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Re: [Amps] HV Diodes

To: "Manfred Mornhinweg" <>, <>
Subject: Re: [Amps] HV Diodes
From: "Carl" <>
Date: Sat, 11 Oct 2014 18:48:02 -0400
List-post: <">>

Dear all,

I wonder why the diode selection is so totally overblown here.

** IMO there is a balance point between using obsolete diodes and overkill. Very few will be suggesting wholesale replacement to 6 or 10A diodes in a 1200-2500W amp which covers just about all of the common high power offerings for the past 50 years which is where the NCL-2000 is located in history as a 1964 introduction.

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.

current will be very peaky, but rectifier diodes are rated to take that.

The cheapy 1N4007 diode is rated for 1000V reverse voltage, and 1A continuous average forward current, when mounted in a normal way that will allow conducting about 1W of heat away, through its leads. Current-wise this diode has all the safety headroom you might need. Voltage-wise you need 4 of them in series, and that's it. For a bridge rectifier you need 16 of those diodes, which cost $0.099 each at Digikey, if you buy just those 16. For $1.58 you get all the diodes you need! Or be generous, use 5 in series in each leg, and spend two bucks on them.

** 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 (-;

I hear some of you cry "and the voltage transients?" Well, what transients? The diode bridge sits right across a big capacitor! Any voltage transients will be clamped to the capacitor voltage.

** Nope; just some will be clamped. Others will possibly blow the diodes if there isnt a path provided around them. 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. A single spike may not cause damage but a string of them will raise havoc to diodes and caps. 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.

Of course some of you will now cry "and the current transients?" Sure, any voltage transients on the power line will translate into current transients when the filter capacitor clamps the voltage. In addition there will be a big inrush current at switch-on, if no step start or slow start circuit is used. So there will indeed be some current transients. But how large can they be? Small transformers have so much resistance in their wire, that just the resistance would limit the short circuit current to about ten times the nominal load current. Larger transformers like those used in legal limit amps have relatively smaller winding resistances, so that the resistance might limit the current to 20 times the nominal value. But then there is leakage inductance, which also reduces the current transients. So I would not expect such a transformer to produce an inrush current stronger than 10 times the nominal current, even assuming a zero impedance power line, which none of us has! So, the inrush current with no step start circuit might be around 11A, considering that a transformer for 0.8A output from a capacitor-input filter needs to be rated for about 1.1A.

** I just measured the secondary DC resistance of a couple of transformers dating over 50 years and all using SS doublers or 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

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.

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.

And a cheapy 1N4007 has a 35A inrush current rating. More than three times what we need.

And how much transient current could we see during lightning? This is harder to tell, because such fast, extremely strong hits are hugely attenuated by line impedance, and clamped by all sorts of electronic devices in the homes, so that the current spike resulting in your amp's rectifier diodes depends a lot on those impedance values. But if such a spike exceeds 35A, it would have to exceed roughly 350A on the 240V power line feeding the amp, which means that in a typical home it would need to far exceed a kiloampere at the service input. The voltage at that point would need to be VERY high, causing flashovers everywhere and thus limiting the transient current into the home. I would expect the final transient in the amp's rectifier diodes to remain well below the 35A rating of 1N4007 diodes. You might see your house catch fire from the flash-overs and the molten wire, before those diodes blow up.

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 1N400x series is actually rated at a 8.3ms peak 30A surge over a half cycle. And all ratings are at 25C ambient resistive or inductive load ONLY. For a capacitive load derate current by 20%.

Of course everyone is free to use 1N5408 or 6A10 diodes, at $0.278 and $0.368 respectively at Digikey. And if somebody wants to use 10 in series in each leg, who am I to forbid that? But technically it's pointless. Strings of 4 to 5 1N4007 diodes in each leg, properly mounted for decent heat sinking, are perfectly safe and sound.

** 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.

In my National NCL2000 I use 1N5408 diodes, but that amplifier has a voltage doubler, so the diodes work at twice the current, compared to a bridge rectifier. While 1N4007 diodes would still have been operating within ratings, the current headroom would have been rather tight. So I chose the bigger diodes.


** Most amps for the past 50 years use doublers, Alpha, Amp Supply, Henry (not all), and Ten Tec were some of the few (based on quantity sold) who used a FWB. Command Tech did it only for the Magnum.

Ive been using 1N5408's in my several (4 in use and one to test and match tubes) and all customer NCL-2000's since the mid 70's. The original round "Black Beauty" diode packages were failing even in the 60's; the flat packs that came next are better but still on the edge IMO. At least they only have 10uF to contend with; I use 100-150uF Snap Ins which fit the clamps perfectly.

Amp manufacturers started using the 1N5408 or a 2.5A alternative in the late 70's or so. They were pretty pricey back then.

IMO continuing to promote the 1N4007 for larger amps is doing a misservice to those that are not familiar with the details.


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