After spending way too much time trying to seperate useful and useless
comments and form a logical response I gave up.
Suffice it to say that the real world of tube type ham amps uses the 1N5408
almost universally no matter what the configuration. If they could save a
penny they would and all those engineers cant be wrong and you the only one
with an alternative. It is time you realize that.
As far as the 1N4007 there is a lot of obsolete technology still in
production and for simplistic applications the price is right. AFIK it hasnt
been in commercial ham amps for over 30 years.
End of discussion
Carl
KM1H
----- Original Message -----
From: "Manfred Mornhinweg" <manfred@ludens.cl>
To: <amps@contesting.com>
Sent: Tuesday, October 14, 2014 3:18 PM
Subject: Re: [Amps] HV Diodes
Carl,
> ** IMO there is a balance point between using obsolete diodes and
overkill.
Are you suggesting that the 1N4007 is obsolete? I just checked, and it's
still
in full production by several companies, such as Diodes Inc, Fairchild, ON
Semiconductors, Vishay, and a few more. It's a highly popular diode,
massively
used, and despite having been available for a long time, I haven't seen any
signs of it becoming obsolete.
To me, a device is obsolete when something better becomes available at the
same
price, or something as good becomes available at a lower price, and thus the
sales of the original device fall, making the manufacturers discontinue it.
As
far as I know, this hasn't happened to the 1N4007. But it has happened to
almost
all tubes...
So, I would look for a balance point between reliability and cost. Given the
low
cost involved, indeed 1N5408 diodes might be that sweet spot, despite being
huge
overkill. My point was to illustrate that technically the 1N4007 diodes
should
be fine for legal limit ham amplifiers using bridge rectifiers, but a bit
tight
for those using voltage doublers. There was no intention to force any of you
to
use them! So, please don't eat me alive... ;-)
>> Let's assume a pretty big amplifer, solid legal limit, CCS, which is
more
>> than any ham needs. The power supply might deliver 3500V at 0.8A. Each
>> diode string in a bridge rectifier will then see a peak voltage that
might
>> reach 4000V in the event of line overvoltage, and an average current of
>> 0.4A at full output.
>
> ** That is an obsolete assumption with RTTY and data modes having many
> users these days. Even AM linear is pushing it.
Sorry, Carl, I don't understand you here. What exactly did you mean by
"obsolete
assumption"? If I base my calculation on a 1500W CCS amplifier, in what way
would this be obsolete, or too weak for a ham running RTTY? In fact it still
has
some headroom, because no ham RTTY operation is continuous in the long term,
even RTTY bulletins might last only 10 to 20 minutes!
Or did you mean that 0.4A average current per diode is too low, for an
amplifier
running 1500W in RTTY? It isn't! If that amplifier runs on 3500V and 0.8A,
giving 2800W input and 1500W output, meaning a 53% efficiency, then each
diode
in the bridge rectifier carries 0.4A average current. That's a fact, not an
assumption! The actual average current will be slightly different, depending
on
the amplifier's efficiency, and the voltage it works at, but will hardly get
much above 0.5A, unless you have a lousy efficiency, run illegal power, or
use
tubes that need a very low supply voltage.
The current will be peaky, due to the capacitive filter, but the average
will
still be just that low value. The peaks might be around 3A, but such
repetitive
peaks are within the safe operating area of a 1A diode, as long as the
average
current is low enough.
> ** I detest when someone who should know better starts championing the
> lowest denominator.....the cheapskates that infest the hobby at the amp
> level. Keep it to QRP where nobody gets hurt (-;
ROFL. Seems that we two represent opposing poles! I absolutely love getting
the
most bang for the buck, and building legal limit amplifiers that cost under
500
dollars, total. And which work well, too. Remember that old adage? "A piece
of
engineering is the product of material and brains. The more you use of one,
the
least you need of the other." I like using lots of brains, and little
material!
And yes, I built QRP equipment for the first several years of my ham career.
The
2SC1969 was my preferred output device, but I also sometimes ran a few
2N2222 in
parallel to deliver one watt... I made many intercontinental contacts using
power levels under 5W, into simple dipoles or quarter wave verticals. Mainly
on
ten meters. And many of them in FM! When the other guy has good ears, you
don't
need a kilowatt. On the other hand, a kilowatt gets heard better in many
situations. As an old QRPer, I do know that!
