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Author Topic: Catastrophic Current Limiting in HV Supplies using resistor - Why?  (Read 30185 times)
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The Slab Bacon
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« Reply #50 on: March 12, 2009, 07:50:17 AM »

Is there any real difference between "energy storage" caps and standard oil caps??

I recently picked up a pair of 35mfd @ 4KV peak "energy storage" caps at a local fester for a very reasonable price. I had planned to keep them for spare filter caps for my 4X1 rig. (3400v plate supply)

They are not much, if any bigger than normal oil caps of that value. I just assumed that they are just standard oil caps made a little more robust to handle the repetetive charge / discharge cycles.

                                                              The Slab Bacon
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KD6VXI
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« Reply #51 on: March 12, 2009, 09:55:03 AM »

No one was in the building when our caps ruptured. So don't know if they spouted like an oil well or not. But oil was all over every inch of the room, it takes about 12 hours with 3 people or more to clean it, a very disgusting job. Ruins clothing. Not PCB oil, these are vintage 1980s GE capacitors.

John,

With the price of CHEAP IP based cameras today, do you guys have any video monitoring of the rooms now?  Would seem like a good idea, if only for "damage review"...  As well as some DAMN fine youtube video footage Smiley

--Shane
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KD6VXI
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« Reply #52 on: March 12, 2009, 09:58:36 AM »

Is there any real difference between "energy storage" caps and standard oil caps??

I recently picked up a pair of 35mfd @ 4KV peak "energy storage" caps at a local fester for a very reasonable price. I had planned to keep them for spare filter caps for my 4X1 rig. (3400v plate supply)

They are not much, if any bigger than normal oil caps of that value. I just assumed that they are just standard oil caps made a little more robust to handle the repetetive charge / discharge cycles.

                                                              The Slab Bacon

Slab,

The energy storage caps (from what I've been told / experienced) are for charging and QUICK discharging.  Whereas standard electrolytics are NOT designed for quick discharge.

Photoflash caps are storage type.

I've used them in smaller power supplies, and they work...  Bought some 450 or 550 volt types off EBay, and they worked FB in a sweep tube amplifier, no noticeable difference (other than larger PEP capabilities, the power supply went from about 45 uF to 150 uf or so of total Cfilter)..

I have heard from others that they didn't work so well, and eventually went south.  So, long term, they might not like being charged all the time (used in power supply work)...  But, my experience has shown them to work fine business.

--Shane
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The Slab Bacon
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« Reply #53 on: March 12, 2009, 10:27:22 AM »

Shane,
         that is pretty mouch what i have found so far. It should be ok to use storage caps as filters, but not to use filters for storage. as they wont hold up to the heavy charge / discharge cycles. Other than that, no one has been able to give me any hard answers as to what the difference is.
 
                                                            the Slab Bacon
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KM1H
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« Reply #54 on: March 12, 2009, 11:03:49 AM »

I find it surprising that so few commercial ham amps include this surge suppressor resistor as it would save a lot of expensive items. The NCL-2000 of 1963 is the first that I am aware of and I was fortunate enough to be an engineering tech on that project and learned more than a few things.

The purpose of the vitreous enamel WW is to limit the surge current until a safety device opens. It is not there to be sacrificial and if it fails it wasnt big enough. I use 25 Ohms @ 50W in the typical 1200-1500W out ham amp and all have survived a bout of tube flatulence.

Carl
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K1JJ
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« Reply #55 on: March 12, 2009, 12:31:11 PM »

Thanks for all the great info on capacitor failures, guys!

Don, I like your idea of having TWO paths on the shorting stick. One through a resistor for the first pop and the next a direct short to be drain the cap completely.  This eases the strain on the caps by using a current limiting resistor first - then they are drained completely using the direct short second. I would then leave the direc short on while working on the rig.

Last night I was working on my QRO 40M dedicated amp and it shorted the HV supply. The limiting resistors did their job and the wire fuses evaporated... :-)     I took out the 20KV  JJ homebrew hi-pot tester and found the problem was the plate coupling cap. It was mounted too close to a grounded bracket and arced through itself.  The evidence was only a TINY spot on the cap - almost impossible to find without the hi-pot tester.  The tester uses a Variac controlled neon sign xfmr that goes thru a 1/2 wave rectifier stack, into a .1 20kv cap...then thru about 20megs, then thru a  meter.

