LED Array Power Source Problem

[davisp2012]

Greetings,
I am rather clueless about this stuff so I'll leave it to you guys to come up with any bright ideas for this. I'm building an array of LED's for my high school that flash to music. I'm using a TIP31C transistor to get this effect. My problem is they're wanting a 1 foot by 1 foot array. I figure i can put about 50 LEDs in it, but I'm going to need to use AC current to get the necessary power. The LEDs i have are rated at a 3.3 - 3.8v forward voltage; they're just cheap green 5mm LEDs. So I know I'll have to convert the AC to DC and use a voltage regulator so I don't burn out my LEDs, but other than that, I'm clueless. What can I do to make this work efficiently and what can I do to prevent it from being destroyed by the power? Also can the transistor handle that kind of current?
Thanks

 

[Sciurus]

Hi, davisp2012. I'm no expert, but I may be able to help a little. Your transistor should have a datasheet (you can probably just google "TIP31C datasheet" to find one), as should the LEDs. You can use the sheets to find out if the current draw from 50 LEDs exceeds the maximum collector current for your transistor.

 

[pantaz]

You should be able to use an inexpensive 12VDC, 500mA wall transformer to run 50 LEDs.
Use http://led.linear1.org/led.wiz to play with series/parallel layouts.

 

[vk6kro]

You could use 49 LEDs in a square of 7 by 7.

It may make sense to drive them in 7 parallel strings of 7 LEDs in series.

Assuming 3.5 volts each for the LEDs that would be 24.5 volts total, plus you need a series resistor for each string.
This should drop about 25% of the total voltage. That is 0.333 times 24.5 volts or about 8 volts.

So, that is a total voltage of 32.5 volts.

Assume 20 mA for each LED and the resistor dropping 8 volts,
then R = 8 / 0.02 or 400 ohms.
So, you could use 390 ohm resistors and you would need 7 of them.
Total current would be 140 mA.

Getting 32.5 volts can be done in a few different ways.

You could bridge rectify a 23 volt AC output from a transformer. A large capacitor across the output would build the output to the peak voltage of 32.5 volts. A bit of ripple doesn't matter much with LEDs.

You could use a voltage doubler from 12 V AC. This requires large capacitors (4700 uF) and diodes that can handle large current peaks (up to 10 amps) but it would probably be a good way to go if you only had a 12 V AC power source.

Any power transistor should be able to switch 140 mA.

You could also switch the display in concentric squares coming from the center outwards or flash the outer and 3rd square from the outside alternately with the 2nd square from the outside and the middle LED.
These would look impresive, but would require a different powering system for the LEDs.