>> Any voltage transients will be clamped to the capacitor voltage.
>
> ** Nope; just some will be clamped.
Do you mean that your filter capacitor is selective, and will clamp some
transients but not others?
> Others will possibly blow the diodes if
> there isnt a path provided around them.
In a bridge rectifier, like in a voltage doubler, there is always a path
THROUGH
diodes, in the forward direction, and that protects the diodes which are
reverse
biased at that moment. Only if you have a center tapped transformer with one
diode string at each end, do I see a chance for large voltage spikes to
appear
on the diodes. But such a configuration would be weird in a modern
amplifier.
You might find it in very old amplifiers that used tube rectifiers.
> Very fast and high spike transients
> will have a repetition rate that could be considered RF and we all know
how
> well electrolytics like RF.
Yes, the electrolytics and associated wiring do have some significant
inductance, that will limit their ability to conduct very fast spikes.
That's
true, although many people believe the effect is larger than it really is.
But
then, your transformer has a significant leakage inductance, which is in
series
with any transients. This prevents such fast transients from appearing on
the
diodes!
Let's imagine a simple model: Power line, transformer, rectifier bridge,
filter
capacitor. The capacitor has some inductance, the transformer also has some
leakage inductance, and the power line has additional inductance, between
the
place the transient is generated, and your amp. For the sake of high
frequency
components in the spikes, this circuit reduces to line inductance, in series
with leakage inductance, the transformer's voltage ratio, the diode bridge,
and
the output of the bridge shorted by the capacitor's series inductance. So,
the
transient voltage appearing on the diodes is basically dependent on the
ratio
between all these inductances. We really need to put numbers to them, to
calculate anything meaningful.
I took a few capacitors and measured their series inductance. A modern 330µF
400V electrolytic with short leads measured 31nH. That's _nano_henry! A
smallish
100µF 350V electrolytic showed 49nH, measured through long leads. The
additional
inductance comes from the leads, not the capacitors. And then I took a big,
old
oil filled capacitor of 10µF, 4000V, and it measured 72nH. So, when you put
8
electrolytics in series, with typical wiring, you can expect maybe 0.4µH
total
series inductance, while a single big oil capacitor will be somewhat better.
Then I measured the total leakage inductance of a transformer intended for a
light duty 1200W amplifier. It has a 1760V secondary. It measured 660mH of
equivalent total leakage inductance, on the secondary side. That sounds like
a
whole lot, but is quite normal for a transformer like this. Instead a good,
generously sized, well designed transformer for a 1500W CCS amplifier would
probably have roughly half that much leakage inductance, or maybe one third.
It
would be interesting if you, Carl, or anyone else who has several suitable
transformers at hand, would measure their leakage inductance. It's done
simply
my shorting the primary and then measuring the inductance of the secondary.
Now, if you have a voltage divider composed by several hundred
_milli_henries on
one side, but less than one _micro_henry on the other side of the bridge
rectifier, any spike voltage will divide by a few hundred thousand times! In
this way, the combination of the filter capacitor and the transformer's
leakage
inductance should be a total, absolute, massive protection against diode
damage
from transient overvoltage!
In practice, of course, things are never as good as one wishes. The fact is
that
the leakage inductance of any transformer is paralleled to some degree by
interturn capacitance. And that capacitance conducts fast transients very
well.
But at any frequency you wish to test, the mix of leakage inductance and
stray
capacitance will always have some inductance left. How much, depends on the
exact design of the transformer. In any case, during 30 years in
electronics,
and being involved in high voltage supplies in scientific applications and
in
industrial, noisy environments, I have never found a transient large enough
to
cause significant voltage overshoot across a bridge rectifier connected to
an
electrolytic capacitor, behind a transformer. To me, this seems to be a
non-issue.
Instead of you have a center tapped transformer and two diode strings, then
there is no clamping effect like in the bridge or the voltage doubler,
because
the transformer leakage inductance isolates one diode string from the other.
In
that situation the diodes can indeed see significant voltage spikes. Instead
of
fixing that by using lots of diodes, or capacitance in parallel with them, I
would rather avoid that configuration altogether. And almost all designers
seem
to think like I do, in this regard.
> A single spike may not cause damage but a string
> of them will raise havoc to diodes and caps.
