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In Reply to: Re: Which PS would you rather listen to? posted by arend-jan on May 8, 2007 at 06:22:06:
Hi!That may well be. I wrote "I think" because I'm not sure. It's been a while since I played with this since I switched to three phase years ago.
Should be easily verifyable by monitoring the volatge across both diodes and see if there is a overlap or a fragment when none conducts.
Then there would a period in which the voltage across both diodes is above the "knee voltage" at the same time.I thought a mechanism like this is in play here: The choke tends to keep the current flowing. Thus the voltage at the end connected to the diode is pulled lower and lower, so the diode keeps conducting. When you scope the voltage at the input side of the choke (or after the rectifier) you will see that it is not just a rectified sinus going from 0 to the crest voltage, but it also sags below zero, which would indicate that the diode would still conduct even if it's input voltage approaches 0. But at a certain point the other diode will start to conduct too.
A scope picture of the secondary voltage points to this direction. It seems as near the zero crossing, the voltage "sticks" for a while. I assumed that is the periode when both diodes are conducting.
But again. I'm just wildly guessing here.
This certainly is the mechanism with three phase supplies. There the insertion of the resistor helps. This causes one diode to stop conduction just before the next begins. Although with this "fix" the current also ceases flowing for a moment, no voltage spike arises.
Best regards
Follow Ups:
All this talk about trees, how are you implementing three phase?!?
Thermionically addicted.
Hi!There is an article in Sound Practices 17. This issue was only released on the archive CD.
There is nothing special about it. Three phase schemes can be found in many electronics books. For example in the first chapters of the RCA transmitting tube manuals.
There's also an article in wikipedia. This is the scheme I typically use:
http://en.wikipedia.org/wiki/Image:Three-phase_bridge_rectifier.jpg
only TV dampers instead of the silicon diodes.
Let me know if you have specific questions.
Thansk Thomas. I understand the use of 3-phase and its advantages but are you using direct 3-phase service in the home or synthesizing it from single phase?
Thermionically addicted.
Hi!3 phase service right out of the fuse box available over here :-)
I remember discussing the 3-phase supplies with you quite some time ago. Short of running a rotary converter it isn't an option for me. But I might have a chance to pick up a bunch of SLA batteries from a UPS that is being replaced. That would give me a 480VDC supply (but I am a little concerned about safety!).So have you ever tried a battery (only) supply for B+ and if so, how did it compare?
Hi!No I haven't. I plan to run a phonostage off batteries. But that's a future project. A friend of mine is using a battery powered phonostage and likes it very much (he uses 3 phase for most of his system tto)
just to see what happens. I'll post the results when I'm done.
Naz
Thomas,
I'm sort of with the "there is a small amount of time where the diodes fight each other" school.Actually what happens is that this:
A diode when conducting has a depletion region of a certain width with a electrostatic field across it - that is it has a capacitance. When reversing the applied voltage you have to discharge that capacitance (the physics bods talk about sweeping out the minority carriers but its easier just to think in terms of discharging the cap). This results in a sharp peak of current FROM the power supply capacitors BACK INTO the diode before it turns off. The size of this "splat" of current depends upon the size of that intrinsic capacitor or more exactly the amount of charge in that capacitor which must be swept out to turn the diode off.
This charge is called Qrr the "reverse recovery charge" and it will not be a surprise to most of you that typical values are:
standard silicon diode - 500nC
Ultrafast Soft Recovery - 100nC
Schottky Diodes - 50nC
Silicon Carbide Diodes - less than 20nC
Tube Rectifier - 0nC
Thomas> > When you scope the voltage at the input side of the choke
> > (or after the rectifier) you will see that it is not just
> > a rectified sinus going from 0 to the crest voltage, but
> > it also sags below zero, which would indicate that the diode
> > would still conduct even if it's input voltage approaches 0.When the diode stops conducting it doesn't care
what the choke wants - it isn't conducting.
Now you have the inductor drawing current that
the diode isn't supplying so then you get the
lowering of the choke's supply side voltage.
The choke is sucking every available electron from
the wire and causing a voltage spike which only
discourages the diode from conducting all the more.
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