# Can I determine resistance and watt of small leds.

Discussion in 'General Electronics Chat' started by Yaşar Arabacı, Nov 12, 2014.

1. ### Yaşar Arabacı Thread Starter Member

Nov 11, 2014
32
0
Hi,

I am learning electronics as a hobby. Today I visited an electronics shop to buy myself some supplies. I now own a breadboard, multimeter, some resistors, leds, switches, cables and a 9V battery.

The problem I am facing is that I don't know how many watts can LED lights handle. Is there a standard for small leds that one can insert into breadboards?

The person who sold me supplies mentioned some numbers but I forgot them as soon as I left the store :/

2. ### MrChips Moderator

Oct 2, 2009
12,228
3,281
You can measure this yourself using the LED, a 9V battery, some resistors and a voltmeter.

Connect the LED in series with current limiting resistors and 9V battery.
Change the resistor value in the range 220Ω - 2200Ω and adjust for desired brightness.
Measure the voltage across the LED.
Measure the voltage across the resistor and calculate the current through the resistor.
Calculate the wattage.

Do not bother to calculate the resistance of the LED since that is not a constant.

3. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,936
1,088
In terms of a small LDE we don't care about the power. Simply always use a series resistor with the LED. And make sure that your LED current does not exceed 30mA and typical LED working current is around 10mA.

4. ### Papabravo Expert

Feb 24, 2006
10,021
1,757
The reason for the series resistor is to limit the current to a value that will prevent the LED from releasing it's magic smoke. Everybody knows that it is the conversion of magic smoke into light that makes an LED do the right thing. Let the magic smoke out and it becomes a DED. That is a Dark Emitting Diode.

bance and cmartinez like this.
5. ### Yaşar Arabacı Thread Starter Member

Nov 11, 2014
32
0
Ok, since I am using a 9V battery (multimeter reads it as 10V which is weird?) a 500 ohm (1/4 Watt) resistor would be enough to keep LED healthy as current would be around 18mA?

Edit: By the way, my LED's does only work with one kind of polarity and not the other way around, are they supposed to do that?

Last edited: Nov 12, 2014
6. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,936
1,088
Yes, everything will be alright don't worry. And yes, fresh battery will always have slightly higher voltage.
But when battery is empty the voltage drop to around 7V
Everything is fine. The LED is a Diode and diode allows electricity to flow in one direction only and blocks the flow in the opposite direction.

7. ### umphrey Member

Dec 1, 2012
39
1
I'd recommend looking at the current - voltage graph for diodes, which is what an LED is. The resistance is needed because P = I *V so even at low voltage the current and power will be very high. A series resistor limits the power and most of the power will be dissipated through the resistor. The forward voltage will be different with different colors.

http://www.oksolar.com/led/led_color_chart.htm

8. ### Erty New Member

Aug 9, 2010
3
0
A quick way of finding the series resistance you need for a LED in a circuit with nothing else in it is to assume it will be drawing exactly 20mA when the voltage across it equals its forward voltage (about 2V for red, 2.5V for green, 3V for blue or white), unless otherwise specified, and then run these numbers on a calculator:
R = (V-Vf) / I,
where R is the least resistance of the resistor you need (it's okay if it's a little bit higher since the decrease in brightness is unnoticeable but too little resistance can be bad), V is the voltage of the power supply or battery, Vf is the forward voltage of the LED, and I is the current which usually is about 20mA