RC Slope Soaring in Hong Kong

今天在飛鵝山送了一些舊LED及電阻給機友，回家用電流表量一下，才知電阻值太細。接到4.8V電源會太大電流。 LED之工作電流最好是40mA, 不要大過50mA，否則LED會很容易燒。 該批LED
紅色LED最好串68 ohms 電阻
藍色LED最好串43 ohms 電阻
綠色LED最好串43 ohms 電阻
下次見到我記得和我取新電阻。有興趣玩夜航的，不介意LED舊，亦可以向我取燈。

我知道電壓(V)=電流(I)x電阻(R)的公式
但我用4.8V的數去算,如何算出40mA至50mA的數
還是你加上令一些參數??????????

There is a voltage drop across LED. Different types of LED (color) has different voltage drop. Red, yellow and green are approximately 2V. Blue, pure green and white are approximately 3.6V. Note that these are just general values. Some have different voltage drop and so you should better check it with a voltmeter.

Example:

Red LED current: (4.8 - 2) / 68 = 41mA
Blue LED current: (4.8 - 3.6) / 43 = 28mA

p.s. I think 10-20mA is bright enough for most of the applications. 40mA may cause some LED to get hot

CM is correct, Current of 40mA will be very high for a LED. But the brightness at 40-50mA is very atttractive. When we make toys, we will still use current at about 40mA, but we add a controller to make the LED flash. This make the LED On & Off at high speed. This give us a very Bright LED wand it also has good life. If we connect the LED directly to Battery without PCB controller, lower currect will give a longer life to the LED. We can try 100 ohms, it should be bright enough at night.