How long... LED on time without resistors?

[Heckler]

Hey Group,

I am building an 8x8 LED matrix device. I am planning on NOT using resistors to limit current to the LED's. In one example I looked at it appears the on time is ~1.5 mS.

Therefore, I was wondering... How does one determine the amount of time that you can safely turn on the LED's without damaging them? I do not have the 8x8 LED modules yet, so I do not know whether they are common anode or cathode. I plan on using one PIC pin to control a ROW and one pin to control a Column.

I realize there are several factors at play here. Such as... supply voltage, number of LED pixels on at any given time (up to 8), etc.

In my case I plan on using 3 volt supply.
I have not determined whether to scan the display rows or columns. One would have only 1 LED lit at a time and the other would have from 1 to 8 LED's on at one time. (I would also be interested in suggestions on the correct way to scan the display?)

Thanks

[mackrackit]

At ~3 volts resistors are not required for the "normal" LED.

[Heckler]

Well that was EASY!!

What if I want to use a 5 Volt supply for testing?

I thought about just ramping up the "on" time until the brightness matches the brightness of a properly current limited LED of the same type.

[Archangel]

Well that was EASY!!

What if I want to use a 5 Volt supply for testing?

I thought about just ramping up the "on" time until the brightness matches the brightness of a properly current limited LED of the same type.

Hi Dwight,
I will give a metaphor to answer the question: how long can I hold a lit cigarette to my skin without a heat shield in between?
It is exactly the same scenario, you are abusing the components both PIC and LEDs, how long before complete burnout? I guess it depends upon the particular batch of parts. I "think" they do sell LEDs with internal resistors, or at least they did. FYI they also sell transistors with internal bias resistors too . . . call them digital transistors. I know it is NOT Uncommon for designers to overdrive components particularly when using PWM, but IMHO it is not the best practice. If this is a home project, and not life critical then who cares right? Then do it, if/when it fails you will fix it.


EDIT:
Here is a link to Mouser's search for 5 volt LEDs: http://www.mouser.com/Optoelectronic...rd=LED&FS=True

This link will allow you to select from whichever voltage and current values you like: http://www.mouser.com/Optoelectronic...rd=LED&FS=True

[rsocor01]

Hi,

No, you shouldn't use 5V in the PIC with a normal LED. You should use 3 or 2.5V for a normal LED. All the PIC digital pins have a small internal resistance that varies with the current drawn from the pin. A "Current Drawn" vs. "Internal Resistance" graph is ussually provided in the datasheet in the Electrical Characteristics section. That small internal resistance is what helps that the LED doesn't get fried.

Robert

[mackrackit]

Read this, it gets interesting...

http://www.picbasic.co.uk/forum/show...t=11575&page=1