Hi All,
My project requires an array of 12 LEDs in parallel in one branch.

Spacing is very tight (picture a light wand but with a very narrow diameter) so I really don't have the space for a current limiting resistor next to the LEDs. Series is not really an option given that I would then have to supply 3.5Vx12 (or maybe that's the best thing i can do?)
I could place a constant current driver/circuit but:
a) I am not sure it would work given that the leds are in parallel and I can fit only 2 wires to supply the LEDs (i.e. can run a wire to the pins)
b) did not find anything suitable for a 12 LED array.
c) some of the ICs I looked at still had a ballast resistor.
My LEDs are white, 3.5V, 20mA, 100mW so nothing too powerful
What are my options? This will be connected to a power supply so I dont have limitations in that sense.
Not worry about the individual characteristics of the LEDs (they are all the same aside from probable manifacture discrepancies) and just run the array with a single resistor (bad) or some limiting circuit?
Use a driver (suggestions pls) and try to fit a resistor for each LED?
Other Ideas?
All of the above are my assumptions so if anything is wrong please feel free to point it out
Thanks a lot.
Antonio

 

Each LED will require a current limiting resistor (not sure where the term ballast got started). They do not have to be next to the LED, wire length is not an issue, but it is required.

With series arrangement you can reduce the number of resistors, and you still have 2 wires per LED.

There is no good design where the LEDs are parallel to each other without the resistor.

LEDs, 555s, Flashers, and Light Chasers

 

How about thin enamelled copper wire to the LEDs and all the resistors somewhere else.

 

Well now, if the LEDs are in series, the wires just have to run from one unit to the next, not all the way down the wand. Only one wire has to come out from the top and one from the bottom. I'd say that the series arrangement is way better than the parallel one, and it also reduces the number of components, with only one resistor for the entire setup, instead of a resistor per LED.

 

Thanks to all for the replies. A lot of my confusion came from the fact that I saw this arrangement (parallel with no limiting res) in a very expensive commercial product and I thought I was missing something. I'll go for an arrangement in series and use a current regulator (any suggestions on which one?), I'll figure something out for the power supply.

BTW I see the term "Ballast" in datasheets at times, must have been someone in marketing that came up with the term.

Thanks,
Antonio

 

http://en.wikipedia.org/wiki/Electrical_ballast

 

While the term ballast is OK, it is pretty misused with LEDs. Current limiting resistors are what they were called 30 years ago, and still applies today. For one thing, it is what they do, the description is the thing. In a fluorescent tube a ballast is a large coil used to limit AC current, a pretty big difference. I use Wikipedia all the time, but I do not always agree with them. Part of the problem I have with using the term ballast with LEDs is they have to be calculated for every circuit, there is no one size resistor fits all needs (quite the opposite).

You want to use LEDs in series and parallel, as my article explains in great detail. Using them in series will use less current from the battery. Neither of these requirements should be onerous for space requirements, and the resistors can be right next to batteries.

 

I recall a ballast resistor being used with tube type devices to compensate for the excess supply voltage applied to series filaments. It developed a voltage drop at a given current so that the filament total and ballast voltage equaled the supply voltage.