and harmonics components, and also want to live to tell it, you need some pretty sophisticated gear.

A simple devider approach won't work at all. You may think your mains carries a steady 120 Volt, but in fact it doesn't at all. Any load you switch on will vary the voltage on all other outlets. And it depends on several factors (wattage of the load, distance to then nearest power distribution point, quality of the wiring, etc.) by how much the voltage changes. These changes are likely to be an --or several-- order(s) of magnitude higher than the noise and harmonics you want to see.

Also, your circuit would have to be able to cope with spikes that occur on your mains circuit. This is also highly dependent on your local situation. 1 KV spikes are rather common. (I don't want to estimate what you get with pole wire feed during a thunderstorm.)

With your approach: you will at least burn your hands. (As 1800 watts gets hot, by any standard.) Probably your PC too. And hope, that you won't burn your house!

Bottom line: any novice should stay away from direct mains attached stuff. If it doesn't run on batteries or a wall wart, don't touch it!

--
Kind regards,
Gerard Bok

Hi Tomi

- a very sincere thanks for the trouble you've taken to help me out.

If I were to go the transformer route, is there something you could recommend that would do the job without changing the frequency spectrum of my AC mains?

How about something like this thing?

[snip]

Thanks for your advice, Gerard.

Would a variable transformer, such as the one below be useful?

--
Kind regards,
Gerard Bok

"Beside Myself" schreef in bericht news: snipped-for-privacy@4ax.com...

That variac is a nice piece of equipment but it does *not* serve your purpose. Your questions make very clear that you are a novice indeed. I even doubt you to understand what you want to achive. The only thing you can get from your audio equipment is a fat 60Hz hum... if you don't blow it. If you nevertheless want to continue, you'd better first make up your will. Add a note to inform this group so we can nominate you for a Darwin Award.

BTW. I don't blame you for being ignorent. Consider it the strongest warning I can think up to stay away from the mains as long as you don't even know what you're talking about.

petrus bitbyter

an audio isolating transformer (eg 600 ohm to 600 ohm) with a large series resistance (eg 1Megaohm) on the input side

It needn't be something bulky or expensive.

--

Bye.
   Jasen

Your proposal to use a transformer or zenner suggests that you might not have the experience needed to do this safely. Best hire a line analyser.

The frequency response of the 600 ohms audio transformers is geenrally the best when they are driven with 600 ohm source and connected to 600 ohm source. Or driven with low impedance source and connected pretty high impedance load.

I don't think 1 megaohm in series the input side would be optimal for good frequency response.

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

--- Here is a _very_ dangerous way to do it, and if you're not familiar with working with AC mains, I suggest you _don't_ do it.

Anyway, view in Courier:

120 HOT>----+ | [120V 100W LAMP] | +------> TO AUDIO CONVERTER INPUT | [3 OHMS] | 120 NEUT>---+------> TO AUDIO CONVERTER GROUND

The lamp is a standard 120 volt 100 watt incandescent lamp, and when it's on it'll be passing about

P 100W I = --- = ------ = 0.833 amperes E 120V

which, when passing through the series resistor, will cause a drop across the resistor of:

E = IR = 0.833A * 3R ~ 2.5V

Then, with the lamp on, the resistor will hbe dissipating

P = IE = 0.833A * 2.5V = 2.08W

So something like a 3 ohm, 5 watt resistor would be OK to use.

Now, there are two procedures which, if you don't follow, could result in you getting killed or your equipment getting fried, or both.

The first thing is to make _SURE_ that you have the circuit connected to the mains _EXACTLY_ as shown and to make _SURE_ that you've positively identifed the hot and neutral sides of the mains.

The reason for that is that if you connect the thing backwards you'll be connecting 120V directly to the sound card's ground, which is not a good thing to do.

An easy way to find out is to connect an AC voltmeter from the ground terminal (the "U" shaped one) in the receptacle to one of the rectangular contacts. If the receptacle is wired properly, you'll read 120V between ground and the smaller contact, and zero volts between ground and the larger contact. That means that the small contact is HOT and the large contact is NEUTRAL. If you find it wired differently or if you don't know how to do the testing, then blow off the project or get an electrician to help you.

The second thing is that you need to have the circuit connected to the mains with the lamp burning _before_ you connect the output from the resistor to your audio converter.

The reason for that is because when the lamp's filament is cold its resistance is pretty low and it'll let a lot of current through which, when it goes through the resistor could generate a high enough voltage to maybe hurt your audio converters. I just measured a lamp, cold, and its resistance came out to be 13 ohms, so when first turned on, the current through the lamp and the resistor would be:

E 120V I = --- = ---------- = 7.5A, R 13R + 3R

and that current will drop:

E = IR = 7.5A * 3R = 22.5V

across the resistor, inititially, and if your converter ws connected across the resistor before the circuit was connected to the mains, that 22.5V would appear on the input to your converter.

As the lamp filament heats up, the current will drop until it gets to 0.83 amps, then the drop across the resistor will be 2.5V and it'll be OK to connect your equipment across the resistor.

There is a third consideration, and that's to keep seriously high line transients from damaging your converter, and to do that you'll want to put a clamp of some type across the mains. Probably the easiest way to do that would be to get a socket strip with integral surge suppressors and a circuit breaker and use the strip to supply the AC for the monitoring circuit. That's also a good idea since the circuit breaker will cut out if you do anything drastically wrong.

So that's basically it, and if you don't understand any of the foregoing, post back with your questions and I'm sure at least one of us will try to help you.

Understand, though, that no matter what, this is a _dangerous_ circuit and it would be a good idea if you had someone with you when you fired it up. Just in case...

-- John Fields Professional Circuit Designer

[snip]

John, thanks so much for your efforts to explain your circuit idea to me. You've done such a thorough job that I don't have any questions off the bat - except where your stroke of genius came from :)

I'm definitely going to give it a whirl. Will let you know how it turns out. If you don't hear back from me...well, I believe one of the responders wanted to nominate me for a Darwin award...I think I'll make it, tho :D

Thanks again!

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:-)