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Old 15th April 2019, 01:24 PM   #101
busaboy is offline busaboy  United States
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Originally Posted by gorgon53 View Post
A spark gap is the least intrusive and simplest OPT protection against kV spikes.

Now days those spark gaps are unobtainium and I make them myself if needed.
Huh? You mean a spark gap that is not shielded against atmosphere? Although there are many types of "open" spark gap devices still sold they would not be reliable in this application.

Gas Discharge Tubes have been around for 100+ years. The GDT is designed to fire at a pre-determined voltage and survive multiple firings. There are billions sold each year and they are dirt cheap. The cheap ones work from 75-2000 VAC and come in various power ratings and with 2 or 3 terminals for differential lines. The cheapest spark gaps are just traces on a PCB. Again not as reliable as a GDT.

In the old days GDTs with a radioactive gas fill were preferred but that is another subject.
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Old 15th April 2019, 04:08 PM   #102
gorgon53 is offline gorgon53  Finland
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Originally Posted by busaboy View Post
Huh? You mean a spark gap that is not shielded against atmosphere? Although there are many types of "open" spark gap devices still sold they would not be reliable in this application.

Gas Discharge Tubes have been around for 100+ years. The GDT is designed to fire at a pre-determined voltage and survive multiple firings. There are billions sold each year and they are dirt cheap. The cheap ones work from 75-2000 VAC and come in various power ratings and with 2 or 3 terminals for differential lines. The cheapest spark gaps are just traces on a PCB. Again not as reliable as a GDT.

In the old days GDTs with a radioactive gas fill were preferred but that is another subject.
Huh, shield air against atmosphere? In what kind of atmosphere do you run your amp?

And what make you think a open air spark gap is unreliable?

On a PCB they may be of limited use and unreliable for sure but properly done they are as reliable as it can get if you know how to do it.

Last edited by gorgon53; 15th April 2019 at 04:20 PM.
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Old 15th April 2019, 04:29 PM   #103
busaboy is offline busaboy  United States
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Originally Posted by gorgon53 View Post
And what make you think a open air spark gap is unreliable?On a PCB they are unreliable for sure but properly done they are as reliable as it can get.
The break-over voltage can vary over a wide range due to mechanical tolerance, humidity, contamination, altitude... Not desirable for electronics. OK for large equipment where +/-50% error won't fry anything. With a GDT being so cheap why bother. Certainly no low cost GDT can match a spark gap for power rating.

I have a surge generator in my shop (5KV@500A). It will blow semiconductors and even wire-wound 50W resistors into pieces.

Last edited by busaboy; 15th April 2019 at 04:31 PM.
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Old 15th April 2019, 07:37 PM   #104
gorgon53 is offline gorgon53  Finland
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Originally Posted by busaboy View Post
The break-over voltage can vary over a wide range due to mechanical tolerance, humidity, contamination, altitude... Not desirable for electronics. OK for large equipment where +/-50% error won't fry anything. With a GDT being so cheap why bother. Certainly no low cost GDT can match a spark gap for power rating.

I have a surge generator in my shop (5KV@500A). It will blow semiconductors and even wire-wound 50W resistors into pieces.
Contamination and mechanical tolerance can easily be handled by a proper design and build up.
When it comes to humidity and altitutude the open air gap has an ADVANTAGE because the transformer will most probable WORK UNDER THE SAME CONDITIONS .
Assume for instance that for some reason the airs breakdown voltage goes down , so will the open air transformers capability to withstand over voltage.
The open air gap break down voltage will go down also and therefore still protect, whilst the fixed voltage GDT wont change a yota and protection is more uncertain.
Or would you prefer a fixed voltage over one that is essentially self adjusting to the transformers capability?

Last edited by gorgon53; 15th April 2019 at 07:58 PM.
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Old 15th April 2019, 08:45 PM   #105
GoatGuy is offline GoatGuy  United States
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Fellows (and lasses)

The “trick” is to keep the dissipated power in a pair of series-connected 600 (or 1000) PIV diodes well moderated during the spikes. The first obvious plan is the pair of diodes (reversed) in series with an inductor, in series with an appropriate resistor. The choke squashes the fast risetime flashover, the resistor perhaps takes the lion's share of the spike(s) dissipation.

It is however not a willy-nilly design spec to optimize. The actual peak-inverse-voltage breakdown needs to be known, and the spec for the chokes has to be both economical and yet high-enough in henries to moderate the spikes. While still passing most of the spike energy thru the resistor-and-breakdown-diode. Balancing trick.

One might argue to increase device dissipation (diode-side) by using a chain of Zeners. Maybe use 70 volt units in series. Good trick. Kind of spendy though if too many are utilized. But hugely predictable.

Just saying,
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Old 15th April 2019, 08:50 PM   #106
TheGimp is offline TheGimp  United States
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http://www.teledynereynolds.com/reso...ark%20Gaps.pdf

Digi Key caries three manufacturers. Lots of devices available.
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Last edited by TheGimp; 15th April 2019 at 08:56 PM.
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Old 15th April 2019, 09:03 PM   #107
GoatGuy is offline GoatGuy  United States
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[url]http:\\http://www.teledynereynolds.com\reso...ark%20Gaps.pdf[\url] (repl \ with slash)

Digi Key caries three manufacturers. Lots of devices available.
Well … yes … then there are the gas-filled units. Basically bomb-proof. If you think about it (and just so happen to have a lot of them) you could easily deploy a series chain of inert-gas voltage regulator tubes, as well. They can handle outrageous abuse of short-lived spikes, if need be.
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Old 15th April 2019, 09:31 PM   #108
busaboy is offline busaboy  United States
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Originally Posted by GoatGuy View Post
Well … yes … then there are the gas-filled units. Basically bomb-proof. If you think about it (and just so happen to have a lot of them) you could easily deploy a series chain of inert-gas voltage regulator tubes, as well. They can handle outrageous abuse of short-lived spikes, if need be.

GDT means Gas Discharge Tube. The key to their power handling capacity is once triggered, the voltage across them is very low. Of course this is the same for a open air spark gap or similar to a Thyristor. A voltage regulator tube cannot handle the anywhere near the same power unless it was massive. A small thyristor cannot handle the current of a GDT which is >5,000 Amps. The downside to a GDT is the 1uS turn on time. There are commercial GDTs with radioactive gas fills and HV bias voltages that turn on much faster. Very pricey.
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Old 15th April 2019, 11:25 PM   #109
gorgon53 is offline gorgon53  Finland
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If someone wants to buy a device instead of DIY and prefers a fixed voltage instead of the always reliable and self adjusting open air spark gap the MOV is the best choice.
If you want diodes use a vacuum booster diode in reverse or at least avalanche diodes.
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Old 15th April 2019, 11:34 PM   #110
gorgon53 is offline gorgon53  Finland
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Originally Posted by GoatGuy View Post
Fellows (and lasses)

The “trick” is to keep the dissipated power in a pair of series-connected 600 (or 1000) PIV diodes well moderated during the spikes. The first obvious plan is the pair of diodes (reversed) in series with an inductor, in series with an appropriate resistor. The choke squashes the fast risetime flashover, the resistor perhaps takes the lion's share of the spike(s) dissipation.
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And how on earth would even a choke damped by a resistor protect the transformer from its voltage spike when exactly the opposite of it, a resistor damped capacitor would be needed? A properly sized cap and resistor makes sense, but a choke would only decrease the current rise time to the diode and during that time the transformer voltage would only rise instead of getting limited.
Please explain how this "spashing" is supposed to work.

Last edited by gorgon53; 15th April 2019 at 11:59 PM.
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