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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Ok, not to jack other threads, there are some things I don't understand about SGTC filter design.
The transformer is a small OBIT rated about 20mA, the voltage rating got erased but I think it's some 12-15kV.
I see some people protect their transformers with MOV stacks, which are expensive and I don't have anywhere to buy such HV MOV's anyway.
So, first thing I know I should do is use safety gaps, and set them properly and tightly, so the transformer *just doesn't arc* over them.
I'd have no trouble creating few hundred pf of capacitance just from things I have laying around, PVC pipe, or etc.
Now, looking at commonly used ''terry filter'',
- Why are the MOV's before, and safety gaps after the resistors? If they are same type of protection shouldn't they be parallel?
- Why are capacitors before the power resistors, and not after as I would find intuitive?
If resistors are there to provide additional impedance to the transformer, shouldn't caps be at the output so they provide low impedance to high frequency voltage?
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Hey, Marko:
When the MOVs fire, they won't last long. They're pretty tiny, unlike the distribution surge arresters some people use to protect their pole pigs. On the other hand, their firing voltage is probably much more reliable than your safety gap. AFAIK, that's why they're used as a backup. They are placed on the NST side of the resistors so that your safety gap gets the first chance at the transients (so the MOVs only sacrifice their lives when absolutely necessary, i.e., when the other protections don't work).
The positioning of the filter caps relative to the resistors seems a little less obvious to me. At the very least, it saves them from self-destruction due to extreme currents when the gap fires. Also, if they were on the other side of the resistors (from where they are now), wouldn't they get involved in the primary tank circuit's resonance? (if the gap were not place directly across the filter output, that is?) By putting them on the NST side of the resistors, their contribution to the primary tank frequency will be negligible, yes?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
The positioning of the filter caps relative to the resistors seems a little less obvious to me. At the very least, it saves them from self-destruction due to extreme currents when the gap fires. Also, if they were on the other side of the resistors (from where they are now), wouldn't they get involved in the primary tank circuit's resonance? (if the gap were not place directly across the filter output, that is?) By putting them on the NST side of the resistors, their contribution to the primary tank frequency will be negligible, yes?
The spark gap is placed so that it *shorts* both the filter caps and the supply itself as long as it is firing, as thus filter cap makes no role in primary tank.
From what I know, the transformer would only see HF HV after the gap quenches and there is still energy in the secondary coil. So I assumed that in order to make a sort of low-pass filter, caps should be after resistors, shorting down the HF current, while resistors block it.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Marko wrote ...
The spark gap is placed so that it *shorts* both the filter caps and the supply itself as long as it is firing, as thus filter cap makes no role in primary tank.
Well, most of the time that's how the gap is placed. But not all the time. And the primary cap isn't truly shorted, anyway; there's the primary L in there limiting its current. OTOH, if the filter cap were place directly across the gap as you suggest, then its current would be limited only by the gap resistance and by its own internal impedance, and so it would probably "fry".
As for the low-pass operation: As long as the HF is shorted *across the transformer*, that's all the matters, right? You're only trying to protect the transformer (and keep the MOVs from firing too often). If there is still some HF across the resistors, who cares?
I'm open to being totally wrong in my analysis as well, though!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
And the primary cap isn't truly shorted, anyway; there's the primary L in there limiting its current.
Sorry if I was unclear, but I didn't mention that primary tank cap gets shorted anywhere.
I'm always referring to design where gap is across the supply.
The caps I'm going to use (pieces of PVC pipe and Al tape) are hardly going to blow up but I'm not sure if it's good to have them discharged into gap all the time.
Maybe I should use resistors both in front and after the caps?
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Marko wrote ...
Sorry if I was unclear, but I didn't mention that primary tank cap gets shorted anywhere.
No, it's my fault. I misread your prior post. Sorry about that!
Marko wrote ...
I'm always referring to design where gap is across the supply.
I assumed that. But I suppose I was trying to point out that the design of the Terry filter doesn't make many assumptions about the configuration of your primary tank circuit.
Marko wrote ...
Maybe I should use resistors both in front and after the caps?
It's not clear to me what is gotten from having the resistors on the NST side of the cap. You just want to spoil the Q of the supply in general and prevent its getting involved in any kind of resonance with the primary tank, right? I suppose I'd only bother with resistors on the NST side if I was worried that some descructive resonance might errupt between the NST and filter cap, but the filter cap will be far too small for that, I think.
Maybe somebody else will chime in with some thoughts, though!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Chris, Aaron, all,
This is especially important as the primary capacitor in a tank can resonate with the leakage inductance of the secondary.
I thought only overvoltage protection (MOV's and safety gaps) can help prevent that, not the low pass filter itself.
So, if I'm not using MOV's, should I put safety gaps on their place (before resistors), or after resistors as they are? I'd like to know 'what is best' and 'why'. It just looks 'safer' to me to have them right on the transformer output... but after all it might not matter.
Caps I'm using are two pieces of PVC pipe covered inside and outside with aluminium tape, so I don't think I'll easily hurt them.
I mean, I'd just love to know what I'm doing while building this thing! Marko
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
You'd put them after the resistors (on the tank side) cause you don't want the voltage spikes anywhere near the transformer. I've always thought that the Terry filter was a bit overkill, as Chris said, all you need is a low pass filter. And a safety gap.
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Filter for a small 20mA OBIT
