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Capacitor Questions

Old Steve

Jul 23, 2015
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I explained that in my post #55:-
me said:
It works perfectly. You have the ground connected to the AC source, then you're measuring from the positive DC output back to ground, rather than to the negative DC output. Make sense?

(I'll leave the next question to Harald when he logs back in.)
 

Harald Kapp

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Its easy for you to say my schematic works AFTER you altered it.
The only alteration was giving names to the nets, so you can see where the output voltage has to be measured. Nothing more.

I don't know what "out P and out N" are nor do I know where or how to access them in LTSpice.
out_P and out_N are symbolic names for the nets, so you can easily identify the nets instead of having to look for cryptic net names like n002. You access them in LTSpice by the little symbol with the "A" right next to the GND symbol in the menu bar. A window will pop up, enter a name and attach the symbol to a net.

Is the out P out N another way of measuring the voltage properties of the simulation.
Not another way, the only correct way. Since GND is connected to the input voltage, any voltage on the secondary side (after the rectifier) referring to GND will see the input voltage's sine as part of the waveform.

Harald Just as an aside. I altered the Grounds on my original as suggested by Steve and then the simulation did show the ripple effect by pointing the "Voltage Pen" on the output.
By moving GND to the secondary you changed teh reference point for voltage measurements.

As an aside :D: Voltage measurements are always between two points (potentials), never on a single point. You can easily test this by touching one pole of a battery with your tongue: nothing happens. Touch both poles and you will will feel the tickling from the voltage (aka potential difference) between the two poles (Note if you really try this, don't use more than a 1.5V battery!)
We (and LTSPice too) most of the time simplify matters by designating one potential as reference, calling it 0V or GND or similar and form there on reference any voltage measurement to this reference potential. Therefore the "measurement" V(n002) is really V(n002,GND). This is how in my "modified" circuit I measured the output voltage aas the difference between the potentials on out_P and out_N as V(out_P,out_N).

Perhaps you could explain why my original schematic did not show the "Ripple effect" and how you got yours to show it.
See item 2 in post #49.
 

Farticus

Jun 21, 2015
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The only alteration was giving names to the nets, so you can see where the output voltage has to be measured. Nothing more.

out_P and out_N are symbolic names for the nets, so you can easily identify the nets instead of having to look for cryptic net names like n002. You access them in LTSpice by the little symbol with the "A" right next to the GND symbol in the menu bar. A window will pop up, enter a name and attach the symbol to a net.


Not another way, the only correct way. Since GND is connected to the input voltage, any voltage on the secondary side (after the rectifier) referring to GND will see the input voltage's sine as part of the waveform.


By moving GND to the secondary you changed teh reference point for voltage measurements.

As an aside :D: Voltage measurements are always between two points (potentials), never on a single point. You can easily test this by touching one pole of a battery with your tongue: nothing happens. Touch both poles and you will will feel the tickling from the voltage (aka potential difference) between the two poles (Note if you really try this, don't use more than a 1.5V battery!)
We (and LTSPice too) most of the time simplify matters by designating one potential as reference, calling it 0V or GND or similar and form there on reference any voltage measurement to this reference potential. Therefore the "measurement" V(n002) is really V(n002,GND). This is how in my "modified" circuit I measured the output voltage aas the difference between the potentials on out_P and out_N as V(out_P,out_N).


See item 2 in post #49.
Harald forgive me if I seem a bit slow but your explanations have given me some better sense of where I was going wrong.
I finally get the bit about moving the pointer between TWO reference points to give me the real voltage differential.
In the diagram I first submitted I had no ground so I had to measure TWO points + to - to get the voltage and ripple.
Would I be correct in assuming that once I had inserted the correct "Ground" (Is this V0??) that LTSpice will default to this ground. This means that when I was using a single point of reference on the "corrected" circuit that Spice automatically used the "Ground" as the second reference point?
 

Harald Kapp

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In the diagram I first submitted I had no ground
But of course you had: the little triangle left on "-" of V1. Without a defined GND No Spice base simulator works. It will always stop and ask you to enter a ground. That is precisely because it needs a 0V reference. Otherwise the simulator (and by the way in reality no instrumenT9 could tell whether a voltage diffference between say points A and B is due to e.g.
V(A)=0V, V(B)=10V => V(A,B)=10V
or
V(A=1000V), V(B=1010V) => V(A,B)=10V

means that when I was using a single point of reference on the "corrected" circuit that Spice automatically used the "Ground" as the second reference point?

Yes, GND is always the reference potential. The two-point measurement method saves you from having to move GND around between simulations to measure different voltages (potential differences).
 
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