That's true, in several ways. One is that a lot of spikes, during RX, might
charge up the filter cap to a voltage well above nominal, and all that
appears
across the diodes. Another is that if there are spikes across the diodes
that
are large enough to make the diodes go into avalanche, then the diodes will
have
to dissipate the power, storing the pulse energy as heat in their thermal
mass,
and then bleeding off this heat slowly. Any repeated avalanches can of
course
overheat a diode much easier than a single event.
> Adding a single 4700pf across each string and another to ground at the
> output provides a non destructive path for whatever gets thru or around
the
> transformer. An oscilloscope will easily show the effect.
Yes, that can be very useful, if any spikes manage to get through the
transformer. But if a choke-input filter is used, those little spike
capacitors
would be essential! Or at least a single small cap across the bridge
rectifier's
output, before the choke.
> ** I just measured the secondary DC resistance of a couple of
transformers
> dating over 50 years and all using SS doublers or FWB....no 866A's and
> 3B28's (-;
>
> NCL-2000 doubler 14 Ohms
>
> Hunter Bandit 2000C doubler 16 Ohms same EI ratings as above to 3-400Z's
>
> Clipperton L doubler 5 Ohms and a well known "thumper"
>
> SB-220 doubler 13 Ohms
>
> Amp Supply LK-550ZC FWB 30 Ohms 3x 3-500Z, or 3 x 3CX800A7's in the
LK-800C
> and other QRO versions. A 46# Dahl C Core very silent
>
> Command Technologies 2500 Magnum FWB 31 Ohms C Core, not a Dahl
>
> Drake L4/L7 doubler 8 Ohms
>
>>From a commercial water cooled pulse amp 7000VAC 2.5A FWB 78 Ohms. For my
> "dream amp"
>
> Ameritron AL-811H 44 Ohms
This data is interesting! I find those resistance values very low, though.
Almost too good to be true. My own transformer for that ICAS 1200W amplifier
shows 92 ohm!
> As you can see there is a fairly wide range of design philosphy across
many
> of them. Obviously the Ameritron was done as cheap as possible for mainly
a
> SSB audience, 45uF of C helps.
Yes, a lot boils down to cost versus performance. But not all. When you have
the
task of designing a 2000V, 1A transformer, to mention any numbers, and you
get a
certain budget for it, there are many different ways of doing it. Which one
a
designer should choose, depends to a large extend on additional data, which
is
dearly needed to optimize the design! For example, one is the duty cycle. A
transformer optimized for being the most efficient when delivering 2000V 1A
continuously, will be different from one optimized for a service in which it
delivers 200V 1A for one hour, and then stays plugged in but idling for one
week. The latter transformer will use a lower flux density, thus more turns
of a
thinner wire, or will use a smaller core and a larger winding package. This
comes from the fact that core power loss occurs continuously, while winding
power loss is directly proportional to the square of the load current.
Most manufacturers of ham amplifiers think ICAS, and in fact the lower end
of
ICAS, that is, at most 50% transmit duty cycle, and no more than about 60%
average current during transmit, relative to the maximum current. Such an
amplifier will require a reduction of power in RTTY, but will run full power
in
SSB, CW.
Other amplifiers are "high end ICAS", they can transmit RTTY at full power
for
several minutes, but not 24/7. Of course there are also CCS amplifiers sold
to
hams, but these are unnecessarily large, heavy, and expensive, in my humble
opinion. The fact is that I won't transmit a key down carrier at full power
for
a week - nor should any ham! So we don't need CCS. High end ICAS is enough!
But I digress...
> Those are DC ohms and not AC reactance and without knowing all the design
> details Im not making any guesses as to flux leakage, etc.
To arrive at the true resistance that's acting in series, you have to
measure
both the primary and secondary resistances, apply a factor according to the
transformer's voltage ratio, and then add them. For example, if you measure
20
ohm on the secondary, a half ohm on the primary, and the transformer is 240
to
2000V, then you have a ratio of 8.33. You have to square this, obtaining an
impedance ratio of 69.44. So, you can calculate your equivalent secondary
total
resistance as
0.5 * 69.44 + 20 = 54.72 ohm,
or the equivalent primary total resistance as
0.5 + 20 / 69.44 = 0.788 ohm
In practice, it's often accurate enough to assume that the transformer was
designed with about the same amount of loss in each primary and secondary,
so
that you can measure either of the two resistance, and multiply it by two,
to
arrive at the total resistance seen from that side.
