[Amps] HV Diodes

Col. Paul E. Cater paulecater at gmail.com
Tue Oct 14 17:27:22 EDT 2014


This really makes one wonder how many times a subject can be covered, in
detail, over and over.

Let's do this.  Here is a little ditty I wrote a long time ago concerning
rectifiers.  At this point it probably is outdated.  Still, it might offer
some insight and some real engineers to come forth and correct it.

I have read many statements, in this thread,  that are nothing but guessing
and speculation.  The bottom line is we are talking a small  difference in
price to do it right and/or over do it.  I always vote for over do myself.

The question of transients is solved using more modern rectifiers or
avalanche diodes.

Paul
WD8OSU


*What About Those Series Rectifiers?*


 The use of several rectifier diodes in a series string is often required
to achieve the Peak Reverse Voltage (PRV) or Peak Inverse Voltage (PIV)
required for building a High Voltage Power Supply (HVPS). It makes no
matter if you are talking about a half wave up to and including a full wave
bridge, one needs to be aware of peak voltages involved and required
component specifications.

 Think about that B+ supply for your dream linear. What are we talking
about here, 2 to 5 or even 6,000 volts DC for that 4-1000A? Woof! That’s a
hugh amount of PIV that can burn up an improperly designed string in a
nanosecond. Having personally experienced a string failure I can tell you
it is not a pretty sight. There is a lot of noise and many projectiles
flying about. I can give you a couple options in your design and you can
decide which will achieve the level of operational reliability and
financial burden you can endure.

 Remember, we are talking about “PEAK” voltage. If you are shooting for
3000 volts DC to your plate the “PEAK” would be 1.41 times this amount or
4230 volts! And, this doesn’t count what they call “mains over voltage”
typically 5%. That brings us to a grand total of 4441.5 volts. That is what
your poor little rectifier string has to be capable of handling.

 Traditionally, normal, everyday, rectifier, diodes were and are used at
the output of the HV transformer to whip out more or less DC to the filter
network. The first requirement is the current that is going to be drawn by
your favorite tube. Don’t forget we are talking about “peak” current too.
So, your tube specifications say 1000 milliamps or 1 amp is the maximum
current your tube likes. Guess what? That’s average not peak current.
Instantaneous current can be quite a bit higher. No worries, diodes are
cheap now days and they have relatively high peak current capabilities. I
like to use a factor of 3 and this is really safe. If you think your tube
is going to draw 1000 milliamps during normal operation then a 3-amp
rectifier will be more than enough to cover the load. They can handle peak
currents much higher than your 4-1000A could stand. Hey, we’re only talking
about maybe 7 cents a piece for the old, reliable, 1N5408.

 Now we run into an area that takes up considerable bandwidth on the
Internet arguing about compensation and/or equalization resistors and
suppression capacitors in parallel with the diodes. There is a bit of a
problem when you start stacking a bunch of diodes in series. Especially
when you use really fast switching diodes, which even the cheapies are
pretty quick now. Each diode does have it’s own personality and really
doesn’t conduct or shut off at the same time as all the others. What does
this mean? Well, basically, it means that there may be voltage differences
across each diode and noise induced in the output of the HVPS that you
don’t want. It can even lead to distortion on your transmitted signal.

 Like I said, folks in the industry still argue about this. But, I always
figure better safe than sorry. You can take care of the unequal voltage
issue by placing equalization resistors in parallel with each diode. This
insures that each diode sees the same, or close to the same, voltage as all
the others in series.

 Now issue number 2, transient voltages across each diode and all the
little bits of noise they make. When the diodes turn on and off, at
different times due to their personalities again, they make a bit of noise
that you don’t necessarily want. There is an issue of unwanted surges too.
Transients are the little creatures that can cause all kinds of problems
and come from any of a number of sources. The bad thing about them is they
don’t always go with the normal flow of current. They are the things, for
lack of a better explanation, that can make your life miserable and cause
your standard rectifiers to give up the ghost. This is what all the
amplifier-building guys are arguing about all the time. How do you stop
transients?

One old school answer is placing a capacitor in parallel with the diode and
a resistor to make a happy family. OK, OK, I know, the Handbook says not to
do this anymore. Well, ah, what can I say? If in doubt, look over
STMicroelectronics application note “AN443” May 2004 and make up your own
mind. To me a few cents of parts that is not really going to hurt anything
might be worth it. Is it really going to make a difference? There’s not a
whole lot of data out there either way. Some say that their semiconductors
are so good now that you need not bother.

 Now, let me give you the ultimate, modern, answer to all the normal
diode’s problems. Introducing the grandson or granddaughter of the 1N
whatever diode family, the avalanche diode. These little guys do what a
normal diode would never do on purpose and survive. Conduct relatively
large amounts of reverse currents on command. What a concept! These dudes
switch on and off on command really quick, don’t introduce noise, and are
all around our power supply friends. Take a look at Dynex Semiconductor’s
Application Note “AN5370” to read all about the little marvels.

 Yea, they do cost a bit more but not that much. As a matter of fact, after
you put the bucks into an equalization network for your favorite 1N5408 it
works out to about the same. Let me give you an example, Fagor makes a
little number that will do 3 amps at 1000 PRV for about a quarter a piece
at Allied Electronics (BYM36E). Not bad!

 So, sooner or later we always get to the math of it all. Let me give you
the basics to get done what you need to do. Remember, it is always PEAK!
Take the plate voltage you are shooting for and multiply that by 1.41 to
get peak voltage. This is the minimum PRV your diode string should have in
a full wave bridge I.E. 3000 VDC X 1.41 = 4230 volts peak. This will work
for normal rectifier diodes.

 Avalanche diodes are a bit different, look out now, a safety factor of 2.4
to 3 times has to be added to the mix. With that in mind, 3000 VDC X 1.41 X
(a minimum of 2.4) = 10152 PIV. That jumped right up there didn’t it? Yes,
you need more avalanche diodes than normal rectifiers. Now, don’t forget
about this little thing called “ mains over voltage”. That pumps in another
1.05% to the equation. Let’s see, we were up to 10152 PIV (10152) X 1.05 =
10659.6. So, you get 11 avalanche numbers is each leg and your set to go.

 You really didn’t think you were going to get all this great avalanche
stuff for free did you? There is always a trade off isn’t there?

 The bottom line is using the avalanche diode is the best way to go for a
very dependable HVPS and at the price they are affordable, even if it takes
more to get the job done.


 *Col. Paul E. Cater, WD8OSU, is a Cambridge, Ohio native that returned to
the area in 1994 after retiring from government service. Mr. Cater is a
veteran and has been a technician, electronics maintenance officer, field
service engineer, instructor, signals analyst, and the director of training
for a major government contractor. He currently resides at historic
Prospect Place Mansion in Trinway, Ohio. *



On Tue, Oct 14, 2014 at 3:18 PM, Manfred Mornhinweg <manfred at ludens.cl>
wrote:

> 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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