Please help -- SPICE simulation and imolementation mismatch

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Could some electronics guru please provide
some hints about what might be yhe problem
in the following setup ?

First I SPICE simulated a simple sertes
circuit consisting of a signal source
(referenced to ground) connected to a
capacitor, follwed by a series resistor
and finally a capacitor to ground. Both
capacitors are non-ideal(includes ESL,
ESR). This is a very simple model of
capacitive coupling. With a signal of 6V
60Hz sine wave, transient analysis provides
very good results. The capacitance values
are approximately 10.0pF and the resistor
is 5K.

Now, as an experiment, I hand constructed
the two capacitors with single layer copper
clad board, separated by a phenolic sheet
3,0 mm thick. The dimensions of the copper
area of Each of the single layer copper clad
pieces is 2 cm x 2 cm and four 2 mm diameter
stainless steel screws hold the three pieces(2
capaciror plates and insulator) firmly together.
Using a relative permittivity of 8 for phenolic,
the capacitance value is 9.44pF for each
capacitor.

I connected a signal generator set at 100 Hz,
6V sine wave to the circuit described for the
SPICE simulation, with my homebuilt capacitors
and a 5K resistor, and read 0.0 Volt AC at the
resistor terminal -- the SPICE simulation showed
5.094 Volts. Given that a capacitor is a short
to AC, what could be the problem ? I understand
that polysttrene capacitors are ideal for AC
operations, but this setup ought to show some
results. I repeated the experiment with a 6-0-6
500 mA output transformer, but there was no
chnage.

Any hints/suggestions as to what the problem might
be would be greatly appreciated. Thanks in advance
for your help.

Re: Please help -- SPICE simulation and imolementation mismatch
wrote in message  

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and a 5K resistor, and read 0.0 Volt AC at the resistor terminal -- the  
SPICE simulation showed 5.094 Volts. Given that a capacitor is a short
to AC, what could be the problem ? I understand that polysttrene capacitors  
are ideal for AC operations, but this setup ought to show some
results. I repeated the experiment with a 6-0-6 500 mA output transformer,  
but there was no chnage.

Quoted text here. Click to load it

This is really a question for the basics newsgroup, but you are making an  
assumption that a tiny capacitor is a short to a low frequency AC signal,  
when it actually presents an impedance (or reactance) of X(c) =  
1/(2*pi*f*C). You must be doing something wrong in LTSpice. You could copy  
your ASC text file for LTSpice if you want someone to try it.

Paul  


Re: Please help -- SPICE simulation and imolementation mismatch
On Sat, 19 Sep 2015 00:40:58 -0400, P E Schoen wrote:

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I do not think he is doing anything wrong in spice. He made a (albeit  
high impedance) voltage divider which in Spice works as such.
As soon however as you construct the circuit and connect a real voltage  
meter to the mid-point, the internal impedance of the capacitive voltage  
divider 1/(2*2*3.14*60*5*10^-12) = 1/(0.36*10^4*10^-12) = 300 MOhm  
(capacitive) is so high that the voltmeter acts almost like a short  
circuit, hence the very tiny, if not 0,measured voltage.

Maybe a capacitor of 1000uF would be a better choice, but is not so easy  
made at home.

joe

Re: Please help -- SPICE simulation and imolementation mismatch
On Fri, 18 Sep 2015 21:24:55 -0700, dakupoto wrote:

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When you typoed "imolementatian" I thought maybe you'd burnt something up  
(immolation, y'know).  Boringly, that appears to not be the case.

a 10pF capacitor has a capacitive impedance of around 160 megohm --  
that's 160 * 10^6.  When we liken a capacitor to "a short circuit at AC"  
we mean AC of infinite frequency.  At any real frequency the capacitive  
impedance is non-zero, and it is inversely proportional to frequency.

Either you did not use 10pF caps in your SPICE model, or you used a much  
higher frequency than  100Hz.

The rule of thumb for using coupling caps (which, since it's a rule of  
thumb, is only usually good) is to choose a capacitive impedance which,  
at the lowest frequency of interest, is 10 times the impedance it's  
"looking into".  You've got a series pair of 10pF caps looking into a 5k  
resistor, so that means you need to do your test with a 25MHz source.

