Led Help

[fred666777]

im kind of new to electronics. i have been able to guess my way around things like this in the past but i dont want to waste money and time with this project.
from what i understand when wiring led lights in parallel you can only have a certain number in each ...row... sorry for incorrect terms... so say i had 10 lights 4v leds running off of 9vdc i can only have 2 in each row?


9vdc
|----O-----O-----|re
|----O-----O-----|si
|----O-----O-----|st
|----O-----O-----|or
|----O-----O-----|s

is this right?

so i have 50-100 lights to wire and most likely a 9VDC 100ma power supply maybe a 24VDC 416ma power supply so on the 24v can i have about 6 in each row or am i completely wrong?
how do i find what resistors i need?(how to do math if possible(not calculator))


---Light being used
Blue, 3900mcd, T1-3/4 (5mm) LED. Max forward current 20mA
continious, 5VDC Max reverse Voltage. 4VDC Forward
voltage drop.

 

[audioguru]

LEDs have a range of forward voltage. Your blue ones might have a spec'd range of forward voltage from 3.2V to 4.6V. So if you calculate a current-limiting resistor for two 4.0V LEDs then they will be very dim if their forward voltages are actually 4.6V and will burn out if their forward voltages are actually 3.6V. You need more voltage across the current-limiting resistor for the difference to be less. Use a 12V supply for two LEDs in series and in series with a current-limiting resistor.

I have never seen a 4.0V blue LED. Mine are 3.2V to 3.6V.

 

[fred666777]

ment for that to be a link
Led

 

[audioguru]

Those cheap blue LEDs do not have a detailed datasheet.
You do not know what is their minimum and maximum voltage.
You do not know their viewing angle which might be very narrow.

 

[electronspeed]

After each element there will be some voltage drop.So after looking the data sheet select your i/p voltage.Next for better glow you need to take care of current.If after going for branches current drop in individual branches is high then go for a buffer at the start itself.