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| 5V/12V LED's - Any need for a resistor? |
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| Ginsu Victim:
From the bottom of the Ultralux page: No need to worry about resistors etc, the LED lamps are designed to be directly connected to the power source. |
| RandyT:
--- Quote from: Samstag on July 22, 2008, 02:20:05 pm ---You are mistaken. Increasing the supply voltage is what increases the current. A resistor will also draw more current as you increase the voltage and will burn out if you exceed it's ability to dissipate the extra heat generated. An LED just happens to be very poor at dissipating heat beyond it's design limits, but it still obeys Ohm's Law. --- End quote --- This is why I gave the conditional "in general". I didn't want folks to misunderstand and think that an LED will only pull as much current as it needs regardless of higher voltages that may be present (I.e. out of spec power supplies, which we have seen a number of in our travels.) A power supply that is off by even .25v (5% at 5v) can shorten the life of an a LED significantly, which is why many will put a small resistor in anyway. If you are going to skip the resistor completely, make sure that your power supply is healthy. RandyT |
| MonMotha:
--- Quote from: Samstag on July 22, 2008, 02:20:05 pm ---An LED just happens to be very poor at dissipating heat beyond it's design limits, but it still obeys Ohm's Law. --- End quote --- Absolutely, positively, NOT true. There is a (positive) relationship between applied voltage and forward current, yes, however it is not linear like Ohm's law would imply. LEDs are not resistors; they are (surprise) diodes. Ohm's law applies to resistors. A standard light bulb is approximately a resistor. If you want to model and LED, you need a model for a diode. A suitable model is the Shockley diode equation. Long story short, the relationship is exponential. That is, current increases exponentially with voltage. This is why LEDs are relatively sensitive to minor fluctuations or differences in voltage and are always driven in some current limited (either constant current, with a resistor, or from an inherently limited supply such as a battery chain) fashion. The purpose of the resistor when lighting an LED like is commonly done is to attempt to swamp the LED's exponential I/V relationship with the linear nature of the resistor. That is, one wants to allow to resistor's behavior to "make up" for minor imperfections in power supply. Most LEDs also have inconvenient "typical" forward characteristics in terms of voltage (1.8-2.5V is common for non-superbright types), so the resistor also lets you drive it from a more "normal" rail like 3.3V or 5V. The idea is that one treats the diode as a constant voltage device (above a certain point, they are very close) and then sets the current using the resistor. A constant current power supply (an item commonly found on EE lab benches) can also be used, but a resistor and constant voltage supply is a lot simpler and easier to come by. Note RandyT's comment about even relatively small differences in power supply specifications substantially affecting lifetime: that's the exponential behavior of the LED at work. In this case, your LED is actually an incandescent replacement module and therefore has everything it needs built in. Just apply 5V at the shown polarity. Your 12V lamp is an incandescent light bulb. Apply 12V AC or DC as you please. I really feel like I've said this sort of things several times over the past week. Perhaps a sticky FAQ on LEDs is in order? |
| SavannahLion:
Early on (ie yonkers ago, before the Internet) I had a hard time understanding how and why LED's behave the way they do and why you had to include a resistor in the mix. I kept blowing, burning and melting LED's in my early experiments. MonMotha's relationship to diodes is absolutely correct but doesn't translate well to someone who is just starting to understand LED's (and by extension diodes). If you understood MonMotha's explanation, great, stop reading my crap and move on to the next post. If you didn't, here's what I was once told (it's kind of corny really). An LED is like a goldfish. If you keep feeding it food (electricity) it will keep eating and eating until it pops. Resist the temptation to feed them too much. Now I feel stupid for writing that. :-[ --- Quote from: MonMotha on July 22, 2008, 03:58:34 pm ---I really feel like I've said this sort of things several times over the past week. Perhaps a sticky FAQ on LEDs is in order? --- End quote --- How about a wiki entry? It might be more useful and easier to include it as a link in posts? |
| MonMotha:
To say what I was saying in the context of your analogy, resistors are like goldfish: if you offer them more voltage, they'll just keep taking whatever you offer them (in the form of current) until they eat too much and pop. LEDs are like a mouse: if you offer it an extra cookie, it won't stop taking things until your whole house is cleaned out. As far as a wiki, one could be written. The wikipedia entry on diodes is actually fairly good, though. |
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