# wm1995

Member Since: January 10, 2011

Country: United States

• So far your tutorials have been brilliant, and I’m sure they’ll continue to be very helpful. Could you cover inductors, transistors and possibly the 555 timer at some point?

• … Sorry, it cut me off.
For example, if the forward voltage drop of the LED is 2V, the power supply voltage is 5V, and the maximum current through the LED is 0.02A, the resistor is calculated as follows:
R = V / I
I = 0.02A (The resistor is in series with the LED and so the current through the LED will be equal to the current through the resistor)
V = 5V - 2V = 3V (The voltage across the resistor will be equal to the total voltage across the LED and the resistor less the forward voltage drop of the LED)
Therefore: R = 3V / 0.02A = 150 ohms
This means that a 150 ohm resistor is the minimum you ought to use in series with the LED. The life span of the LED may be extended if a larger value resistor is used. However, a larger value allows less current to flow, meaning that the LED is going to be dimmer.
To address your final concern about adding more LEDs: to put more LEDs in parallel with the current LEDs, put a resistor in series with each new LED.
Hope that helped.

• I apologise in advance if the following explanation is a bit simple, but I don’t know what you do know and what you don’t.
When using LEDs, you place a resistor in series with the LED to limit the current that can flow through the LED. This is because the LED has quite a low resistance and a maximum current. You can run LEDs without resistors, but they can get dangerously hot because you are allowing more current than the LED is designed to handle to flow. This is because the LED is dissipating more power (P = I x V, I is large therefore P will be large). If the voltage is high as well, then the power dissipation will be even larger, which could cause the LED to fail.
To calculate your own resistor for your LED, you need to know a few things first: the voltage that you are using to power the LED; the forward voltage drop of the LED (which can be found in the datasheet) and the maximum current through the LED (also found in the datasheet). You also need to know the formula R = V/I.

• We use V for voltage and I for current in the UK (or at least as far as A level (exams at age 18) we do).

No public wish lists :(

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