Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx> At 03:52 PM 3/21/2006, you wrote:
Regarding those 100W resistors, do they need to be that big?
The power dissipation is about 80mA ^2 x 1000 ohms which is 6.4 watts. But we are discharging about 10 watts from the filter caps and MOVs stored energy too. So roughly 16 watts.
The problem is at 100 watts the resistors are at 300C!! They are basically space heaters.... At 16 watts they are at about 75C which is very comfortable. The larger size also helps voltage standoff and adds a little inductance. 50W resistors run at 120C.
The filter is rated to 240mA so there is a lot of margin there too. You can run smaller resistors, but they get very hot very fast.
So I many cases you can use smaller resistors especially if you are planning on them getting really hot. The first designs had smaller resistors, but folks complained about how hot they got. The big 100W resistors solved that ;-)
Cheers,
Terry