Trouble shooting LED moonlight parallel wiring

[jiffy]

Ok, so I decided to make an LED moonlight. I got all my parts and started my crash course in wiring and soldering. I decided to wire it in parallel so that it would hold a steady voltage across the circuit. I also used resistors on each of the LEDs on the positive lead.

It all went pretty good until I noticed that my 3rd (which is the last) led in the circuit was dimmer than the rest. Also, even though you shouldnt do this, If I touched the positive and negative leads on the last LED, both the last one and the middle one would turn off.

So, does anyone have an idea:
1) as to why the last LED is dimmer than the rest

2) and why if I touch the leads the last on the 3rd LED, the 3rd and 2nd turn off

Thanks!

 

[Lateral Line]

Unless you pay for specially matched LED's, production variation will invariably produce variable results, it is the principle reason why people do not wire them in parallel, (the second being you need three resistors instead of one). I posted the serial resistor calculation in this thread. You'll get a better result doing it that way.


Simplistically, the current is finding the path through the dimmest LED more difficult, so it is going through the other 2 in preference.

The "last" LED is not a significant factor, as far as the circit is concerned, the three LEDS are all the same.

Why they should fail in the manner you describe suggests a dry joint somewhere.

 

[jiffy]

Thanks for the tip on the dry joint. I took apart some of my connections and resoldered them and it it lighitng up properly now.

The reason I didnt want to use series wiring is because voltage drops with each LED. I am running these off of a 4.5 volt ac-dc adpapter and wiring in series woudlnt allow me to run 4 LEDs off of this.

Parallel wiring keeps the same voltage across the entire circuit, and even though you need more resistors, those are alot cheaper than getting a new power source.

 

[Lateral Line]

What you could try to do is replace each resistor with a trimmer pot., then connect them up one at a time to the PSU. Using an ammeter, set the pot high, then trim it down until you get the optimum current through the LED, once you have done them all, re-assemble it and see if the brightness across the group is the same.