# What do I need to know to design an LED lamp

So I had an idea for an LED lamp. I've got some old flashlights that have older model CREE LEDs.

My idea is that I'd put the LEDs in parallel, alternating polarity so that for each phase of AC, half of the LEDs will be conducting.

The array of LEDs would be in series with a resistor and a fuse to drop voltage from 110 VRMS from the outlet to 3VRMS over the LEDs and prevent the LEDs from breaking if something catastrophic happens (and to compensate for my own failings.)

The LEDs have a 5V reverse voltage and a 3.3V forward voltage at 3A at 80 deg C, so I figure if I limit to 3 VRMS I should't hit the reverse voltage and make badness happen.

With this in mind ... I am only passingly familiar with anything more advanced than kirchov's and ohm's laws. I can't apply ohm's law to the diodes, so what do I need to know to figure out the resistor value for n parallel diode pairs (Or maybe n sets of 5 in series and put those in parallel)? or is it just a bad idea?

• If you are only passingly familiar, throw a transformer in the mix. Transformerless designs are inherently dangerous. Oct 18, 2013 at 19:16
• I assume you mean to use a transformer to step the voltage down to the 3.3VRMS, but I'd still need a resistor to limit current wouldn't I? Why is it dangerous? Oct 18, 2013 at 19:18
• Not necessarily step the voltage down to 3.3V, yes you'd still need a resistor. But you want the transformer for galvanic separation from the mains. Mains power can potentially kill people. An added safety is that you have to deal with a lower voltage, which is safer too. You want a voltage below 40V, that is commonly regarded as a safe voltage. Oct 18, 2013 at 19:22
• Your LED's are really rated for 3A? Oct 18, 2013 at 19:25
• @jippie: The XM-L's, yes. Oct 18, 2013 at 19:28