BTW, I have used  "PhotoFlash" caps (55 uf @ 5kv types) in many of HV supplies and love 'em.... and still do.  Never had a failure yet over 20+ years. 


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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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KA1ZGC
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« Reply #56 on: March 12, 2009, 01:58:58 PM »

If you're talking about an independent unit for discharging loose capcitors, maybe. If you're talking about your rig's Jesus Stick, absolutely not!

A transmitter's shorting stick should be a shorting stick, period. Nothing more, nothing less. Straight path to chassis ground. You do not want to add any parts which may fail. That shorting stick is your last stop before The Great Beyond. It's not just your life at stake, either. Someone else may wind up with your rig after you die, and they'll need to depend on that shorting stick, too.

If the choice is blow a capacitor or die, hell... I'd rather blow a cap.

Now, there's a way to do what Don described within a power supply. Harris did just that in the power supply in my avatar (FM-25K).

Here's how they did it:

Mounted on one wall of the cabinet are two electrically-identical solenoids. Their field coils are tied to the 120V cabinet interlock. These solenoids are arranged vertically such that they form normally-closed switches (they have 1" dia. brass slugs on the end, as do the stationary contacts below).

Of course, "normally closed" means closed when power is cut or interlock is open. The rest of the time (from Filament On to Filament Off + 2 minutes), these two solenoids are held open.

The stationary contacts are perfectly level with respect to one another. One contact leads to one side of the cap bank through a 500 ohm (IIRC) carborundum resistor, the other stationary contact is a straight shot to the same spot.

The (non-stationary) solenoid contacts are both tied directly to the opposite side of the capacitor bank.

All seems pretty obvious at this point, right? But, you're probably asking yourself where they reliably derive the time delay between the resistive short and the dead short?

It's so simple it's brilliant: they simply mounted the dead-short solenoid coil 1" higher than the resistive-short solenoid coil! So open the interlock (or lose the mains), both solenoids drop at equal velocity, the resistive-short contact gets there first, followed by the dead-short contact a few tenths of a second later. As long as you pay your gravity bill, this step-stop configuration doesn't have too many things that can go wrong with it!

We've dumped the mains on this transmitter with the plate on at 11 kilowatts output. No bangs, no flashes, not even a slight "snap". Just the "chuh-chunk" of the solenoids falling, and the HV meter instantly falls to zero.

I have close to 100 pictures of this supply, not sure I took any of the step-stop. I can certainly take one the next time that supply is opened up (probably next week, since we need to take it off the air for some finishing touches very soon).

I like the "What Would The Egyptians, Mayans, Incans, or Aztecs Do" approach to these things. You're far better off putting your life in the hands of the laws of physics than in the hands of human engineering!

...for what any of that's worth.

--Thom
Kilovolts Alter One Zorched Gate Crasher
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Patrick J. / KD5OEI
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« Reply #57 on: March 12, 2009, 10:54:11 PM »

Is there any real difference between "energy storage" caps and standard oil caps??

I recently picked up a pair of 35mfd @ 4KV peak "energy storage" caps at a local fester for a very reasonable price. I had planned to keep them for spare filter caps for my 4X1 rig. (3400v plate supply)

They are not much, if any bigger than normal oil caps of that value. I just assumed that they are just standard oil caps made a little more robust to handle the repetetive charge / discharge cycles.

                                                              The Slab Bacon

Slab,

The energy storage caps (from what I've been told / experienced) are for charging and QUICK discharging.  Whereas standard electrolytics are NOT designed for quick discharge.

Photoflash caps are storage type.

I've used them in smaller power supplies, and they work...  Bought some 450 or 550 volt types off EBay, and they worked FB in a sweep tube amplifier, no noticeable difference (other than larger PEP capabilities, the power supply went from about 45 uF to 150 uf or so of total Cfilter)..

I have heard from others that they didn't work so well, and eventually went south.  So, long term, they might not like being charged all the time (used in power supply work)...  But, my experience has shown them to work fine business.

--Shane


The typical storage cap is not intended to remain at its rated voltage for very long and might break down if done that way. 75% for continuous filter service is what I was told by a laser technician, but with so many kinds of caps and as many educated opinions, only your datasheet vendor knows for sure.
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