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发表于 2013-5-22 05:50:18
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如果假定LED的正向压降是2V,那么其功率是不可以用1mA*2V进行计算的!
There is some truth to that.
We calculated, using the "incorrect" method, the power dissipation over the positive cycle is 2.5mw.
Here is a simulation, for the same set-up, over the positive cycle, but we let the computer calculation the integral for us.
The top trace, in red, is the power dissipation over D1 during the positive cycle (first 10ms). The average power dissipation is, as you can tell, 2.35mw, very close to the "incorrect" figure.
If you had used a led with Vfwd=1.8v and a resistor of 100k, you would have a power dissipation of 3.6mw. so I think the 4.4mw figure estimated earlier for a 2v Vfwd led is fairly accurate.
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