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RHS will be using 220v 400watt bulb

2 simple questions 1- does the 360ohm resistors has to be of high watt too ? 2- all i can find is 5-10watt resistors, can i use them ? would they snub correctly ? (will use 3 in series to get the ohm i need) what do i do ?

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  • \$\begingroup\$ The 360 Ω resistors are just for the trigger circuit (not the snubbing circuit). 1/2 W should be fine. If in doubt run the circuit with lamp off for one minute, isolate mains and feel resistor temperature. Repeat test with lamp on. You can always use a higher wattage than specified. It will just run cooler (and take up more space). \$\endgroup\$
    – Transistor
    Commented Jan 16, 2016 at 13:05
  • \$\begingroup\$ @transistor and for the 39 resistor, how much watt do i need ? \$\endgroup\$
    – Andrew
    Commented Jan 16, 2016 at 13:12
  • \$\begingroup\$ It may very well have to do with the voltage rating of the resistor. A regular low power resistor is often rated for 200V max. across it. Resistors with higher power rating often come with a higher voltage rating too. Check its datasheet. \$\endgroup\$
    – jippie
    Commented Jan 16, 2016 at 13:27
  • \$\begingroup\$ @jippie ill check the voltage ratting when i know the appropriate resistance power that will handle the bulb \$\endgroup\$
    – Andrew
    Commented Jan 16, 2016 at 13:35
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    \$\begingroup\$ The bulb will not change the power rating of the transistors. The TRIAC does depend on the bulb used. \$\endgroup\$
    – jippie
    Commented Jan 16, 2016 at 15:21

1 Answer 1

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The resistors on the optotriac carry essentially no current at all, only the brief pulse to trigger the triac. after that the triac carries all the current. use 0.25W or whatever is convenient, use resistors rated for 400V or more.

The 39 ohm resistor carries about 0.3mA while the lamp is off. I-squared-R puts the power at less than 5uW if you're using phase control peak currents could be higher resulting in more power consumption but quarter watt should be fine there too.

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