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Re: [Amps] HV Diodes

To: Jim Thomson <>, "" <>
Subject: Re: [Amps] HV Diodes
From: Glen Zook via Amps <>
Reply-to: Glen Zook <>
Date: Mon, 13 Oct 2014 07:29:29 -0700
List-post: <">>
Who is paying 9.99-cents for a 1N4007?  I pay 2-cents for them in 100 quantity. 
 I keep a drawer full to use anywhere I need a silicon power diode.

I was paying like 10-cents each for 1N5408.  However, they recently went to 
12-cents each.  Again, in quantity of 100.  I have another drawer full of those 

Being retire, and on a "fixed budget", I don't like to squander money.  
However, $2.00 for a quantity of 1N4007 and $12.00 for a quantity of 1N5408 is 
not going to "kill me"!  Being able to open a parts drawer and have something 
to do the job at hand, is certainly worth it!

As to which diode to use:  It depends!  I don't like to have to repair 
equipment so I do tend to "go overboard" sometimes with the parts used.  But, 
not that far from the boat!  If, for some reason, I don't have the desired part 
right then, I may go ahead and used a "heavier" part just to get things going.  
However, that is for convenience and not for any financial reason. 
Glen, K9STH 


On Sunday, October 12, 2014 8:08 PM, Jim Thomson <> wrote:

Date: Sat, 11 Oct 2014 16:35:39 +0000
From: Manfred Mornhinweg <>
Subject: Re: [Amps] HV Diodes

Dear all,

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

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 
event of line overvoltage, and an average current of 0.4A at full output. The 
current will be very peaky, but rectifier diodes are rated to take that.

##  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 
208vac,  3 phase input  will not start to clamp until the ac line V has 
For a 3500 vdc no load B+  supply, use 10 kv piv per leg. 

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 
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 
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.

### 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 
with the string of 1N5408s.   A 6A10  runs warm to hot with just 2A CCS 
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. 

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

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 
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 
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.

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

###  Nice try.  My dahl xfmr has a 6 ohm dc resistance across its 5200 vac 
and only  3 ohms across the 2600 vac tap.  Pri resistance is just .002 ohm.
  My L4B xfmr has a 10 ohm sec winding.
The big B+ caps across the B+ will only clamp the excess V  so much. See carls 
That 35A inrush rating is for a half cycle of 60 hz.... or  8.3 msecs  to be 

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 
  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.

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.

###  I use 6A10s  for the HV supply, also use 6A10s  for the bias..and also use
em for safety diodes, between B- and chassis.   I have seen 6A10s  short out
when used as safety diodes,  so now use 3-4 x 6A10s in parallel, to increase the
surge rating.   Since I use strings of diodes  for the FWB / FWD..and also 10-60
of em in series for bias.... tapped of course to vary the bias...and also for 
safety  diodes.... its dirt cheap to just buy 200-250 of the damned things..
6A10s..and be done with it.   I also put a rvs connected 6A10  across each HV 

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.


##  If you cant afford to buy 1N5408s  or 6A10s.... you shouldn’t be in this 
Even if you are in the poor house,  you can still afford something better than 
$.0990  1N4007 pos diode. 

##  I spend way more for coffee + 94 octane every day.   How cheap can you get ?
Blow up some 1N4007s  and you will be  cursing that you didn’t use a real diode.
Having to pull an amp all apart, unsolder some stupid  1N4007s, then replace em
with 1N5408s is a pita.    And don’t even think of doing something silly.... 
replacing with more  1N4007s.   Even if your time is free, its still a pita. 

Jim   VE7RF
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