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

Discussion in 'Homework Help' started by ManeeshaN, Apr 13, 2010.

1. ### ManeeshaN Thread Starter New Member

Feb 17, 2010
8
0
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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2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,785
771
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.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
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.