About wiring up some LEDs?

Question

Forgive me, I'm new to this electronics lark :)

I want to have a string of LEDs, I'll be using the ultra-bright blue ones (about 3.5V each) with the appropriate resistor(s).

If I just connect them in series, will my power source need enough voltage for each LED? Eg if I have 8 LEDs I'd need about 28V?

If I do them in parallel I'd need loads of wires but less voltage, so I could even use batteries. Right?

Also, if I use an AC adaptor do I need to worry about the current it gives out? Voltage seems to be the important one.

I'll be experimenting but haven't bought the LEDs yet, plus I don't want to blow a load of them.

Any help/guidance appreciated!

Answer 1

The voltage doesn't matter but the current does. If you put more than the forward voltage (about 1.4 Volts)  across the LED it will light up. If you put a high voltage the LED blows. I've done that to kill time before. You need to fit a resistor to either one or all the LED's connected in parallel so that the current is limited to 20mA. You take the voltage and divide the current 0.02Amps and then you get the resistor closest to that.  

Answer 2

You need to control the current to the LEDs.  I'm not sure what you mean by "ultra-bright" but the Philips LXK2 (Blue or Royal Blue) can take up to 1.5 Amps at about 3.5 V, providing the junction temperature is kept down.  These "ultra-bright" LEDs  must be cooled properly by using a heat sink on the back  or they will quickly fail. Thermal management is the most difficult part of using these powerful LEDs.  The power supply should be a constant current type using a switched mode for efficiency.  You could build a simple linear regulator if efficiency is not important to you.  These LEDs are extremely bright when operated at their maximum ratings, bright enough to be dangerous to your eyes if you stare directly at them.  They are still very bright when operated at much lower current (even 5% of the maximum) which would eliminate many of the cooling problems.

  You really should read the data sheet on the LEDs you will be using.  That will answer most of your questions.

Answer 3

It's unclear from you statement whether or not the LEDs have built in resistors, I'll assume not.

With 8 LEDs in series you will need more than 28 volts as you will need a series resistor to limit the current.

In parallel, you will need a resistor for each one, so would need a 6 volt battery. Plan on dropping at least 2 volts for each LED.

Re AC adaptor, just get one with enough or more current than you need, and the correct voltage.

.

Answer 4

If you run them in parallel, you will can run them at 3.5 to whatever voltage your power supply outputs - three 1.5V batteries wired in series will give you 4.5V, but I don't know how long they will last. The downside is you need to use a current limiting resistor for each LED.

You can also do a mixture of series and parallel. If you have a 12 V source, then you can easily supply enough current to 3 blue LEDs (Forward voltage of 3.5). Assuming you want to supply them with 20 mA of current, then you will need one 75 Ohm (or next higher one available) resistor for all three.

R = (V_Supply - V_Forward)/I

If you want to use an AC adapter, you just have to make sure that it can supply enough current for all of the LEDs that you want to run. For example, if you're running 10 LEDs at 20 mA each, then the AC adapter needs to supply 200 mA, so just check the specifications. I bought a universal AC adapter from radioshack for $15 that has a span of voltages from 3-15V selectable by a switch and is has a maximum output something like 1 A of current.