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In Reply to: Re: Okay, let me ask a real dumb question posted by Dave Cigna on May 8, 2007 at 13:05:22:
"It seems to me that if we consider an ideal choke input supply with very high inductance and perfect diodes (zero dropout voltage) then the current through the choke would be constant, not sinusoidal."I look at it this way.
The current through the choke in a choke input PS is constant and sinusoidal. Constant only meaning that the current never stops flowing.
On the other hand. Look at how low the amplitude is of this current waveform. That's almost constant. That's what 250Hy will get you.My 2 cents.
Follow Ups:
Simulate for 26ms after a delay of 1 second.Then try ...say a 3H choke, a 0.47uF cap, and a 2K8 load resistor and see what I(T1) looks like. By adding another normal sized LC filter you can get the ripple reduction and voltage regulation acceptable and still keep the I(T1) looking about the same.
Not that I know if this is a goal worth chasing mind you:)
Hey Russ,I'm not sure that it matters either, but it sure is fun to look at. ;-)
When all of the talk of RF hash being created and broadcast, i thought it might be interesting to look for the source. This is when i noticed that the current waveform for the "real choke input" seemed to have the most HF content.
How this content gets out is a whole new can of worms i'll save for a later discussion. What i do want to point out is that a distorted current drawn through a resistance gives a distorted voltage. luckily we can consider the line voltage to be a very low impedance source so this would seem to minimize the damage, but remember we do need to add the DCR of the windings as series resistors and suddenly we have the mechanism in place for the creation of a distorted voltage.
still not sure what that means in sonic reality, but we are determining merit by simulation dammit and i say the prettiest picture must sound best.
I'll also add that it seems logical that both undistorted current and voltage waveforms are desired when dealing with filter systems.
"This is when i noticed that the current waveform for the "real choke input" seemed to have the most HF content."I don't understand. Are you saying that this will throw more RF than the one in the link below?
There are two "spikes" per cycle in both waveforms and these are much smaller than the spikes without the choke.
Thanks, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
In this scale, RF content is represented by a vertical line. the more the line approaches a pure vertical, the higher the frequency content. The magnitude of the signals has nothing to do with this. By adding a small choke in front of the first C you end up greatly reducing the narrowness of the current draw "spike" netting what appears to me to be a current draw with much less high frequency content than even a "real" choke input filter.
"By adding a small choke in front of the first C you end up greatly reducing the narrowness of the current draw "spike" netting what appears to me to be a current draw with much less high frequency content than even a "real" choke input filter."In this example, what value choke would be "best".
Thanks, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The best choke would be the one chosen by ear, but since we are in sim land you first have to decide what you want the current to look like then decide what looks best. How that translates to what sounds best is still up for discussion.
Anything that adds resistance/impedance between wall power and first cap will help increase conduction angle. Any increase in conduction angle moves you closer to a real choke input supply.Play around....up the transformer DCR...go to FWCT and a 5U4....try an RC filter stage. There is no best here. It is more of a mental exercise and to see if what looks "pretty" on the scope/sim.....and/or.....to see/hear if drawing current at a steady constant level (that reverses each 1/2 cycle) is better than drawing current in sine wave manner. Don't forget to increase load as needed.
Being who and what we are here, I reckon when we see an evil square wave we freak and think "sum of odd order harmonics".
but russ, that Evil R instantly converts distorted current into distorted voltage.
I didn't mean best in that way. When I add a small choke it lowers the amplitude but doesn't change the shape. Am I missing something?Thanks, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
if it lowers the amplitude, it has to change the shape. Assuming the same power into the load, the area under the plots must remain the same independent of filter type, so if you decrease amplitude, you must widen the trace at some point to keep the area constant.
It appears that the total magnitude is higher for green, but green drops off faster/more with increasing frequency. I'm a little hesitant to mindlessly trust the sim results. It would be interesting to build just these two simple supplies and measure the actual noise.
Yes, the FFT plot was an eye opener. I'll take it as gospel but it wasn't what I expected!So if we are going to draw power from a transformer and feed it into reactive parts how do we get some same voltage and current waveforms that we would if said transformer supplied power to a resistive load? I suggested one way but perhaps you have another you'd like to share? Also, have you played around with putting line reactors (chokes) before the power transformer? Maybe if we "contained" all the bad looking stuff to only the PT we are doing fine???
Anyhow, interesting discussion and I am glad to see I am not totally off base.
Russ
P.S. It looked like you were a little "kind" with the cap input filter. Perhaps we could come up with one that had worse spikes and see how that models.
actually the green trace was for a Low L input filter... it sort of takes the edge of the full C input.
Did the choke and cap models include parasitics? I've been playing around quite a bit with Spice sims of supplies and the effects are an eye opener above the first cap self-resonance.
Thermionically addicted.
Even on a picture-only basis, we're looking at less than half. Step Tre's 250H/220uF load and see what happens to the current.
Thermionically addicted.
I think people are getting bit confused here. I am referring to the current through the power transformer primary which represents how current is drawn from the wall.
I see what you mean.It seems to me that the theoretical limit of choke input (infinite inductance choke, etc.) would make the current waveform in the transformer a square wave. Obviously, that's got tons of hf content. Cap input has spikes which also have hf content. If a little inductance at the input helps the cap input then maybe critical inductance would be just right. Spread the cap input current waveform out until it just barely conducts from start to finish.
Unfortunately, that would work at only one average DC current. Change the load and you lose that balance. (Hmmm... I've heard people report that they like swinging chokes....)
It also assumes that the critical size inductor works as well at high frequencies as the deliberately subcritical one. That's a practical issue.
Eh, I suppose I could sim it myself, but it's a really nice day and I just don't feel like it. Maybe later. Maybe. :)
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