by EWBrown » Mon Jun 05, 2006 6:29 am
1/4 watt may be a tad small, I like to have about 300% of actual dissipated power for safety margin in resistors. I'd go with no smaller than half-watt, and one watt sould be ideal. Bend the leads around to fit the pads as needed, and keep the resistor bodies off any "metal bits" of the PC board, or at least space them at least 1/8 inch away from etches.
The "stock" 330K SMD resistors supplied on the boards are 1 watt devices, and are a nice clean fit betweem the pads on the solder side of the PCB.
As an example, a 330K resistor connected across 330VDC will consume 1 mA, so the power dissipated through it will be 330 mW or 0.33 W. A 1/4 watt resistor is definitely too small, 1/2 watt is a slim margin, and 1W gives the 300% "safety margin".
In the SDS ST-70 or MKIII or similar boards, on which the caps are in a series / parralell arrangement, the caps (and resistors) will see about 240-250VDC across each, so the power dissipated across each resistor (at 250V) will be 250V X 0.75 mA pr about 0.188 watts, call it 0.2 watts just for round numbers. 1/2 watt resistors would be a bare minimum, but and 1 W would still be the best chooice, for long resistor life.
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