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Old 27th September 2011, 09:38 PM   #1041
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Klewis, find a diagram of the wiring system of a house and you will have an idea of where the ground goes. I don't know why you would get no current through that resistor, unless your voltage source had too high a frequency or too low a voltage. Or you had accidentally given one of your caps a low value parallel resistance instead of series resistance.

For the inductors you need to have a coupling parameter, for instance

K1 L1 L2 .9

Where .9 is the coupling coefficient; 1 is perfect but not realistic. You will need to flip the inductors depending on whether the coupling is common-mode or differential-mode. I think the chokes in line filters are common-mode (not sure).

Here's a schematic for my Feller filter, with improvised house wiring (bad guess probably). Response graph is probably unrealistic too.

- keantoken
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Old 28th September 2011, 04:14 AM   #1042
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keantoken is correct, the sine source needs to be ground referenced. for 6 amps, make R3 20 ohms. if you need to simulate noise on the line, you can add a noise source in series with the sine source (for the frequency response test make the AC parameter of the noise source 0V). all voltage sources in LTSpice are zero output resistance by default.

i think the resistance between neutral and ground is more realistically around 100-200 milliohms. there's quite a bit of footage of copper in those walls, and a lot of the integrity of the ground and neutral runs depends on the skill of whoever installed the wiring. i once tried to help somebody figure out a wiring problem in their house, and the neutral and ground at the end of a string of outlets had about 1 ohm between them (with a 10 amp load, the difference between neutral and ground was 10V, and 110V between neutral and hot). now that's extreme (and dangerous) (fortunately, tightening up some screws on some of the outlets in the string brought it down to about 0.2 ohms). likewise, poorly installed grounds can be a hazard if something shorts to ground.
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Old 28th September 2011, 05:37 AM   #1043
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That's interesting, maybe I should take apart all my sockets and tighten (solder?) them. But tell me what's the solution when none of your outlets are grounded? We have a grounding post outside but that seems to be a luxury exclusively for the phone box and electric meter.

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Old 28th September 2011, 03:14 PM   #1044
klewis is offline klewis  United States
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Quote:
Originally Posted by keantoken View Post
That's interesting, maybe I should take apart all my sockets and tighten (solder?) them. But tell me what's the solution when none of your outlets are grounded? We have a grounding post outside but that seems to be a luxury exclusively for the phone box and electric meter.

- keantoken
Thanks for the help. Kean, no don't solder your outlets. The reason the screws seem to loosen is because when you draw significant current through an outlet, the wire heats up and expands. With the expansion, either the screw stretches, or the wire compresses. Either of these actions are permanent, so, the screw to wire contact is reduced. This bad contact then increases the heat when current is drawn and the process repeats until the contact is pretty weak and the screw is quite loose. In modern house wiring the neutral and ground are joined at the fuse/circuit breaker panel. It's a safety thing, two ground paths are better than one and also better in case their is current on the neutral in the particular circuit. In older houses, the neutral is also the ground and is connected to the earth ground at the panel.

Ken
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Old 29th September 2011, 12:31 AM   #1045
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soldering is a bad idea, because if the solder melts and oozes down inside the outlet, it can cause a fire. you don't want anything in house wiring that can melt and migrate.
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Old 29th September 2011, 02:52 AM   #1046
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Right, I thought it was weird they used screws to clamp house wiring...

Thanks,
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Old 10th October 2011, 03:23 PM   #1047
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Quote:
Originally Posted by Nelson Pass View Post
I use SPICE to good effect, but I don't rely on it to give very
accurate results for non-linear circuits. I have to smile if someone
quotes parts-per-million distortion figures from a simulation.
True!
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Old 10th October 2011, 03:54 PM   #1048
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Quote:
Originally Posted by michaelkiwanuka View Post
True!
Mike,

I don't disagree with what Nelson said in the quote, but that is far, far from supporting your overly-general statements that SPICE is of no use in THD simulations. The truth is that when you get into parts per million (i.e., below 0.001%) there are MANY real world things that are not modeled in a typical SPICE simulation, like magnetic coupling and other physical design related things.

But for an amplifier that is well implemented, SPICE distortion simulations are very useful down to better than 0.01%.

You are throwing the baby out with the bathwater when you make your very general claims like this, as you have on several other threads.

Buy my book, read the two chapters on SPICE, use my models and then see what you get. If you go to my website, you can be up and running with LTspice in about 10 minutes, with good models freely available to boot.


Cheers,
Bob
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Old 10th October 2011, 04:32 PM   #1049
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Quote:
Originally Posted by Bob Cordell View Post
Mike,
[snip]
The truth is that when you get into parts per million (i.e., below 0.001%) there are MANY real world things that are not modeled in a typical SPICE simulation, like magnetic coupling and other physical design related things.
[snip]
Cheers,
Bob
Hi Bob,

As for magnetic coupling, it's quite easy to simulate the effects. Just insert small inductors into the traces/leads who are involved and define the coupling coefficients. Admittedly, it's a bit of guesswork if you don't have specialized software. In that case one have to revert to simple rules of thumb, but it can be done (or use my magic little pot to cancel it out ).

Cheers,
E.
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Old 10th October 2011, 05:02 PM   #1050
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So where can I these "rules of thumb"?

Thanks,
- keantoken
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