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Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Hi all.
I’m rebuilding my BIG MOT Tesla coil, and I have a question about RC filters on the high voltage side of my MOTs.
I have searched the internet for schematics and ideas, and I found this:
Terry Filter (Can this be used for 4MOTs?)
Terry Blake’s filter
The MOTs are external ballasted to 2KVA, the output is 9800V 200mA. And the frequency of the Tesla Coil is about 100 kHz My coil runs with a 200bps SRSG, and 0.085uF tank cap.
I have 15 2k 75W resistors that I could use for this purpose, but I need doorknob or ceramic discs caps for it.
Is there any calculations on how big the resistance and capacitance should be in a filter circuit?
Does anybody have a suggestion or an idea for a filter?
Registered Member #530
Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178
What I used for my 3kVA 12kVDC MOT supply are 2 - 5 Hn chokes on either leg of the outputs and 2 - 15kVDC, 1nF rated - blue ceramic disc capacitors from ebay with a safety gap. No resistors used...
On a large power system - 4 MOTs in series - I'd just simply use a static safety gap with 15kVDC rated ceramic caps to help with any "RF" surges. Run the power supply away from the base of your tesla coil and all should run well. You should run these trannies under oil too!
Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Terry,
I thought your filter would work if I centre tapped my MOTs like an NST.
But after searching more on the internet I’d have to agree with EVR, because there aren’t many who use filters on their MOTs.
So if I set it up with a couple of safety gaps, I should be fine
My only concern is if I kill a MOT in my supply, it would be a pain in the a%& to find a new one. I can’t buy old microwave ovens at the recycling place anymore, because the municipality has made some new rules about recycling in my area
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
I forget the name of the place, but there is an EBAY seller who has a ebay store which sells MOTs (new) quite inexpensively. Like $15.00 a pop. Sure, its not free from the trash, but its still relatively cheaper than a NST.
The Terry Filter was only made to handle less than 1/4 amp of current. A MOT can hit 2 amps and would just burn it up. I would just use a safety gap and for sure fuses on the input side. MOTs have very low secondary resistance so the high frequency noise should not be a problem there. I really am not sure a safety gap would do much good, but that might depend on exactly how it is setup. Fuses will be the main protection so definitely use them.
If you system is ballasted to 200mA, then the Terry Filter might be OK and might not really hurt anything. I just have never considered using it with MOTs at all before so I don't know for sure. I know that odd things can happen, so I hesitate to say it is OK without really knowing for sure.
Registered Member #530
Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178
I had my 17KV rated diodes die from RF kickback into the power supply - using safety gaps with some added inductance took care of the nasty surges and sent them to ground. The gap fired once every 3 seconds.
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Filter for MOTs?
