LED array

[Andrew Davie]

OK, so here's some thoughts on the LED array. I've used an online wizard, and scouted some suitable (?) LEDs on eBay, and here's what I've been presented with.



The above was generated by the LED array wizard at http://led.linear1.org/led.wiz. I retrieved the specifications of the LEDs from the eBay advertisment at http://cgi.ebay.com.au/50-PCs-UltraBright-White-LED-5mm-55-000mcd-Free-R-NEW_W0QQitemZ320075590744QQihZ011QQcategoryZ105796QQrdZ1QQcmdZViewItem -- these are advertised as "50 PCS UltraBright White LED 5mm 55,000 mcd", and they come in a 50-lot for AUS$24 (about US$16) delivered which seems like a good deal to me.

THe specs in the ad seem to be inconsistent (DC forward current) but I used the higher value in the wizard parameters.

Would anyone care to comment either on the suitability of these LEDs, the circuit design from the wizard, the suitability of the 6x3 array and the brightness I'm going to get with this. Or anything else

If I don't get any comments in a day or two, I'll go ahead and buy one of these LED bulk lots.

Cheers
A

[Klaas Robers]

Andrew,

This is rather cheap. Be aware that white LEDs need about 4 volts each. You may run them at 20mA for white. The club-PCB LED driver is designed for 40 mA peak white, so you should make no more than two series of LEDs in parallel. Put one equalisation resistor of 220 ohm in each chain. This will add another 4 volts for the chain at a current of 20 mA, but it will assure that both chains burn at the same current.

Then calculate the voltage that you need: (N x 4) + 8 extra for the resistor and the driver transistor. Example: 2 chains of 10 LEDs each: you need a voltage of 48 volt. This may be (somewhat) higher, but not lower.

There is quite a big difference in the voltage per LED. White LEDs need about 4 volts, orange LEDs need just 2 volts.

[Andrew Davie]

I did end up making a 6 x 4 LED array, using two groups of LEDs arrays of 3 x 4, each consisting of 4 strings of 3 LEDs, with a 75 Ohm resistor in series with each string. The whole is documented in http://www.taswegian.com/NBTV/forum/viewtopic.php?t=11