Bad arc in transformer

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James Henry
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Bad arc in transformer

Post by James Henry »

Just had a really bad arc in transformer. I have been using this GE dental trans for a couple of months without a problem, only at 20kv and ZAP.
Blew the Diodes and 2 meters, and it shut down the computer, small EMP! I am getting another transformer, once mounted i will pump down for a couple of weeks. I have a question on the Diodes i have read all the FAQS. What is every one's favorite Diode? Please chime in.
James
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Dennis P Brown
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Re: Bad arc in transformer

Post by Dennis P Brown »

I buy a chinese diode that handles 20 kV and is claimed to handle 1 amp; however, I never push it pass 200 ma (and that is a spike only, steady load is under 40 ma) and use two in series for my 30 kV x-former per diode leg. The key for me is getting lowest reasonable cost but then being careful to run it well below its claimed values.

So very sorry about your x-former. Its possible the diode killed the x-former by failing first; thus creating a runaway current that killed the x-former, maybe?
James Henry
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Re: Bad arc in transformer

Post by James Henry »

Dennis
I am set up the same way. I was using 20kv, 2amp Diodes, two in series for each leg. Not sure what happened, could be what you said.
Anyways i will start out with a fresh transformer and go from there.
Thank You
James
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Dennis P Brown
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Re: Bad arc in transformer

Post by Dennis P Brown »

Not trying to be insulting but your variac and transformer were each protected by separate fuse systems? When my diodes went, the x-former fuse went but not the variac's.
Alex Aitken
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Re: Bad arc in transformer

Post by Alex Aitken »

Dennis, are your diodes potted black things? Only I think I have a few of those and figure they are just 1007s in series.
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Dennis P Brown
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Re: Bad arc in transformer

Post by Dennis P Brown »

Yes, they are rather large potted affairs. Easier to show a pic. My x-former, bridge and ballast are all arranged in a simple configuration. I use to keep the diodes under oil but realized it was pointless due to the huge air separation between contact points. I like the cooling aspect of oil for the ballast resisters. I use a large fan to keep the x-former cool - once, long ago, I actually use to make 30 - 45 minute runs so heat buildup was a bit of a concern. I had a water cooled chamber then, too.
Attachments
I arranged them in a classic diode bridge because - well, easier to keep track
I arranged them in a classic diode bridge because - well, easier to keep track
20 kV, 1 amp Diode
20 kV, 1 amp Diode
Emma Black
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Re: Bad arc in transformer

Post by Emma Black »

I am also using the exact same diodes, in the same config as Dennis 4 pairs of 2, one pair blew last week. The result was big current draw from the transformer, I have circuit breakers and a couple of large bulbs as ballast, the breaker for the transformer popped but not the variac.

They don't like to be pushed voltage wise it seems, mine failed pretty much immediately at around 45kv, may switch to three in series.
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Richard Hull
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Re: Bad arc in transformer

Post by Richard Hull »

Microwave oven diodes are all typically rated 12kv-14kv @ 500ma. Put them in series. They work fine and they are relatively inexpensive. Again, do not push. For 20kv rms rectification = 28 kv PIV, I would try three in series or 4 in series if paranoid. I lucked onto 25 in new individual packs for $20.00 a couple of years ago at a hamfest.

Note: They are not suitable for ultra-fast switchers. Mostly 5khz and below. Naturally, in microwaves they are 60hz and the diodes just lazy along comfortably.

Richard Hull
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Dennis P Brown
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Re: Bad arc in transformer

Post by Dennis P Brown »

Circuit breakers are slow; my HV power supply has a master circuit breaker for the entire system but both the variac, and the low voltage side of my transformer each have their own fuses. Most fuses will go fast and protect a given device. Using a fuse that is rated very close to the limit of the device is the best practice - a quick calculation can give you what that value should be.

I am unfamiliar with 'bulbs' as ballast resisters. Could you explain that a bit more?

Richard is spot on - when a device is too valuable to risk, over kill on diodes is a good practice. Also, not surprised at all that Richard came across a really great deal on diodes - he certainly knows when to pouch on a given item at those hamfest's.
Last edited by Dennis P Brown on Sun Jul 31, 2022 5:22 pm, edited 1 time in total.
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Richard Hull
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Re: Bad arc in transformer

Post by Richard Hull »

high watt incandescent lamp were commonly used for ballasts in primary circuits of transformers feeding uncertain or highly variable loads on their secondary. Ancient battery chargers for autos were a very common use. Naturally, they are for 60hz systems only and only in the transformer primary circuit.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.
Emma Black
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Re: Bad arc in transformer

Post by Emma Black »

Hey Dennis, yes so the resistance of an ordinary incandescent bulb is very non linear depending on the current, increasing quite rapidly as the current grows. I have a couple of 500w bulbs between the transformer and the variac, in normal operation the filaments stay dark and resistance is low, soon as the current starts to be pushed, the bulbs dimly glow. In major fault conditions the bulbs light fully.
Unsure how well they will work long term, but it is simple and inexpensive, there is some more information about the technique here:

https://www.eetimes.com/use-light-bulbs ... -limiters/

Also circuit breakers can be slow depending on the type, everything plugged into the mains here in the UK also has a fuse in virtually every plug.

