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Re: What wattage resistors for NST protection?
Tesla List wrote:
> Original Poster: "William Parn" <parn-at-fgm-dot-com>
>
> What wattage resistors are required for NST protection?
> I think I have been seeing wattage resistors that
> are a lot lower than the actuall watts of the NST.
> Can someone explain how this works.
I'm using 25W resistors with my 15/60. I couldn't get the 50W units I
wanted, so I just made do. They work fine, but they get hot after a few
minutes of run time. Like most of the coil components, you can run parts
over their ratings if you keep the runtimes down and allow a cool down
period. It's a duty-cycle kind of thing. ;) I've run a small NST bank of
3 units on my small variac, I estimate I've run it at 2x its rating. But
it doesn't seem to mind. It growls at me, but it works fine as long as I
let it cool off. I've got some bigger variacs now, I decided I didn't
like hearing it growl. ;)
> I have also been reading about 1/2 watt bleed resistors
> on each cap in an MMC. How does this work when the wattage
> on a 15kv-at-60ma could be 900 at some given point in time.
The resistors disappear from the circuit when the AC is flowing. The
shortest path to ground for AC when a resistor and a cap are in parallel
is through the cap. The resistors are 10Meg, so they present a high
impedence to the incomming AC. When the AC turns off, the resistors only
have to disipate the charge from the cap. And only for the one cap they
are connected accross. So in my MMC each resistor/cap combo sees 1000V
(15KV/15 caps in series). They don't really see all that much current
(I=V/R so I=1000/10000000 or .0001A.), thus the wattage isn't as bad as
all that (.1W if I haven't screwed up all my numbers yet). That leaves
me with .4W to spare on my 1/2W resistors. And my resistors are 12Meg,
not 10, so I get a little more. ;)
Travis