--  
www.wescottdesign.com

Re: Please help -- SPICE simulation and imolementation mismatch
On Sat, 19 Sep 2015 00:10:59 -0500, Tim Wescott wrote:

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For all the critiques you're getting -- it's GOOD to experiment.  Your  
experiment has just presented you with wildly unexpected results, which  
you're going to remember _forever_.  Even though an experienced engineer  
may say "of course that'll fail", a lot of the good ones (myself  
included) have just that sort of "failed" experiment in their past.

--  
www.wescottdesign.com

Re: Please help -- SPICE simulation and imolementation mismatch
It'll be even more memorable and useful if the OP calculates it for himself. Experoments are good for developing intuition fast, and algebra is even faster.  

Cheers

Phil Hobbs

Re: Please help -- SPICE simulation and imolementation mismatch
On Sat, 19 Sep 2015 00:42:28 -0700, Phil Hobbs wrote:

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Apparently he still has to learn that, and this was a good lesson.

joe


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Re: Please help -- SPICE simulation and imolementation mismatch

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  Exper-o-mentally speaking, of course.  :-)  Hhehehehe

Re: Please help -- SPICE simulation and imolementation mismatch
On 09/19/2015 09:24 AM, DecadentLinuxUserNumeroUno wrote:
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Phone keyboard and fat fingers.

Cheers

Phil Hobbs

Re: Please help -- SPICE simulation and imolementation mismatch
On Sat, 19 Sep 2015 12:06:25 -0400, Phil Hobbs

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  Expeditiously speaking, of course.  ;-)

Re: Please help -- SPICE simulation and imolementation mismatch
On Saturday, September 19, 2015 at 1:12:56 AM UTC-4, Tim Wescott wrote:
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Thanks to each of you for pointing out  
the capacitor reactance, that I overlooked,  
and has caused much misery.  The Ngspice  
file is listed below:

.PARAMS PERMFS=8.854187817p RELPERM=8
.PARAMS LEN=0.02 THK=0.005 ; METERs

.SUBCKT NICAP 1 2
* 1 IN
* 2 OUT
C0 1 3 '(PERMFS*RELPERM*LEN*LEN)/THK'
L_ESL 3 4 4.5nH
R_ESR 4 2 0.5
.ENDS

.SUBCKT CC 1 2
* 1 IN
* 2 OUT
XC0 1 3 NICAP
XC1 4 2 NICAP
R0 3 4 5K

R0 1 2 1.0
VSIG 1 0 DC 0.005 SIN(0, 6.0, 60.0, 0, 0, 0)
VTEST 3 0 DC 0.0 AC 0.0
XJ0 2 3 CC

.OPTIONS NOPAGE METHOD=GEAR
.IC V(2)=0.0
.PROBE
.TRAN 50ms 500ms 10ms UIC
.PRINT TRAN V(2) I(VTEST)
.END


Re: Please help -- SPICE simulation and imolementation mismatch
On 20/09/15 05:05, snipped-for-privacy@gmail.com wrote:
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There's an .ENDS missing in there.

Didn't you mention that you were doing this with LTspice?

Jeroen Belleman

Re: Please help -- SPICE simulation and imolementation mismatch
On 19/09/2015 05:24, snipped-for-privacy@gmail.com wrote:
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If your voltmeter had infinite impedance (like the spice probe tool)  
then you would see something but the 1Meg or 10Meg input resistance of  
your real-world voltmeter is simply too big a load for such a tiny  
current. Use spice to measure the current flow in your series 5K  
resistor and tell us what the current is.

piglet


Re: Please help -- SPICE simulation and imolementation mismatch
On 19/09/15 10:18, piglet wrote:
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*That* is your problem. A capacitor is *not* a short for AC.
It's an impedance Z=1/(j*2*pi*f*C). At 100 Hz, a 10pF capacitor
would be j160 Mohm!

Another thing you may be unaware of is that Spice uses base
units throughout. Capacitor values are thus given in Farads!
Fortunately, it understands (most) multiplier prefixes, so a
10pF capacitor value can be written as 10p.

Finally, LTspice .asc files are simple text files. Next time
you want to discuss simulation results, append the .asc file
to your message, so that other people can see what you did.

Jeroen Belleman

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