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Old 29th April 2013, 08:56 AM   #141
Funker is offline Funker  Germany
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Quote:
Originally Posted by Rod Coleman View Post
Maybe you can get away without, but balancing resistors can help reliability - and suitable parts can be procured to make it easy.

For example, I use the Welwyn MH37 series, which are rated for 3500V, and are almost as cheap as ordinary resistors:

MH37-470KJI - WELWYN - RESISTOR, H/V 0.5W 470K | Farnell United Kingdom

One more subtle reason for using them: Before the diodes break down destructively, you can sometimes get transient breakdown/leakage effects, that may not destroy the diode, but cause a lot of noise.

Especially if you choose to avoid balancing resistors, I would recommend a fuse in the PT secondary winding circuit - rated to be able to manage the HT voltage. A microwave cooker fuse is made for 5kV, and may be useful, depending on the VA rating of the PT:

Microwave Oven High Voltage Fuse 800mA 0.8A 5kV | eBay

A fuse in the secondary gives closer control, since the power-ON surge needs no accounting for.
Hi ,
if you employ separate transformers for HV and the rest of all required voltages, you can secure the circuit with a fuse on the primary side.
To protect the HV windings from arc over due transients , attach a small value capacitor across. Any capacitor betweeen 1 to 5 nF / 5kV will be suitable.

And why a bridge rectifier from single diodes? This one here delete all discussions about reverse voltage balancing.

HV bridges are availble here:

http://ixdev.ixys.com/DataSheet/uge.pdf

And if you like to fiddle with diodes , 4 pcs microwave oven diodes can complete a bridge without any further measures.

73
Wolfgang
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Old 29th April 2013, 01:26 PM   #142
nhuwar is offline nhuwar  United States
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Quote:
Originally Posted by Rod Coleman View Post
Maybe you can get away without, but balancing resistors can help reliability - and suitable parts can be procured to make it easy.

For example, I use the Welwyn MH37 series, which are rated for 3500V, and are almost as cheap as ordinary resistors:

MH37-470KJI - WELWYN - RESISTOR, H/V 0.5W 470K | Farnell United Kingdom

One more subtle reason for using them: Before the diodes break down destructively, you can sometimes get transient breakdown/leakage effects, that may not destroy the diode, but cause a lot of noise.

Especially if you choose to avoid balancing resistors, I would recommend a fuse in the PT secondary winding circuit - rated to be able to manage the HT voltage. A microwave cooker fuse is made for 5kV, and may be useful, depending on the VA rating of the PT:

Microwave Oven High Voltage Fuse 800mA 0.8A 5kV | eBay

A fuse in the secondary gives closer control, since the power-ON surge needs no accounting for.
Its common practice to put a high wattage wire wound resistor in series with the plate line. Something around 50 ohms works well. It doesnt create much of a voltage drop during normal operation. But if something were to arc over it would quench the arc since an arc draw massive amounts of current. The main thing to remember is you do not want the arc to sustain its self long enough to discharge the filter capacitors. That is were the real damage happens.


A fuse does help. And the one mentioned would be great. but high voltage fuses can sometimes be a bit difficult to source. At least in low current levels.

Then theres the option of using a small metal film resistor as a fuse. I seen the menstioned many times but I have never done this.

Since tour amp will be running at such a high voltage, you will need some metthod of protecting you investment in those output transformers. Id suggest a fuse and a resistor in series with the plate.

The resistor acts as your primary safeguard and the fuse as a backup. And or course theres the circuit breaker on the primary. But thats not really usinful in protecting things. Mainly just preventing fires.

Nick
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Old 29th April 2013, 01:34 PM   #143
Magz is offline Magz  United States
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Thanks Rod and Funker and Nick,

I have been thinking about fusing the HV separately - it is from a dedicated transformer, so that should be a simple matter. It also will have a soft start circuit capable of 20A on the primary, so that should help with turn-on transients. I'll consider the small series resistor, Nick.

Funker, I guess I didn't just buy a pre-made HV bridge for a couple reasons. One, I kind of like the challenge of making my own. Two, I would like to avoid the reverse recovery spike from a standard rectifier; I use Schottky diodes whenever I can. I even thought about using Xenon rectifiers in a bridge but the added filament xrmr, weight, and four hazardous HV top caps per unit pushed me to SS. These monos will be HUGE and HEAVY enough as is...

Last edited by Magz; 29th April 2013 at 01:37 PM.
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Old 29th April 2013, 01:38 PM   #144
Magz is offline Magz  United States
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To all who have expressed concern - thank you and relax, please. I'm doing a few experiments to generate some data to help answer the question about the need for balancing resistors. Let's see what the data tell us; frankly, this is the part of this hobby that I like the most. I guess it's hard to take 30+ years as a research scientist out of my blood, and I've always been much more of an experimentalist than a theorist.

