View Full Version : LED type vs Resistor Values
01-15-2008, 01:20 AM
Hi, i have a heap of purple 5mm LEDs which im hoping i can use for my LEDTriks Board.The values are similar to the reds as stated for the board except they are 3.2-3.4V. They are 1500mcd. Is there anything that would need to be changed as i dont really want to go and buy another 800 LEDs since i have 9000 left in the shed which i am slowly using to convert minilights with.I have red, yellow blue green and purple, all Ultra Bright 5mm...but i think the rest are over 3000mcd except for the purples.
The chips have constant current outputs. the 1k resistor on pin 23 sets this at 20mA aprox. This means that the actual led voltage figure should not matter.
It is the mcd value of the led that needs to be considered which will be influenced by the size, viewing distance or day/night use of the sign and of course cost. What ever the decision it will be a tradeoff.
01-15-2008, 03:48 PM
thanks for that, the purples will be fine for the sign as they are'nt as overbearing as the other colours
01-16-2008, 03:07 AM
yep, I locked the MBIs at 20mA with the three resisitors.
There is a graph in the data sheet for the IC and it gives you data if you want to change the I led to a different value.
01-16-2008, 04:28 AM
Thanks John, i got the boards today also so will start organising the components.All the LEDs are 20mA so it wont be a problem then. Is it possible to get a BOM for the PCB's you have as i have used the BOM in the Wiki and some of the components are unavailable or obsolete. For someone like me who isnt that electronic savvy it gets very confusing trying to find a substitute component.
01-16-2008, 02:30 PM
Can you let me know which parts were ob or unavail.
I'll update te new parts once I have the info.
01-16-2008, 05:19 PM
On the DCSSR part numbers 571-3902614, 538-398900302,660-CF14C471J,660-CF14C681J
On LEDTriks 649-65474-002,571-3902618, 571-3902614,
On the Grinch 571-3902618,
Think i have all the ones that show up as not available
01-16-2008, 08:04 PM
Having just been through the boxes of bits i have in the shed i have come across a couple of packs of 1.2k resistors.been through the datasheets for the MBI and also Allegro chips and if i were to use the 1.2k's i would drop the current to approx 15mA with the MBI and 17mA with the Allegro. Can this affect the LEDs appearance being 20mA LEDs or should i just order the 1k resistors with the rest of my bits and pieces?
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