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Aluminum wire was Re: [TCML] pole pig protection
Just a thought here guys, aluminum is a poor conductor of RF, but a good
conductor of 60 Hz (if of the correct diameter.) Could one
theoretically use a length of aluminum conductor (actively cooled) to
attenuate the RF and allow the 60 Hz to pass freely, this would also
effectively squelch the MHz primary harmonics nicely. Trouble is I
don't know how much conductor you would need (miles of it perhaps) I
have not done the math, this is just an idea.
Scott Bogard.
Jim Calvin wrote:
> Gary,
> The division ratio will not be the same at all frequencies. The RF
> choke's impedance increases with frequency (hence a low pass action) also I
> am fairly certain the inductance of the HV winding is not 1000 henries. In
> fact, the impedance at the HV winding should be the impedance at the low
> voltage input times the turn ratio (1/65). This of course is going to be
> the source impedance plus the ballast impedance.
>
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
> Of Lau, Gary
> Sent: Wednesday, September 10, 2008 9:51 AM
> To: Tesla Coil Mailing List
> Subject: RE: [TCML] pole pig protection
>
> Correct - a choke in series with the HV winding does create a voltage
> divider. But -
>
> 1) The division ratio is constant for all frequencies, 60 Hz as well as the
> tank frequency. Filters usually imply that the attenuation is
> frequency-dependant. We want a filter that attenuates high frequencies
> more-so than low frequencies - a low-pass filter.
>
> 2) Let's say that the HV winding L1 has an inductance of 1000 Henries. If
> we put a 10 mH inductor L2 in series with this, the voltage seen by the HV
> winding is L1/(L1+L2):
> 1000 / (1000+.01) = 0.99999 of the unfiltered value. No difference.
>
> Regards, Gary Lau
> MA, USA
>
>
>> -----Original Message-----
>> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
>> Behalf Of Jim Calvin
>> Sent: Wednesday, September 10, 2008 8:53 AM
>> To: 'Tesla Coil Mailing List'
>> Subject: RE: [TCML] pole pig protection
>>
>> As a point of fact I have destroyed a pole transformer last year with this
>> very same coil. So they are not indestructible.
>>
>> Gary, you are right, an inductor by itself is not a filter, but in series
>> with the HV winding of the transformer forms a voltage divider with an
>>
> input
>
>> impedance of greater than 5Kohm (at 90Khz). I have not put a scope on it
>> with my HV probe so I can not say for certain it is helping.
>>
>> Can you email me this filter DC. This is exactly what I am looking for.
>> My email is jcalvin@xxxxxxxxxxxxxxx
>>
>> -----Original Message-----
>> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
>> Behalf
>> Of DC Cox
>> Sent: Tuesday, September 09, 2008 11:52 PM
>> To: Tesla Coil Mailing List
>> Subject: Re: [TCML] pole pig protection
>>
>> I disagree with some of this.
>>
>> If there were zero oscillations then we would not have to use Terry
>>
> filters
>
>> even on NST circuits as the spark gap would be shorting out the
>>
> oscillations
>
>> there also. If you throw a scope on the circuit you can see some
>> oscillations coming back from the tank circuit directly toward the xmfr
>>
> ---
>
>> yes, even thru the shorted spark gap (conduction resistance is
>> constantly changing and not a fixed value so it is not always at zero
>> value).
>>
>> and yes, one should employ a simple air core inductor and also a resistive
>> filter (12 pcs 600 Ohms in parallel, 200 watts each) even on pole
>> xmfrs. again, if it is scope you can see the non-60 Hz oscillations.
>>
>> pole xmfrs are testing usually for a few single shots at 125 kV BIL (basic
>> impulse level) using a Marx generator, however lightning oscillations tend
>> to
>> be short-lived and certainly not continuous as with a LC tank circuit. I
>> know many experimenters who do not use a simple filter but these may be
>> the same guys who drive around without auto insurance!
>>
>> the filter is simple and will save your $300 pole xmfr, plus the $250 it
>> cost to ship it if in another state. The filter costs approx $30 ---
>> $300/$30 risk/reward ratio.
>>
>> I have the filter circuit that I use if anyone wishes to contact me
>>
> off-list
>
>> I will email it to them.
>>
>> better safe than sorry and out a few hundred bucks.
>>
>> Dr. Resonance
>>
>>
>>
>>
>> On Tue, Sep 9, 2008 at 6:59 PM, miles waldron
>> wrote:
>>
>>
>>> These comments are in response to Gary Lau, who writes on this thread.
