# Simple Resistors Question

I have no experience in electrical, and as such I have no idea about what resistors to use for these LED’s; here’s what I know:

Forward Voltage (V) : 2.0~2.4
Forward Current (mA):20
Static Sense:Yes
Luminous Intensity: 15000mcd

But I don’t know what that means…

So here’s my question: How can i figure out what to use for this, as in, i don’t just want an answer of XXXX ohm resistors (i’d rather not pester this board for basic stuff like again… and i guess you wouldn’t want me too either) I’m planning on making a matrix of 5x20 LEDs, which will use LED drivers as well. If there are any guides out there that you know of for this stuff to help me learn, or if you have any insight about this, i’d appreciate it greatly.

Well determining the proper current limiting resistor value for a LED is pretty straight forward. You take the source voltage you are going to use to power the LED, 5vdc, 12vdc, whatever as long as it’s more then the forward voltage drop of the LED. Next subtract the LED’s forward voltage drop from the source voltage, so say 5vdc - 2vdc = 3vdc. Now use ohms law to figure out the resistance value that will allow 20ma to flow at 3vdc. So 3 volts divided by .020 amps equals 150 ohms.

Make sense?

Lefty

thanks a bunch for your help! another simple question: would the voltage be constant (5vdc) through an LED diver for each output pin from the driver?

would the voltage be constant (5vdc) through an LED diver for each output pin from the driver?

I would need more information about what you are calling a driver. There are constant current drivers and constant voltage drivers and sometimes open collector output drivers that depend on external components to set voltage and current values.

So what device(s) are we talking about? An Arduino output pin would be an example of a constant output voltage driver, but it’s limited to a maximum of around 40ma or current.

Lefty

I’d be using the driver as given in this example: http://www.arduino.cc/en/Tutorial/LEDDriver

sorry if i was unclear; i mean, i put the resistors in before the LED and after coming from the driver (which is hooked up to a pin on the arduino) again, i know it’s basic, but i’m tryin’ to learn =P

The 4794 driver uses open drain transistors to drive the LEDs, Fig 6 of the data sheet shows how to wire the LED’s. So the calculations I gave apply and you only need to determine what source voltage value you are going to use to drive the LEDs with. Using 5vdc from the Arduino board should work fine.

Lefty

right-o, that just about ties up all the loose ends, thank you for your help! can’t wait to get started now