✔ 最佳答案
USB供電5v ~ 5.5v
如果電壓5v 直接接電阻70Ω :
I = V / R = 5 / 70 = 0.071A
P = V x I = 5 x 0.071 = 0.355W
但係, 你用
5v - 3v = 2v
I = V / R = 2 / 70 = 0.029A
P = V x I = 2 x 0.029 = 0.058W
來計, 所以. . . . . .
你應該用
R = V / I = 5V / 0.025A = 200Ω
P = V x I = 5 x 0.025 = 0.125W
= (1/8w) 你可以用1/4W 就冇問題
或者試試
P = V / R
= (5 x 5) / 70Ω = 0.357w
= (5 x 5) / 200Ω = 0.125w
由於你未計電流
P = I x R
= (0.025 x 0.025) x 70Ω
= (0.025 x 0.025) x 200Ω
2009-12-25 23:32:44 補充:
仲有好多好多
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