I have no intention of killing myself or anyone else with this project. It's called the Midlife Crisis, not the Death Wish.
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Old 29th April 2013, 01:48 PM   #145
iko is offline iko  Canada
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350V will kill one as readily as 900V. So, precautions apply to all of us doing tubes. I'm sure that you have a healthy respect and fear for the high voltage, or else you wouldn't do this.
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Old 29th April 2013, 05:46 PM   #146
hpeter is offline hpeter  Europe
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discussion is about semi-diodes, but how about gas rectifiers... can withstand up to 10k
i know they are PITA because of filament requirements, but one can do with thick wire few turns on big toroid 230/12...
and you get 12v "for free", for your pwr managment circuits
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Old 29th April 2013, 05:55 PM   #147
nhuwar is offline nhuwar  United States
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Quote:
Originally Posted by iko View Post
350V will kill one as readily as 900V. So, precautions apply to all of us doing tubes. I'm sure that you have a healthy respect and fear for the high voltage, or else you wouldn't do this.
120 volt ac IS the most dangerous. Higher is actually a bit safer. To a point though. If your a linemen for the power company it doesnt matter what your working on, LOL.


I didnt think youd hurt yourself. Just passing on different option to you to use on your final build.

Nick
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Old 29th April 2013, 08:22 PM   #148
Magz is offline Magz  United States
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Quote:
Originally Posted by hpeter View Post
discussion is about semi-diodes, but how about gas rectifiers... can withstand up to 10k
i know they are PITA because of filament requirements, but one can do with thick wire few turns on big toroid 230/12...
and you get 12v "for free", for your pwr managment circuits
I mentioned in Post#143 that I did consider Xenon gas rectifiers, but I had to make some choices to try to keep these things in one chassis per mono, and going to SS rectification was one of the choices.
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Old 30th April 2013, 02:48 AM   #149
Magz is offline Magz  United States
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Experimental results:

I had previously checked the DC voltage across each diode in all four legs of each high voltage diode bridge with 166VDC output from the bridge, and the % difference between the high and low diodes in each leg ranged from 0.6% up to 4.5%.

That told me about low voltage behavior, but there was no guarantee that behavior would be the same at high voltage, so I hooked up my Hammond 733A transformer to a variac, connected it to one of the bridges and ran up the DC voltage out of the bridge from 100V to 1040V, in ~100V increments, measuring the DC across all the diodes in one leg of the bridge at each point. The results appear in the graph below.

As you can see, the five diodes appear to share voltage superbly all the way up to 1000V+ bridge voltage, or 40% of the way to the target 2500V. The relationship is very linear, with R-squared = 0.998. At 100VDC bridge voltage, the difference between high and low diode is .16V, and at 1040V it is 1.6V. It's almost like I made the numbers up! But I didn't.

Now, the only caveat I can think of is that this was a naked bridge with no filter and only a multimeter for a load, so the total current was only on the order of a milliamp or so, but the voltage sharing behavior appears to be near perfect. Higher forward currents would cause the temperature to rise a bit, which could affect the results, but I can't see it getting to the point where there would be a problem, especially with (5) 1200V diodes in each leg.

Am I missing something here?

PS: The wider gap in voltage after 600V is where I shut down for a moment and switched to a 5000V multimeter probe, in case you are interested.

.
Attached Images
File Type: jpg 833 Bridge Rectifier Diodes Reverse Voltage Sharing.jpg (99.2 KB, 571 views)

Last edited by Magz; 30th April 2013 at 02:55 AM. Reason: added PS
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Old 30th April 2013, 10:00 AM   #150
Funker is offline Funker  Germany
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Quote:
Originally Posted by Magz View Post
Experimental results:

I had previously checked the DC voltage across each diode in all four legs of each high voltage diode bridge with 166VDC output from the bridge, and the % difference between the high and low diodes in each leg ranged from 0.6% up to 4.5%.

That told me about low voltage behavior, but there was no guarantee that behavior would be the same at high voltage, so I hooked up my Hammond 733A transformer to a variac, connected it to one of the bridges and ran up the DC voltage out of the bridge from 100V to 1040V, in ~100V increments, measuring the DC across all the diodes in one leg of the bridge at each point. The results appear in the graph below.

As you can see, the five diodes appear to share voltage superbly all the way up to 1000V+ bridge voltage, or 40% of the way to the target 2500V. The relationship is very linear, with R-squared = 0.998. At 100VDC bridge voltage, the difference between high and low diode is .16V, and at 1040V it is 1.6V. It's almost like I made the numbers up! But I didn't.

Now, the only caveat I can think of is that this was a naked bridge with no filter and only a multimeter for a load, so the total current was only on the order of a milliamp or so, but the voltage sharing behavior appears to be near perfect. Higher forward currents would cause the temperature to rise a bit, which could affect the results, but I can't see it getting to the point where there would be a problem, especially with (5) 1200V diodes in each leg.

Am I missing something here?

PS: The wider gap in voltage after 600V is where I shut down for a moment and switched to a 5000V multimeter probe, in case you are interested.

.
Hi ,
here some useful informations about rectifier diodes.

Frequently Asked Questions about VMI's High Voltage Diodes"


73
Wolfgang
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