Richard I am dead jealous of some of your deals after looking up the new price of some of them!
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Richard Hull
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Re: Bad arc in transformer

Post by Richard Hull »

As I noted years ago, I use to attend 13 different large hamfests each year, by drawing a 150 mile travel radius around Richmond! That is 13X the opportunity of a person who is not willing to travel thusly to attend ones beyond his own home town.

No more, however. Due to covid fears and the new hams not willing to get off there duffs to setup a venue in their town, a lot of hamfest are no longer extant or haven't started back up yet. Those old greatest generation guys had the verve needed to keep many fests going. The new guys, not so much. I am lucky now to attend 4-5 fests each year. Of those, only 3 might be considered large with good yields of goodies to repay the travel effort and expenses.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.
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Dennis P Brown
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Re: Bad arc in transformer

Post by Dennis P Brown »

Light bulbs contain argon gas at essentially atmospheric pressure. If the filament in the bulb suddenly experiences a huge current surge, part of the metal can vaporize - i.e. turns to plasma (runaway situation) - and this vaporized metal will easily ionized some of the argon. This conductive path could support a very large current surge conduction for a short time (till the gas cools.) Until then, this surge will continue stressing the transformer. A fuse, on the other hand, simply melts and breaks the conduction rapidly. It is possible the bulb filament, by vaporizing in argon gas, could have allowed enough current to cause the transformer to arc. I would suggest that bulbs not be used in lieu of a simple, reliable and well understood, fuse.

For instance, my breakers support 15 amps each and are slow. My variac fuse can handle 15 amps and is faster. My HV x-former has a 10 amp fuse on its primary. The power my x-former will see is 10 amp x 120 volts or 1200 watts (continuous duty) before the fuse will fail. The x-former can handle far above that (30 kV @ 0.2 amp or 6000 watts for a few seconds.) The 10 amp fuse will go before the x-former see's such a dangerous current (yes, micro-secs surges are far higher and occur but this is a rough estimate.)

I would never trust a light bulb to protect my valuable x-former - there is a reason all modern devices only use fuses. Also, there is no way a bulb is less expensive than a fuse - I can get a box of fuses for less then a light bulb here. Besides, when my diode shorted to ground, my 10 amp fuse blew and my x-former had no issues. It did work.
Emma Black
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Re: Bad arc in transformer

Post by Emma Black »

As I said, this was an as well as approach, I could just use another ammeter. Fuses and or breakers are still going to be essential, these transformers are tricky and expensive to get.

May be mistaken but the response time for a breaker would also surely depend on the type used? In the case of the thermal type which work via a bimetallic strip heating up, it takes many seconds to pop. Which I believe is designed to protect against overload rather than dead short type conditions. The type that uses a coil and solenoid have a very fast response, obviously not as quick as a fuse.
A lot of breakers have both types of protection in one unit, but I am unsure what is the normal in different places.
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Dennis P Brown
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Re: Bad arc in transformer

Post by Dennis P Brown »

I'm a bit confused. I'm not talking about Breakers per se. We all know the breakers have slow response time. I'm talking about simple fuses. Which have nothing to do directly with breakers. I like fuses because they fail quickly and protect your devices. Breakers like we both agree are rather slow.
Emma Black
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Re: Bad arc in transformer

Post by Emma Black »

Yeah sorry for the side track on breakers, not too sure where I was going with that. I do agree, not much can go wrong with a simple fuse to protect things, plus very cheap.
Frank Sanns
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Re: Bad arc in transformer

Post by Frank Sanns »

Fuses are after the fact.

Once dielectric breakdown has occurred, the component is history. It may take other components with it due to that electrical stress or the rapidly collapsing magnetic field in a transformer.

Think car auto ignition with points. The coil is charged. The HV spark comes when the circuit is OPENED. Snap!
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Rich Feldman
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Re: Bad arc in transformer

Post by Rich Feldman »

This is not about thermal behavior of fuses or ballast lamps, but that of XRT's.

Dental x-ray generators normally run for less than 1 second out of every minute.
Bigger XRG's, as for imaging human chests or spines, might support exposure times as long as a couple seconds. Even CAT scanners have duty cycles much less than 50%.

The limiting parameter is typically the heat capacity of anode in Coolidge tube.
http://sprawls.org/ppmi2/XRAYHEAT/
Not applicable to, for example, tubes with water-cooled anodes for X-ray diffraction work.

Point being: if the dental/chiropractic XRT were designed to work continuously, at anywhere close to rated current,
it would be needlessly large and expensive for the XRG design. So when I get to actually loading any of my XRT's,
will be sure to derate the secondary current substantially. Oil immersion plays an important part in the thermal recovery between shots, but not (I think) the temperature rise deep inside primary and secondary windings during a full-current pulse.
All models are wrong; some models are useful. -- George Box
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