The voltage drop of a transformer is given basically by the series
combination
of these resistance, with the leakage inductances. Of course, the total
equivalent leakage inductance can be calculated by the same method, after
measuring both, or just one.
> The cheapy SB-220 diode is basically a 1N4005 string of 14 and is the
> weakest link in the PS and those diodes are known for going up in smoke.
> Spikes dont help nor do gradually leaking filter caps.
The SB-220 uses a voltage doubler. Indeed in that case the current is too
close
for comfort to the limit of the 1N400x series, and that's why in my NCL-2000
I
used 1N5408 diodes. The NCL-2000 comes with diodes rated at 750mA in the
schematic, and 1A in the parts list. Probably these are also stacks of
1N400x
diodes. This current capability is marginal for an amplifier using a voltage
doubler, but it's fine for one using a bridge! The diodes in those amps go
up in
smoke because they work too hot from the current and slowly degrade, and not
because of any spikes.
> The 1N400x series is actually rated at a 8.3ms peak 30A surge over a half
> cycle.
Datasheets are a wonderful thing. Indeed the table of maximum rates says
exactly
that, but the graph relating number of cycles to allowable current gives the
30A
level for a full cycle, not a half cycle! That would make it about 35A for a
half cycle, extrapolating the curve. Which one is true, I don't know...
I checked the datasheets from Fairchild and Diodes Inc, and both contain the
same inconsistency!
> And all ratings are at 25C ambient
Except the current rating of 1A, which is for 75°C ambient temperature, and
the
leads held to that temperature at 9mm from the case.
> resistive or inductive load ONLY.
> For a capacitive load derate current by 20%.
Indeed the current rating has to be derated for capacitive loads. But where
did
you get that 20% figure from? Obviously the true derating factor depends on
the
capacitance value relative to voltage, current and frequency, and the source
impedance of the line and transformer. In some cases the derating factor can
be
so small that we can ignore it, while in other cases it could be as high as
50%!
20% makes sense as a typical, average derating factor, but it's somewhat
dangerous to appy it, instead of calculating the actual factor that counts
in
the particular situation at hand.
A typical example of this happens in the abovementioned amplifiers using 1A
diodes in voltage doubling supplies. It's marginal, but might survive for
many
years as long as the filter caps are small, like the 10µF total of the
NCL-2000.
When after some decades these filter caps die a natural death, and a
well-meaning but unknowledgeable ham replaces them by caps with the same
physical size but a much larger capacitance rating, the necessary derating
factor for the rectifiers increases too, and the small diodes die soon after
the
capacitor retrofit.
> ** Most 1N4007's are floor sweepings of no particular tolerance or QC.
Used
> mainly in LV consumer goods where a little leakage is OK. The 1N5408 and
> bigger are commercial/industrial quality and subject to QC and tolerance.
Now where did you take that from??? ROFL! Come on! It's the exact same
companies
that make 1N4007 and 1N5408 diodes, and I don't see why the quality
standards
should be different! Both types are used in lots of consumer and industrial
equipment. The difference is their current ratings, and thus their size,
capacitance, leakage, cost, etc. Not their quality!
Did you perhaps buy a bag of of ultracheap 1N4007's on some web site, which
turned out to be re-labelled 1N4004's ?
> IMO continuing to promote the 1N4007 for larger amps is doing a
misservice
> to those that are not familiar with the details.
I have no intention of promoting any specific part. The only reason to write
my
post a few days ago was to try and make people see what the real working
conditions of those diodes are, to help them decide what diodes they want. I
even wrote quite clearly that I have absolutely no problem with anyone
prefering
to use larger diodes than necessary - that's the freedom everybody has! I
just
wanted, and still want, to increase precisely this familiarity with those
details, among the general ham population, and amplifier users specially!
That's
also whu I'm taking the time to write this long message, instead of simply
hitting the "delete" key.
Jim,
## Commercial broadcast HV supplies will typ use TRIPLE the piv rating
for
each leg of a FWB. The theory here is.... the MOVs across the 240 vac
input,or
208vac, 3 phase input will not start to clamp until the ac line V has
doubled. For a 3500 vdc no load B+ supply, use 10 kv piv per leg.
