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But assuming you have all your LEDs in parallel, each with a series current limiting resistor, and assuming each is pulling only 40 mA...question, is this a measured or calculated current draw? Because if your circuit is only a 320 mA load, the 7805 should be more than adequate.
Are there other circuits also involved that you didn't mention?
If not, then this is a troubleshooting issue, not a design issue. You need to verify the polarity of your LEDs. Also, what is the input voltage source? Batteries? Or a rectifier and transformer? Is the rectifier properly rated?
If this is a battery voltage source, is it new? Have you tried replacing it? I'm thinking likely your battery is going dead.
If none of this is the case, then verify the bias of all your diodes and make sure none of them are faulty. You might remove each diode one by one to see if one is shorted. Even a shorted resistor isn't an unreasonable failure. Do a physical check, touch each resistor and diode to see if one is getting too hot.
Use a multimeter. Take a measurement of all your voltage drops and find one that is behaving differently.
But assuming you have all your LEDs in parallel, each with a series current limiting resistor, and assuming each is pulling only 40 mA...question, is this a measured or calculated current draw? Because if your circuit is only a 320 mA load, the 7805 should be more than adequate.
Are there other circuits also involved that you didn't mention?
If not, then this is a troubleshooting issue, not a design issue. You need to verify the polarity of your LEDs. Also, what is the input voltage source? Batteries? Or a rectifier and transformer? Is the rectifier properly rated?
If this is a battery voltage source, is it new? Have you tried replacing it? I'm thinking likely your battery is going dead.
If none of this is the case, then verify the bias of all your diodes and make sure none of them are faulty. You might remove each diode one by one to see if one is shorted. Even a shorted resistor isn't an unreasonable failure. Do a physical check, touch each resistor and diode to see if one is getting too hot.
Use a multimeter. Take a measurement of all your voltage drops and find one that is behaving differently.
measured current is closer to 37 mA, no batteries, the whole circuit is as i atached in the pic above. and as i mentioned in my first post, they turn on for 10 secs then the voltage measured across the leds drops to 1.8V. the regulator goes to something like 2.3V (i dont remeber and im not in the mood to check again cause i might just smash it)
i have a 7805 feeding current to 8 blue leds each with 47ohm resistors. when i turn it on, my voltage drops to 1.8 after 10 seconds? any ideas????
oh and each led only is drawing 40mA
First, I know it won't effect the normal operation of the circuit, but I think you should consider placing the resistors on the anode side of the LEDs. If you have a short circuit in your current configuration between and LED and resistor you have nothing to protect the diode from overcurrent. The resistors might be able to handle a short where the diodes won't.
Have you checked your filter capacitor? Take it out of the circuit and see if the problem goes away.
First, I know it won't effect the normal operation of the circuit, but I think you should consider placing the resistors on the anode side of the LEDs. If you have a short circuit in your current configuration between and LED and resistor you have nothing to protect the diode from overcurrent. The resistors might be able to handle a short where the diodes won't.
Have you checked your filter capacitor? Take it out of the circuit and see if the problem goes away.
That doesn't seem right. (28V - 5V) * 0.32 = 7.36 Watts. It should burn your fingers without a heat sink! What is the input voltage to the 7805 when it's output voltage has dropped to 2.3V?
Assuming 2.0 volt drop across the LED, you must current limit for 3 volts. Normal LED current limiting is around 50 ohms / volt. Minimum resistance would be 150 ohms with 5 volts supply. That would be a little over 20mA. Most LEDs are rated 30 mA max. Computer boards usually operate LEDs with 330 ohm resistors on the 5 volt line.
Now, with 47 ohms and 40mA (m/l) it looks like right at 2.0 volts drop across the diode, so that assumption is valid. 40mA is pretty hot for LED current. Look for very short lifespan there.
The regulator appears to be going into "fold back", reduced voltage for current limiting in a failure mode. They will do the same thing if they get hot. There are 8 LEDs loading 320 mA. Appx 1/3 load. If your supply to the regulator is very high, it must drop more voltage which will cause some heating. The usual rule of thumb is to use 8 volts. Using 12 or more volts to supply a 7805 will generate considerable waste heat.
The TO-220 package is nominally rated at 1.0 amp / 1000mA. That's with a heat sink. Figure about half, 500mA without a sink. Now you're approaching the upper limits of capacity. Add a hot supply and it comes down further.
I would speculate you're getting thermal fold back. Try a heat sink. Something as simple as four square inches of aluminum roof flashing can make a difference.
alright, thanks guys, i found the problem. i didnt think the leds would cause it to heat up so much. i put a heat sink (and with my fan connected to the 110 side of my transformer) it works great. just need to look so a small but beter heat sink. thanks again
It would be more efficient to put the LEDs into groups of series connected LEDs and run them from a higher voltage regulator like a 7815, 7824 or LM317 so that you are wasting less power in the regulator.
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