I tried to make a PWM AC dimmer circuit which found in instructables.com as below:
but the 100k 1W resistor got hot. Could using a pull up (or maybe pull down) resistor for optocoupler solve the problem?
Electrical – pull up or pull down resistor for optocoupler
opto-isolatorpulldownpullup
Related Solutions
You shouldn't need to pull down the MOSI or Clk lines.
Master Out Slave In is an output from your controller, as is the Clk line. Unless you do something that turns those pins to inputs, they should be just fine without either pull-down or pull-up resistors.
On the other hand, Master In Slave Out is an input to the controller. When none of your ADC chips are selected, that line does float. It needs the resistor that you added.
As you can see in the datasheet, this optocoupler uses a (Schottky-clamped) transistor between the output and ground pins:
This output is designed to be used with a pull-up resistor.
The chip requires that its supply voltage (VCC relative to GND) stays constant, and the output transistor is switched on by raising its base above GND, so it is unlikely that you could manage to do anything interesting with the GND pin.
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Best Answer
That's quite a large electrolytic capacitor.. it will take some time to discharge through R1 to a safe voltage. 10uF would work as well and would be smaller, safer.
Nominal power dissipation is about 1W regardless of whether the opto is on or off.
If you want to stay with this, use a physically larger resistor rated at a few watts, and it won't get as hot (but of course it will be dissipating the same amount of power). Preferably a MOF type that is flameproof and rated for mains voltage.