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Registered Member #61654
Joined: Fri Jun 16 2017, 01:13PM
Location:
Posts: 2
I'm currently undertaking the construction of my first SGTC (static gap), powered off a 9kV@30mA NST. I've read that these transformers are quite fragile, so require a Terry filter between the HV output and the coil.
I've been struggling to find the details of how much voltage overhead I need to allow on the filter capacitors - right now I have 24* 3.3nF 700VAC polypropylene caps (so total AC rating 16.8 kV), but I'm now concerned that, should a voltage spike occur, these will fail before the MOVs can pull the overvoltage to ground.
Is it strictly necessary for the 1k resistor to be of the large, ceramic type? Has anyone else used the aluminium-enclosed heatsink-attached resistors, such as these?
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Hey, welcome to the forum.
Those resistors with the heatsink are not suitable for high voltage applications, it will simply arc through the heatsink and destroy the resistor, because they have very small clearances.
You can use a series string of cheaper ceramic wire resistors instead of a single large one.
I believe the AC rating of your 24 cap string is sufficient, but I'm not sure if the final capacity is high enough (137.5 pF). I have successfully used 300 pF ceramic caps in the filter circuit of a tiny SGTC (protecting a little bug zapper transformer), so it will probably be fine.
Registered Member #58522
Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50
As a point of reference, I just built a Terry filter for my dual MOT Tesla coil with voltage doubler, which measured gets about 14 kV peak voltage (it should have about 12 kV, but its a little higher for some reason).
I used 18 2200 pF, 6.3 kV ceramic capacitors
with three parallel chains of three series capacitors so that 2200 pF @ 18.9 kV is achieved, each connected to RF ground. The resistors are eight 1.2 kOhm 5 W sandblock resistors arranged as four parallel strings of two series resistors for 600 Ohms @ 40 W dissipation.
I use 10 1800 V MOVs in series to protect the diodes
I think something similar could work for your NST tesla coil.
Registered Member #61739
Joined: Wed Aug 23 2017, 04:43PM
Location:
Posts: 44
Om my 15kv 120 ma i kust bough two 30kv 500pf ceramuc caps 15$ and used those instead of a string of caps then i used two 150 watt 1k ohm wire wound resistors 7$ from ebay as my resistance i added a saftey gap and that was that
I feel the mov string is just redundant
I use this coil as a demo coil, its been dispayed at various high schools and repeatedly abused by supervised students with there hands on my variac and no damage as of yet the two caps barley get warm and the resistors heat only a little
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Terry filter design details
