[Amps] HV Diodes
Carl
km1h at jeremy.qozzy.com
Mon Oct 13 12:22:22 EDT 2014
I use the 1N4007 for simple things such as replacing the 0Z4, 6X5, 6X4, 12X4
in auto radios as well as a few test equipment, house radios and others that
are known for 6X4 and 6X5 problems.
For items such as the HT-32/HT-37 family I use the 4007 in LV and the 1N5408
in HV since those transformers are known for the 5V rectifier winding
shorting to the HV winding due to poor insulation.
I AM NOT a wholesale replacer of tube rectifiers with SS as they can cause
other problems.
A string of 1N5408's are fine for replacing troublesome 866A's and becoming
expensive 3B28's in vintage amps as the voltage drops are almost identical.
Carl
KM1H
----- Original Message -----
From: "Glen Zook via Amps" <amps at contesting.com>
To: "Jim Thomson" <jim.thom at telus.net>; <amps at contesting.com>
Sent: Monday, October 13, 2014 10:29 AM
Subject: Re: [Amps] HV Diodes
> 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 diodes.
>
> 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
>
> Website: http://k9sth.net
>
>
> On Sunday, October 12, 2014 8:08 PM, Jim Thomson <jim.thom at telus.net>
> wrote:
>
>
>
> Date: Sat, 11 Oct 2014 16:35:39 +0000
> From: Manfred Mornhinweg <manfred at ludens.cl>
> To: amps at contesting.com
> 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 the
> 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
> 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.
>
>
> 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.
>
> ### 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.
>
>
>
>
>
> 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.
>
> 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.
>
> 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
> winding,
> 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 comments.
> That 35A inrush rating is for a half cycle of 60 hz.... or 8.3 msecs to
> be exact.
>
> 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.
>
> 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 lytic.
>
> 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.
>
> Manfred
>
> ## 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 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.... like
> replacing with more 1N4007s. Even if your time is free, its still a
> pita.
>
> Jim VE7RF
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