Re: What wattage resistors for NST protection?
Subject: Re: What wattage resistors for NST protection?
From: Terry Fritz <twftesla-at-uswest-dot-net>
Date: Tue, 15 Feb 2000 12:31:16 -0700
A typical MMC cap may see 2000 volts across it (maybe an EMMC). A 10Meg
resistor will dissipate 0.4 watts if that were DC. Since it is AC, the
power is about half that. The real reason we (I) use 1/2 watt resistors is
to get enough size in the resistor body to hold off the voltage. The 1/2
watt YAGEO resistors I use will arc across their bodies at about 3500
volts. So the 1/2 watt 10MEG resistors just seem to work out right. They
also cost slightly over a penny each and are easy to get so their cost is
The protection filter resistors will each have to dissipate about 10% of
the coil's power*. I use the tubular Ohmite type for this. They are
already large enough to provide good voltage standoff and they have good RF
*They actually dissipate 5% of the coils power but I get them twice the
size to keep them from getting to their "normal" 300 degree Celsius
temperature. That's too hot for me...
Protection filter resistor values should be selected to dissipate about 5
to 10 percent of a system's total energy to get good filtering without
reducing the power too much.
At 01:32 PM 02/15/2000 -0500, you wrote:
>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.