I was wondering about making a white LED torch. I found some 5mm 3200mcd white LEDs at Rapid Electronics for under £1 each. Here are some more specs: VF: 4v VR: 5v Rated Current: 20mA View Angle: 20o I was going to use 7 leds in a cluster (6 leds all around a central led) I can use the led calculator to work out that I need a 100ohm resistor with a supply voltage of 6v (4 battries) but do I need to use 7 resitors or can I use one big one? Also what would be the power consumption of all 7 leds? Would it drain batteries quicker than a fillament bulb when considering brightness? Would it be too dim in the first place?
You could use one resistor, but it'd need to be really powerful, probably a 1 or 2 watter. Using one resistor per LED means you can get away with using 1/4w resistors. Also, you screwed up the calculations on the resistor value. (6v - 4v) / .14A = 10.7 ohms*. Also, 3 AAs = 4.5v, no need for a fourth battery. By getting rid of the extra battery, the resistance you need goes down to a mere 3.57 ohms*, the nearest standard resistor value being 3.9 ohms. At that resistance, you can use a 1/4w resistor** and still have plenty of headroom current wise. *(Vin - Vf) / I = R **(P/R)^.5 = I (Maximum current that can be handled by a resistor)
The rated current of the LEDs is 20mA not 140mA! If I used only a 3.9ohm resistor I wouldn't have any LEDs left! I want to use 4 batteries though because it's easier to find a battery pack for them and they'll last longer than three. Anyway what I really want to find out is just how long the leds will keep burning. I don't know how much power will be used with all those leds and resistors.
He's talking about wiring them all through one resistor, i.e. resistor then the 7 LED's in parallel. That way the current drawn through the resistor would be 7 times that through each LED... Rob.
But whypick1 said I'd need a really powerful resistor. According to the calculation though the resistor needed for 7 leds in parralel has a much lower rating than using separate resistors. I'm confused .
Power and resistance rating are very different. If you use a 4.5V battery to power a 4V LED witha resistor the resistor will disipate 1/8th of the power of the LED, 20mA x 0.5V = 10mW Using 7 LED in parallel the resistance of the resistor needs goes down (because the voltage drop over it has stayed the same and the current running through it has increased 7 fold), but it has to dissipate 7 times as much heat, 20mA x 7 x 0.5V = 70mW. With a 4.5V source this isn't an issue, but if you'd used a higher voltage source you may have need to buy a beefie resistor. Rob.
