Re: What wattage resistors for NST protection?

• To: tesla-at-pupman-dot-com
• Subject: Re: What wattage resistors for NST protection?
• From: "Gavin Hubbard" <ghub005-at-xtra.co.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Wed, 16 Feb 2000 06:57:47 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Bill,

You can get away with it because the resistor only sees the low current of
the charging circuit. Remember, the output impedance of the NST is very
high , so the NST can only supply a relatively small current to the
charging circuit.

Say you have a 60mA NST to charge your tank tank circuit and you want to
insert a resistor (say 100 ohms) as part of a filtering circuit to protect
your NST

There are 0.06 amps (RMS) flowing through the resistor. So the power
dissipation is: 

P = I^2 * R 
= 0.06^2 * 100 
= 0.36 watts

Not a lot of power. 

Of course the resistor sees large current spikes from time to time (thats
why you want to filter the circuit!). Fortunately these spikes occur
infrequently. Although the peak power dissipated by the resistor is very
high during one of these spikes, the total power is still quite low. Most
coilers find resistors rated at 150 watts to be adequite for filter duty in
NST powered systems.


WRT to the MMC bleed resistors. The tank current almost flows entirely
'through' the caps in a MMC as the resistors have such relatively high
resistance (typically 10 M_ohm). Again, this keeps the power dissipation
very low (typically less than 0.5 watt).

Safe coiling,

Gavin Hubbard


P.S. The reason you don't need special HV resistors for the filters or MMC
is again the small current these devices 'see'. For the first example

V = I * R
= 0.06 * 1000
= 600 volts

In a 20 turn wire-wound resistor this is only 30 volts per turn. The
insulation will easily cope with this.



At 12:06 PM 2/15/00 -0700, you wrote:
>Original Poster: "William Parn" <parn-at-fgm-dot-com> 
>
>
>Greets,
>
>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 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.
>
>Cheers,
>Bill
>
>
>

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