That makes a lot of sense, for power supplies without a filter capacitor
connected directly to the rectifier. With that cap, I think triple
overrating is
overkill. But then, of course, the cost is so low, that probably they do
this
for peace of mind.
### A 1N5408 runs pretty damn warm to hot with 1A CCS flowing, when I
tested em for bias use.... using a variable dc power supply + resistor in
series
with the string of 1N5408s. A 6A10 runs warm to hot with just 2A CCS
flowing. And that?s with full lead lengths on each end of each diode. Try
running 1A CCS through a 1N4007, and see hot hot it gets.
Using these diodes with full lead length is foolish. The data sheet
specifies
current rating with 9mm leads, and that should be taken as an absolute
maximum
length. When I use those diodes near their full current ratings, I make the
leads short, and solder them to nice big circuit board areas. That way they
work
very much cooler than with long leads!
Also these diodes, like all silicon semiconductors, can take some
significant
heat. In fact they are rated for 1A, with 75°C ambient temperature. At that
ambient temperature, they will burn your finger even at zero current!
Working at
1A in a 75°C environment, the silicon will be at roughly 120°C, which is hot
enough to instantly vaporize any water, and cause nasty burns, but is safe
for a
pretty long time for the diodes. It still leaves some thermal headroom for
spikes!
So don't assume that a diode that burns your finger is working too hot.
### Nice try. My dahl xfmr has a 6 ohm dc resistance across its 5200 vac
winding,
and only 3 ohms across the 2600 vac tap. Pri resistance is just .002
ohm.
That sounds like a transformer of roughly 20kW, maybe more. Indeed with such
a
transformer you need diodes MUCH bigger than what we are talking here, and
at
least a 15kW power tube. At that point surely we aren't talking ham radio
anymore! When I write on this forum, I have ham amplifiers in mind. That's
1500W
output, strictly, with no concessions. If anybody wants to run more, that's
his
problem, but I think it shouldn't be assumed that we are talking broadcast
transmitters in a ham forum!
And the resistance ratio of that transformer clearly suggests that the
primary
is for 120V. Do you really run such high powered amps from 120V? I wouldn't.
## If you cant afford to buy 1N5408s or 6A10s.... you shouldn?t be in
this hobby. Even if you are in the poor house, you can still afford
something better than the
$.0990 1N4007 pos diode.
I have to say that I find this quite a bit rude. First, ham radio is
supposed to
be a hobby for people interested in radio, without any exclusions based on
wealth. Secondly, I'm not too poor to afford a few 1N5408's, but I'm enough
of
an engineer, in addition of a ham, to try to figure out when it makes sense
to
use a larger part, and when a cheaper, smaller, lighter, is plenty. And with
ham
legal limit amps I drew the line at the power supply configuration: Bridge
rectifiers feeding 1.5kW amps at a 3kV level can use 1A diodes, while
voltage
doubler circuits would be marginal with them, and thus should use larger
diodes.
And I also wrote that it's perfectly fine with me if some fellow ham elects
to
use larger diodes than technically required. Be it just for peace of mind,
or
because he has them by the hundreds in his junkbox, or because he finds them
cheap enough to not lose a thought on it, or because he wants his amp to
survive
the the next several nuclear wars including EMPs. It's his business, not
mine,
but on this forum we can hopefully all express our views of technical
matters,
without being thrown out of the hobby!
Personally I have an issue with people who believe that heavier is always
better, and that more expensive is always better, and that rules of thumb
work
better than actually calculating, but I respect their right to live as they
please, and enjoy themselves!
## How cheap can you get ?
There is no limit! ;-) When you get the hang of it, there is a huge thrill
in
doing things as cheaply as you can, and at the same time BETTER than some
other
people manage by throwing lots of money at it!
Blow up some 1N4007s and you will be cursing that you didn?t use a real
diode.
I have been using them for 36 years, and can't remember having ever blown up
any! It's strange, actually, as I have sometimes blown up some other
components.
I have been using bigger diodes too, almost for the same time, in circuits
that
need them. The largest I have used in my own designs were rated for 400A,
and
the smallest for 15mA. It's not like one size would fit all applications.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
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