# Power rating of the Resistor used in a Opto-Interrupter Ckt

#### ManeeshaN

Joined Feb 17, 2010
8
Hi,

The emitter side of the opto-interrupter is connected to a +15V supply via 820ohm resistor.

To determine the reqd wattage for the resistor, shall I consider only voltage and R value or do I need to consider any other parameter of the opto interrupter.

If I consider only R value and V for calc. wattage, it comes : V^2/R = 0.274
Is this ok..?In that case is it ok if I go for a 0.25 W Resistor or should I choose one with a greater wattage 0.27W?

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#### R!f@@

Joined Apr 2, 2009
9,740
Wattage is equal to voltage drop times Current. It is easily calculated, and you should use any value higher than what you calculated.
The problem in the circuit is that you need to calculate the resistance with the interrupter max current rating in mind.

#### beenthere

Joined Apr 20, 2004
15,819
The wost-case situation is a short to ground, resulting in a dissipation of on the order of 270 milliwatts. Use a 1/2 watt rated resistor.