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In Reply to: Ray, the whole point of using a choke input filter... posted by Allen Wright on May 6, 2007 at 03:18:51:
How right you are. I tried to show this with screen shots, from the PSUD, of the huge charging spikes present in a supply with an input choke that does not meet critical inductance.No one seemed to notice. I know people respect you so I am showing this screen shot again in hopes that people will take notice.
A choke input supply with too small a choke is not a choke input supply at all. It will not have the regulation of a choke input supply and it WILL have the spikes of a cap input supply.
This circuit is drawing about 80ma. but we have large (over 350ma.) charging current spikes. More important than the amplitude is the shape. Those sharp spikes will, as you say, create RF that radiates through out the amp.
With a proper choke in place and the circuit adjusted to draw the same 80ma. DC, the charging currents are held down to just over 25ma. peak and are Sinusoidal, no spikes = no RF.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Okay, I'm convinced! I wasn't crusading against choke input filters, just asking a question, which now seems to have been well and truly answered. Thanks, everybody!
Solve one problem, create another !! Boo and G-d bless Ivan303 who I've plagerized.
In general, how many Henries do you need to really get critical inductance? I'm guessing it depends on current draw?
The formula is printed in the Radiotron Designer Handbook 4th edition, and depends on both the load presented to the power supply RL = voltage of power supply/current drawn by load, and the AC frequency. For a 50 Hz supply,L = RL/940
For a 60 Hz supply,
L = RL/1130.
These are approximate values to achieve continuous current flow according to the RDH. If I read the text correctly, this also suggests that below the critical inductance the current flow is not continuous during the whole cycle.
then too much should be just about right!I don't know what Jeff's doing or hearing, but it makes NO sense from any understanding I have - no more than his "crazy" grid stopper concepts.
And this isn't a critque of Jeff as such - I just wish his ideas came with some understanding.
Can't speak for Jeff, but he's expressed some, well, I would call it comtempt, for the RDH in some past posts. It seems his answer for any criticism is "just listen to it."
What Henry showed us, not Jeff, was that the right combo of small first choke (L1) and small first cap (C1) made it possible to charge C1's voltage to more than the normal max of 1.4 times the secondary RMS value. When a more "normal" LC stage followed it, regulation proved to be very good. Tuning is rather touchy (load dependent too). Personally I was troubled by the very high voltages developed in the first choke but Henry felt it was a limitation on Duncan's sim. I have not bothered to build it nor model anymore. Call me old school but I have no problem with a regular choke input filter.
Choke value in henries = ((voltage /current) + resistance of choke))/1000Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
At any given rail voltage, there is a minimum (critical) inductance that's required for proper choke I/P operation. Frankly, I don't know the formula for critical inductance.Once the critical inductance criterion is satisfied, a minimum current draw (the critical current) is needed for proper operation. A good approximation for the critical current, in mA., is V/L. When a bleeder resistor is used to ensure the critical current gets drawn, it turns out that 1 KOhm of resistance is appropriate for each Henry of inductance present in the 1st position.
Eli D.
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