Re: [TCML] Terry Filter for OBIT's

[mddeming@xxxxxxx]

Hi Paul,

Sounds like you have a power supply transformer of the type that might be found in a 300W VT transmitter. While they are much more robust than an OBIT or an NST, (they operated at or near their rated values for hours at a time),  they usually were not designed with the expectatiuon of transients 2-3 times their voltage ratings, while pole pigs and a few "exotic" transformers are designed with the expectation of occasional transients 8X-10X their nominal ratings. If you do have a power supply transformer, they usually were not current limited and so would require an external ballast for TC operation. 
If you use a well-built safety gap and no Terry filter, you are less likely to kill it than you would an OBIT or NST. How much less likely is between you and whatever deities may watch over your equipment. ;^))  It has been my experience that any screw connection exposed to temperature fluctuations will work loose over time (like safety gaps) and at some point, the human tendancy to "open it up just another hair" becomes almost irresistable for most coilers.  A safety gap with a loose or poor connection may be "invisible" to transients. 
If you build your coil to the characteristics of this transformer, it is unlikely that an OBIT or a pair of them will be a direct drop-in replacement if and when it needs to be replaced, and it MAY require more rework of the circuit than just changing the tap point.
(different V, different Z ==> different MMC ==> different Fres ==> different Csec needed, etc.) 
Transients are not a once-in-a-lifetime occurrance for coilers, so the question becomes "How MANY cheap-transformer fairlures can occur before the cost of a Terry filter would have been justified?" Each person's cost-risk tipping point may be different.
The situation with NSTs is a little different in that the type of NST that has been the workhorse of most coilers for the last century are being phased out and may become extinct in the not too distant future. 

All TCs are a balance of conflicting considerations. I hope this gives you some insight to find what you're comfortable with.

Matt D

-----Original Message-----
From: Paul Brodie <pbbrodie@xxxxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sun, Jan 3, 2010 1:25 am
Subject: Re: [TCML] Terry Filter for OBIT's


Good point Matt but you still didn't address the need for a filter with this type of transformer. I am assuming, from your other comment about using a Terry filter to Andrew, that you espouse the use a filter with all transformers??? 
 
In regard to the cost factor, I failed to mention that OBIT's are almost always available on eBay for about $20 plus $10 for shipping and this still makes replacement quite a bit less than the full cost of building a Terry Filter. 
 
This leads to my next question. About 5 years ago, I had two previous TC's based upon neon sign transformers and out of ignorance never used anything but safety gaps and never used a filter at all. One was a 15kV 30mA TC and the other one was based on a 12kV 30mA neon sign transformer and both used multiple spark gaps with very conservative gapping. I didn't run my coils for long periods of time, usually not more than a few minutes at a time, and I never had a transformer failure. I have had several other types of small Tesla Coils but they were based on very small power supplies and I do not feel they are relevant to this discussion. 
 
So, my question is whether you believe a filter is an absolute necessity with all but pole pigs or is it simply added insurance? Please note that I have no opinion or even a dog in this fight. I am just asking. 
Paul 
Think Positive 
 
----- Original Message ----- From: <mddeming@xxxxxxx> 
To: <tesla@xxxxxxxxxx> 
Sent: Saturday, January 02, 2010 9:09 PM 
Subject: Re: [TCML] Terry Filter for OBIT's 
 
 
I finally got in my "OBIT" and it isn't an OBIT at all. It also isn't 9000 volts and 50mA. It is actually a power transformer built similar to a large MOT and is 6000 volts and 50mA. I'm pretty sure the 9000 volts was a typo. The guy I bought it from just would deliberately give the wrong information. It is also a very hefty, 21 lbs. The core measures about 4" by 4.5" by 6" and it is apparently wound with the primary underneath the secondary, as there are two wires that go from the primary binding posts to the center of the windings and the secondary binding posts are mounted on top of the windings. It is NOT wound like most MOT's I've seen with the primary and secondary windings separate. The secondary is also center tapped. 
 
Since I now have very little to go by about how substantial this transformer is, I think it may be wise to simply go ahead and use a filter but since I only paid $20 for it, I'm having difficulty justifying the expense to do so. Maybe I should just use a safety gap? 
 
The justification for a Terry filter is not what you paid for your transformer, but rather the cost you will have to replace that transformer 
with another if and when it fries. If you won a car on a $1 raffle ticket would you not put good brakes on it because they cost more than you paid for the car? 
 
Matt D. 
 
= 
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