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Old 30th April 2013, 10:05 AM   #151
JLH is offline JLH  United States
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I would still use resistors to ensure voltage balance. Why play games with this stuff? Just use the resistors and be done with it. The peace of mind will be well worth the extra few bucks you spend. I can't speak for anyone else on here, but every time I've cheaped out on something it has came back to bite me.
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Old 30th April 2013, 12:28 PM   #152
Magz is offline Magz  United States
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Originally Posted by Funker View Post
Hi ,
here some useful informations about rectifier diodes.

Frequently Asked Questions about VMI's High Voltage Diodes"


73
Wolfgang

Interesting. From that reference:

Question: When diodes are used in a bridge or string configuration is it necessary to place equalizing resistors across the diodes? Also, is it necessary to place capacitors in parallel with the equalizing resistors to reduce noise and high voltage spikes?


Answer: Balancing resistors and capacitors are generally not required when using VMI diodes. A resistors connected in parallel with a rectifier is intended to balance the reverse bias voltage across the series rectifiers. As such, the resistor value needs to be selected such that the current through the resistor is fairly large compared to the reverse leakage of the diode, thereby making a stiff divider circuit. If there are relatively high leakage currents expected in the diode, this can result in a significant power loss in the balancing resistors.

Capacitors connected in parallel to resistors/diodes can be used to balance voltages across diodes during transient conditions, or ringing. Care must be taken to select the capacitors small enough so as not to impact the rectified waveform at the output of the diode.

VMI's diodes are well-balanced for Ir and reverse recovery time (Trr).

Many of VMI's diodes have up to twenty junctions in series. The series junctions operate under many kinds of applications, and all sorts of conditions without compensating resistors or capacitors with no problems.



My diodes aren't VMI diodes, but that does agree with my experimental results for the Cree SiC Schootky diodes.
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Old 30th April 2013, 01:24 PM   #153
Magz is offline Magz  United States
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Originally Posted by JLH View Post
I would still use resistors to ensure voltage balance. Why play games with this stuff? Just use the resistors and be done with it. The peace of mind will be well worth the extra few bucks you spend. I can't speak for anyone else on here, but every time I've cheaped out on something it has came back to bite me.
I understand your sentiment. It's not about being cheap - hell I'm spending $10K on parts here! It's about adding an additional level of complexity and additional failure modes to the rectifier bridge. My concern is that a "belt and suspenders" approach of adding resistors because they "can't hurt" may actually decrease the reliability.

Looking at the 1000V data, I get 1.6% difference between the highest and lowest diode, or let's even round up to 2%. If at full power I end up with a total of 500V across each diode that's a maximum difference of 10V in the stack, while each diode has 700V of headroom. If operating at higher currents and temps changes the relationship between the highest and lowest diode by a factor of 10X I still have an extra 600V headroom to spare.
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Old 30th April 2013, 06:48 PM   #154
Funker is offline Funker  Germany
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Originally Posted by Magz View Post
Interesting. From that reference:

Question: When diodes are used in a bridge or string configuration is it necessary to place equalizing resistors across the diodes? Also, is it necessary to place capacitors in parallel with the equalizing resistors to reduce noise and high voltage spikes?


Answer: Balancing resistors and capacitors are generally not required when using VMI diodes. A resistors connected in parallel with a rectifier is intended to balance the reverse bias voltage across the series rectifiers. As such, the resistor value needs to be selected such that the current through the resistor is fairly large compared to the reverse leakage of the diode, thereby making a stiff divider circuit. If there are relatively high leakage currents expected in the diode, this can result in a significant power loss in the balancing resistors.

Capacitors connected in parallel to resistors/diodes can be used to balance voltages across diodes during transient conditions, or ringing. Care must be taken to select the capacitors small enough so as not to impact the rectified waveform at the output of the diode.

VMI's diodes are well-balanced for Ir and reverse recovery time (Trr).

Many of VMI's diodes have up to twenty junctions in series. The series junctions operate under many kinds of applications, and all sorts of conditions without compensating resistors or capacitors with no problems.



My diodes aren't VMI diodes, but that does agree with my experimental results for the Cree SiC Schootky diodes.

Hi ,
it is always advisible to make themself a mind about things before starting to work on a project. This will safe time & money.

I understand your approach. But the don´t overegg the pudding.
ordinary rectifier diodes like a common 1N4007 withstand a reverse voltage of 1000V , the forward loss is appr. 0,7V at 1A.

Several years ago I build an Power supply for an rf amp (ham use) .
The plate voltage was also 2500V d.c. I made a h.v. bridge with 1N4007 , each leg employed 7 e.a. of the latter. So 7 diodes share 2500V reverse volts . I had no resistors and no capacitors, just the 28 diodes.
The power unit run from the first time without any troube so far.

In an audio amp , snubber capacitors across each diode make sense to subdue noise etc, if you use ordinary diodes. With your schottky´s are such capacitors not neccessary.


good luck with yor amazing project.

73
Wolfgang
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Old 3rd May 2013, 05:17 AM   #155
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The caps on this thing could cause ball lightning.
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Old 3rd May 2013, 11:31 AM   #156
nhuwar is offline nhuwar  United States
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Originally Posted by miragem3i View Post
The caps on this thing could cause ball lightning.
Uh no. Ball lightning is shrouded in myth but I don't think he's running that kinda b+.
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Old 3rd May 2013, 02:33 PM   #157
Magz is offline Magz  United States
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Originally Posted by miragem3i View Post
The caps on this thing could cause ball lightning.
Only 378 Joules of storage per mono...
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Old 3rd May 2013, 04:00 PM   #158
Magz is offline Magz  United States
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BTW, I'm working on the capacitor banks now. (7) 150uF, 450V for C1 and (7) 1000uF, 450V for C2, in series. Each cap will have a 47kohm balancing resistor, so the two banks will each bleed about 7mA of current. Total time to bleed down to 30V is about 8 minutes.
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Old 3rd May 2013, 07:51 PM   #159
nhuwar is offline nhuwar  United States
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Are you going choke input or will that be a pi network?

with that kind of capacitance at that voltage you definitely want a low resistance high wattage resistor between the supply and the output transformer. If it arced over under chassis or worse in the output it would destroy pretty much all of it.

Lol, maybe you should look into a crow bar for it. To protect your investment. It would probably be a good idea. What's your b+ again?

Nick
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Old 3rd May 2013, 08:27 PM   #160
Magz is offline Magz  United States
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Originally Posted by nhuwar View Post
Are you going choke input or will that be a pi network?

with that kind of capacitance at that voltage you definitely want a low resistance high wattage resistor between the supply and the output transformer. If it arced over under chassis or worse in the output it would destroy pretty much all of it.

Lol, maybe you should look into a crow bar for it. To protect your investment. It would probably be a good idea. What's your b+ again?

Nick
LCLC Choke input on all circuits (I posted the PSUD charts a few pages back). 35H choke on the 2300V 833C B+, 13H choke on the 450V 6E5P B+, 2.5mH choke on the 10V, 10A 833C filament supply. Soft start circuit on the 833C B+ transformer primary.

I've been looking into a crowbar circuit for OPT protection. Those Monoliths deserve some protection.
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