Hi Carl,
I always chuckle at some of your responses.....
> Which is 180 degrees away from any other source Tom. You certainly enjoy
> providing your own view of reality.
> Tens of thousands Tom.???? A bit of an exaggeration IMO unless you
> or Martin are willing to release statistics.
> Keep an open mind Tom, the capacitor benefit with 1997 components
> is still in question. I would like to here more on this subject.
> Which goes back to Day 1 on their use.....absolutely nothing new
> there.
> As an owner of vintage muscle cars also I fail to understand your
> comment. I suggest that you do a bit of research into the development of
> the 1N54xx series and "modern" process control before you start condeming
> it out of ignorance.
Since you are always so fuzzy-warm and helpful, let me ask
you this. Besides the power rectifier design engineer at Motorola,
and the semiconductor design engineers at H-P and Micro-semi, I
spoke with, who do you suggest I call Carl? I'll call them!
>I still have heard ( heard..not confirmed) suggestions that
capacitors
> across diode strings can cause problems. From some early conversations a
> few years ago I get the impression that the problem is cumulative.....a
> few diodes/caps are OK but a long string with caps can be destructive. I
> will try and get some professional engineering advice on this before I
> saw off the limb!
It's a simple series circuit Carl. Every engineer seems to agree that
junction punch-through is not a problem. The only problem is one of
current, resistance, and time. From that we can calculate
dissipation, if that is the failure mode (as semiconductor
diode experts seem to claim).
Making a series circuit longer does not aggravate any problems
related to voltage division, unless one component happens to come
along that is greatly out of tolerance with the rest of the string.
The reasons I use caps is simple.....
1.) The diode is a non-linear component with different recovery
times. Let's picture one diode in the string having much faster
recovery time. This goes hand in hand with that diode having less
voltage breakdown. Slowing the rise time of voltage across that
diode (by having the capacitors reactance be much less than the
diodes impedance near breakdown) prevents one FASTER diode from
taking a big hit in voltage cycle after cycle.
2.) When the diodes do switch, the waveform is not a sine wave. Being
non-sinusoidal means the waveform contains harmonics. These harmonics
can be heard in receivers, and most ham stations have receivers. Even
if the PA does not have to pass CE tests for powerline conducted EMI,
why risk generating trash that can affect the receiver?
3.) Bypassing is almost always often less than perfect in high power
PA's. Capacitors prevent RF from being rectified in the diodes, and
generating harmonics.
Finally I'd like to offer this.
I don't believe transients are a paramount reason for using caps
across the diodes unless the supply is transformerless, is wired
poorly, or has a very poor or unusual transformer design.
Normal 60 Hz power transformers have very poor transient response.
The magnetic field in the core can not respond to very fast
transitions, and the layered windings have large amounts of shunting
capacitance and series inductive reactance .
The most common way a high-frequency transient passes
through the transformer is by capacitive coupling from the primary
to secondary. Low loss capacitive coupling would occur only if the
hot side of the primary winding was located NEXT TO the secondary
winding. If the neutral side of the power line was connected to the
primary winding layer next to the secondary, transient voltages
would be limited to near the level of those on the neutral
wire. If it is a balanced winding and line, transients would still
be reduced by the winding impedance.
The usual surge problem in transformer coupled power supplies with
large transformers is low frequency transients. Small caps don't
help this, nor do line by-pass filters. Only MOV's or other hard
clamps across the line terminals can greatly reduce low frequency
transients.
If transients of a frequency that can easily pass through the
transformer are a source of great concern, I'd use power line MOVs. A
filter or small caps across the diodes simply won't do much!
Any technical comments or personal flames?
73, Tom W8JI
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