Page 1 of 1
two led in series
Posted: Tue Jul 23, 2013 11:37 am
by cacoolee
How to join more than one led with lm 317 in current regulation mode? I join a 50 ohm resistor to lit the white led. The single led lights up . Then i connect two led in series it wont gives light. If i connect in parallel it will ad dim . Please help me.
________________________________
cute video ~
video library ~
commercial video
Posted: Wed Jul 24, 2013 3:14 am
by admin
LEDs do not behave as a resistive load. They do sink current like a resistor however, its not a linear function. For this reason, LEDs cannot be dimmed in a conventional fashion. In fact, they go out completely when the voltage across them drops below a threshold. A regular diode has a voltage drop of somewhere around 0.5v and the drop for LEDs vary. So, when you put 2 LED's in series with a current limiting resistor, you may drop the voltage below the threshold for the LEDs to light. All this means that calculating a value for a current limiting resistor for an LED requires more than Ohm's law. You need to know some values such as the current and voltage drop for the particular LED. Take a look at
https://www.sparkfun.com/tutorials/219