>>>
>>> Gary is a very nice person. The following opinions are mine, and for the
>>> record I agree completely with everything Gary has said. Someone please
>>>
>> let
>>
>>> me know if I have missed the point entirely, and if that is the case
>>>
> then
>
>> I
>>
>>> am sorry.
>>>
>>> Terry filter protection for a pole pig is pointless. A pole pig is
>>>
>> designed
>>
>>> to be struck by lightning over and over again during its lifetime. Have
>>>
>> you
>>
>>> ever been hit by lightning? As equipment goes, a pig is typically used
>>>
> to
>
>>> avoid transformer limitations. You can do just about whatever you want
>>>
>> with
>>
>>> a pole pig, and it will still be there (not hindered in any way) after
>>>
> the
>
>>> explosion.
>>>
>>> Miles Waldron
>>>
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
>>>
>> Behalf
>>
>>> Of Lau, Gary
>>> Sent: Tuesday, September 09, 2008 6:25 PM
>>> To: 'Tesla Coil Mailing List'
>>> Subject: RE: [TCML] pole pig protection
>>>
>>>
>>> Opinions vary on this topic. Here is mine.
>>>
>>> First off, in contrast to NST's and MOT's, pole pigs are extremely
>>>
> rugged
>
>>> and well designed. They are designed to be undamaged by lightening
>>> strikes.
>>> While it is not unheard-of, damage to pole pigs is a very rare thing.
>>>
>>> As to protecting the pig from RF - I assume that your TC circuit has the
>>> main gap in parallel with the pig HV secondary. Please consider that
>>>
> the
>
>>> ONLY time there is RF oscillating in the TC primary or secondary, the
>>>
>> spark
>>
>>> gap is conducting. If the spark gap is conducting, it is essentially
>>> shorting out the pig's HV secondary, so there's not a lot of RF to
>>>
> protect
>
>>> against.
>>>
>>> Now, it's not entirely true that the gap is conducting continuously
>>>
> during
>
>>> each bang. Each time the current through the gap passes through zero at
>>> the
>>> resonant frequency, the gap arc extinguishes briefly for a few(?)
>>> nanoseconds. During this brief time the primary rings with its
>>> self-capacitance, in the tens of MHz region. The initial magnitude of
>>>
>> this
>>
>>> oscillation is Vbang, and since it is in series with the tank cap, also
>>> initially at Vbang, the combination of the two in series (what the HV
>>> winding sees) is 2Vbang. This, IMHO, is the thing that needs to be
>>> filtered, at least for NST's and less sturdy transformers. And this is
>>> easily and effectively attenuated by the R-C filter in the Terry filter.
>>>
>>> Simply adding a choke in series with the pig's HV terminal does not a
>>> filter
>>> make. The pig's secondary may be viewed as a many thousand HENRY
>>>
>> inductor.
>>
>>> Adding a few mH in series with it accomplishes nothing. Low pass
>>>
> filters
>
>>> are constructed of typically two or more elements - L-C, R-C. L-C-R,
>>>
> etc.
>
>>> Just adding a choke inductor does not achieve any filtering. You might
>>>
> as
>
>>> well string together cloves of garlic or St Christopher medallions.
>>>
>>> I apologize for being so wordy and blunt, but the use of chokes to
>>> "protect"
>>> transformers is a practice that was once done, just because it "seemed"
>>>
> at
>
>>> first glance like a good idea, and persists despite strong logic not to.
>>> It
>>> has since been shown through rigorous circuit analysis, simulation, and
>>> measurement, to be ineffective at best and counterproductive when done
>>>
>> with
>>
>>> bypass caps.
>>>
>>> Regards, Gary Lau
>>> MA, USA
>>>
>>>
>>>> -----Original Message-----
>>>> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
>>>> Behalf Of Jim Calvin
>>>> Sent: Tuesday, September 09, 2008 3:07 PM
>>>> To: 'Tesla Coil Mailing List'
>>>> Subject: [TCML] pole pig protection
>>>>
>>>>
>>>> I have a 25KVA pole pig I am using for my large tesla coil (the coil
>>>> only uses about 5-6KVA). I am using a hand wound RF choke to protect
>>>> the RF from the coil from getting back into the pig. It is wound with
>>>> 16 gauge wire onto a 8 inch sonatube. I measured the inductance with
>>>> RLC meter to be 10mH. My pole pig has two HV leads with one grounded
>>>> and the other connected through this choke to my coil. I have not
>>>> used the coil in awhile (been a busy summer). I was going to start
>>>> using it again but I wanted to make sure this is sufficient to protect
>>>> my piggy! Any advice would be helful.
>>>>
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