Hello to all the experts here. I have a simple electrical circuit question. Qing Da Jia Duo Duo Pang Chu. Thank you.
Kelvin is given a 5W LED. He is told that 9V battery will blow the LED, but unless there is a resistor connected in series with the LED. Calculate the possible range of the resistor.
If a 3V battery is used instead of the 9V battery, no resistor is required. Explain how this is possible.
Note: I could not recall is the LED value is 3W or 5W. All other values are true.
hi,
I'm not sure if this is correct... I shall give some hints first
1) http://en.wikipedia.org/wiki/LED_circuit
2) Not sure if you would have to consider internal resistance of the battery.
Originally posted by OP90:Hello to all the experts here. I have a simple electrical circuit question. Qing Da Jia Duo Duo Pang Chu. Thank you.
Kelvin is given a 5W LED. He is told that 9V battery will blow the LED, but unless there is a resistor connected in series with the LED. Calculate the possible range of the resistor.
If a 3V battery is used instead of the 9V battery, no resistor is required. Explain how this is possible.
Note: I could not recall is the LED value is 3W or 5W. All other values are true.
first you need to understand 2 formuli
P=V*I
V= I* R
Thus I = V/R
What causes the LED to blow is the high current due to high voltage low resistor.
If we assume 3V is the maximum voltage that can be applied without resistors, then the maximum current the circuit can take is 5W/3V = 1.67A
Subsequently the resistance of the circuit is 3/1.67 = about 2ohms
With a 9V batt and a resistance of 2 ohms, the current is about 4.5A which is over the limit of the system.
to reduce the current to 1.6 and below, you can use 9V/R = 1.6 and solve for R.
Lol hope i am correct....learnt this stuff like almost 7 years ago.
Originally posted by eagle:hi,
I'm not sure if this is correct... I shall give some hints first
1) http://en.wikipedia.org/wiki/LED_circuit
2) Not sure if you would have to consider internal resistance of the battery.
lol not sleeping yet?
busy mah