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 Chief Delphi (Off Season) Blinking LED Problems
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#1
06-14-2006, 06:46 PM
 Alex698 Sparky FRC #2449 (Out of Orbit Robotics) Team Role: Mentor Join Date: Jan 2005 Rookie Year: 2005 Location: Arizona Posts: 114

I am having a problem with some LED's that I am using for a backlight (they draw less power for my project). The LED's are powered by a 5V regulator then into a 10 ohm resistor then paralleled together with a common ground. The LED's work fine for about seven hours then they begin to blink (fail). Any ideas or solutions?
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#2
06-14-2006, 07:10 PM
 DonRotolo Will machine for food. FRC #1676 (Pascack PI Oneers) Team Role: Mentor Join Date: Jan 2005 Rookie Year: 2005 Location: Montvale NJ Posts: 5,793
Re: (Off Season) Blinking LED Problems

Quote:
 Originally Posted by Alex698 I am having a problem with some LED's that I am using for a backlight (they draw less power for my project). The LED's are powered by a 5V regulator then into a 10 ohm resistor then paralleled together with a common ground. The LED's work fine for about seven hours then they begin to blink (fail). Any ideas or solutions?
What color light are the LEDs? That's important.

As a first guess, even if they are white or blue, your 10 Ohm resistor is not enough. You are overpowering the LEDs, causing them to fail.

Asuming 3.5 volt operating voltage at 20 mA, you need to drop (5.0 - 3.5 = 1.5) volts. To do that, R = V/I = 1.5 / 0.020 = 75 Ohms. I'd use the next size larger, for just under 20 mA. You can re-do the calculations with the manufacturer's specs (voltage, current) for the LEDs you're using. (Or just go to the Panasonic site and find something equivalent).

Don
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#3
06-15-2006, 01:08 AM
 Mike AA Programmer and Mentor AKA: Mike Aalderink FRC #3458 (Code Blue) Team Role: Programmer Join Date: Jan 2003 Rookie Year: 1999 Location: Holland, Mi Posts: 686
Re: (Off Season) Blinking LED Problems

Quote:
 Originally Posted by Alex698 I am having a problem with some LED's that I am using for a backlight (they draw less power for my project). The LED's are powered by a 5V regulator then into a 10 ohm resistor then paralleled together with a common ground. The LED's work fine for about seven hours then they begin to blink (fail). Any ideas or solutions?
What is the regulator powered by? you may wish to hook a multimeter to the LED leads and see if it is the LEDs which are doing this or if it is your power supply ( the regulator needs a heat sink?).

-Mike
#4
06-15-2006, 07:35 AM
 Al Skierkiewicz Broadcast Eng/Chief Robot Inspector AKA: Big Al FRC #0111 (WildStang) Team Role: Engineer Join Date: Jun 2001 Rookie Year: 1996 Location: Wheeling, IL Posts: 9,334
Re: (Off Season) Blinking LED Problems

Quote:
 Originally Posted by Alex698 I am having a problem with some LED's that I am using for a backlight (they draw less power for my project). The LED's are powered by a 5V regulator then into a 10 ohm resistor then paralleled together with a common ground. The LED's work fine for about seven hours then they begin to blink (fail). Any ideas or solutions?
Alex,
A little more info please...Is the 5 volt regulator getting hot to the touch? Can it survive the 5 second rule? (can you hold your finger on it for five seconds?) Typical 7805 type regulators have thermal and over current protection which cause them to shutdown or scale back on current under certain conditions. How many LEDs are you trying to power? Also, as Don has pointed out above, the resistor may be sized wrong. I like to use the rule of thumb of 2 volts drop across a standard LED and a 20 ma forward current for calculating the series resistor for one LED. Under these conditions, a 150 ohm resistor is the calculation. Standard values of 120 or 180 should work fine. If you are paralleling all the LEDs than you need to modify the resistor for the parallel current. If you are using a super brite white or blue LED these may have up to 4 volts forward drop and the calculation needs to be modified for those particular devices. Know the beast you are dealing with.
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#5
06-15-2006, 08:25 AM
 Alan Anderson Software Architect FRC #0045 (TechnoKats) Team Role: Mentor Join Date: Feb 2004 Rookie Year: 2004 Location: Kokomo, Indiana Posts: 7,163
Re: (Off Season) Blinking LED Problems

Putting LEDs in parallel is rarely a good idea. Any slight discrepancy in voltage drop among them will cause the current to go primarily through the one with the lowest voltage, and a shared current limiting resistor will not have the desired effect.

How many LEDs are you using, and what voltages are available to you? Put as many in series as you can, and use multiple sets of series-connected LEDs each with its own resistor.
#6
06-15-2006, 12:53 PM
 Alex698 Sparky FRC #2449 (Out of Orbit Robotics) Team Role: Mentor Join Date: Jan 2005 Rookie Year: 2005 Location: Arizona Posts: 114
Re: (Off Season) Blinking LED Problems

Thanks to everyone for all the help, I will try the series circuit and the higher ohm resistor (in different trials). Thanks!
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