電阻一問

請問如果有粒LED最大係7.2V, 我要用一舊7.4 V LIPO去推, 咁我要加多OMH電阻呢?

150-200 Ohm

點計出黎架, 可唔可以教我呀?

V= I x R
Or ,Voltage = current x resistance

Or, R = V / I

Led typically can handle 50 mA max

So,  R = 7.4 / 0.05 = 150 ohm, approx.

LED is in fact a non-linear device. If you make reference to the LED specification, you can check the forward voltage and current value. If I remember correctly, the typical forward voltage is around 3.2V with a  forward current of 20ma. So, if the supply voltage is 7.4, the resistance R = (7.4-3.2)/20ma = ?? (no calculator on hand, do it youself then)

voltage drop across the LED should be 7.4-7.2 = 0.2V in this case......