Led circuit help? - Engineering Forums

Forums / Public / Engineering / Led circuit help?

Author	Message
Omnious Members 346 posts Location: Singapore Occupation: Steward or stewardess Age:	#177255 2008-05-20 09:19 GMT
	The battery I will be using is 8.4V and supplys 700mAh current. The leds I plan to power has a foward voltage of 3.2 and forward current of 30 mAh each. So far i know that in a parallel circuit, voltage is constant and current splits....so does this mean I have to hook up 23 leds in parallel so each led gets around 30mAh without resistors? The leds are ultra bright white leds.


TemprementalExplosion Members 369 posts Location: Andorra Occupation: Quilter Age:	#177256 2008-05-20 09:30 GMT
	The voltage is what drives the current. Hook up 3 LED's in series for 3.2x3 = 9.6V. They will be a little dim, but still fairly efficient. Put lots of these circuits in parallel across the battery for as much light as you want. You could also put 2 LED's in series in each circuit, with a dropping resistor to get exactly 3.2V on each LED. But, battery voltage varies with temperature, state of charge, and load, so an active voltage regulator would be cool to maintain exactly the right voltage. I would go for the 3 in series. If you can't live with the [lack of] brightness, you could also try putting the 2 LED's in series across the battery. They may have enough margin to be OK, but you might significantly shorten their life. Give it a try!

thruthelookingglass Members 347 posts Location: Peru Occupation: Bassoonist Age:	#177257 2008-05-20 09:43 GMT
	yes u can,, but would suggest to use resistors to reduce vtg across led to 3.2 V Frm 8.4 V

Biohazard Members 391 posts Location: French Southern Territories Occupation: Tutor Age:	#177258 2008-05-20 11:56 GMT
	When hooking up multiple LEDs, it is not a good idea to try to parallel them directly. Each one needs its own resistor. Here is why: LEDs work on current, not on voltage like a light bulb. With a light bulb, half voltage means half (or reduced) brightness. With a LED, half voltage means NO brightness. Please try to think of a LED as needing 30mA, not 3.2 Volts. Voltage drops between individual LEDs are not always the same (you cannot rely on it being exactly 3.2 Volts) - consequently, if you parallel LEDs, some will be real bright and some will be real dim. Yes, all LEDs need to be in parallel, but each has to have its own dropping resistor to get the current in each parallel leg down to 30mA. You cannot put 3 of them in series with an 8.4 Volt supply. They will not even light. You can probably get away with putting 2 LEDs in series, and using a smaller dropping resistor, but that's the best you will be able to do. Good luck! BTW, mAh is a measure of energy, not current. Current measurements are in A or mA.

CreamPetals Members 350 posts Location: Cote D'ivoire Occupation: Urban planner Age:	#177259 2008-05-20 13:53 GMT
	When using LED's, it is very easy to compare it as a normal incandescent lamp. When designing circuits using LED's, the following points should not be overlooked. 1. LED is basically a diode. The light it emits is bonus. Hence all specifications related to diodes apply to LED's as well, i.e. minimum forward bias voltage, forward voltage drop, maximum forward current (If), maximum reverse voltage (Ir) the junction can handle etc. 2. A current limiting resistor in series is always advisable as any junction breakdown or short circuit of the terminals in any of the LED's connected in series may cause the other LED's to fail due to increased forward current. The resistor helps to limit the current. Larger the current, larger the voltage drop in the resistor.