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| BobA:
Don't think of an LED as a resistive load. It is a diode which conducts current and destroys itself if it does not have an additional resistor to limit the current. You can put multiple LEDs in series with a single resistor, it is just not the ideal way to arrange them as the voltage across each LED can vary due to internal variation. If all LEDs are the rated the same it usually works fine. |
| RandyT:
--- Quote from: bfauska on May 09, 2007, 04:00:37 am ---I think I have been oversimplifing LEDs in my head. If I am understanding what you are saying... even if you had a perfectly stable PS you would not be able to run 2 2.5v LEDs in series on a 5v PS w/o a resistor? --- End quote --- I don't think you've oversimplified, rather you just haven't taken into account the fact that perfect circumstances don't really exist in practice. Theoretically, if you had a perfect power supply of 5v and two perfect 2.5v LEDs, yes, you should be able to use them in series without a resistor. The problem is, electronic components have tolerances associated with them (as do power supplies.) That means your supply isn't putting out exactly 5v (usually up to 10% is considered a reasonable deviation) and your LEDs aren't operating at exactly 2.5v. Obviously, just because there is a deviation it doesn't automatically indicate that your LEDs will be damaged in the configuration you propose. It's likely just as possible that the output of the power supply will be lower than the 5v, and in that case things would be fine. But it's a gamble, and who's to say that the output of the supply will stay where it is as the loads change or the components start to age? So it's always better to use a resistor, even if it's a very small one, to account for tolerances and variations in supply outputs. And then there are other things to consider, like modulation. If an LED is modulated, it can usually handle greater currents. So the type of application enters the equation as well. The long and short of it is that your LED will last longer if you you run them a hair dimmer than they are rated. Sometimes this happens by default, as the exact resistor values one needs for a circuit aren't always easily available. In that case, the next highest resistor value is used so more current is blocked. LED's are cheap, but your time probably isn't. For most, installing new LEDs in 8 years instead of the 11 years they are usually rated for isn't going to be a big deal from a cash standpoint (especially since super bright LED's will probably be very cheap and commonplace by then) but replacing all of them as they burn out might not be much fun. Better to take a few minutes extra to add a resistor when you first install them and possibly get many more years of service. RandyT |
| bfauska:
Thanks for the replies everybody. I suppose I will have to go with the resistors when I use LEDs, it's not like they cost anything. I am going to have to study up on some of the components in basic electronic circuits, I have a fairly strong understanding of basic electrical ideas, I wire motors and lights and motor control hardware as part of my job, but resistors, diodes, caps, coils and the like are still a little fogy for me. The idea of current needing to be blocked sounds totally odd to me, I was fairly sure that current was something available from a power supply and drawn out of it by the item using the power, not something that the power supply could force upon the parts of the circuit. |
| polaris:
--- Quote from: bfauska on May 10, 2007, 12:12:31 am --- The idea of current needing to be blocked sounds totally odd to me, I was fairly sure that current was something available from a power supply and drawn out of it by the item using the power, not something that the power supply could force upon the parts of the circuit. --- End quote --- youre gonna kick yourself . lights etc are fused to prevent power surges, so of course youre aware that power needs to be blocked . its just more finite control in electronics as opposed to big ass fuses in lights. :cheers: |
| SavannahLion:
--- Quote from: bfauska on May 10, 2007, 12:12:31 am ---The idea of current needing to be blocked sounds totally odd to me, I was fairly sure that current was something available from a power supply and drawn out of it by the item using the power, not something that the power supply could force upon the parts of the circuit. --- End quote --- That's only part of the equation. I don't like to use the water analogy (since it doesn't paint the whole picture, but for our purposes it will suffice) and I don't have it pinned down exactly myself. I think what you're thinking of is amperage. How fat the "pipe" is for the water (electricity) to flow through. The bigger the pipe, the more water that can pass through. You can use a power supply that pumps out, say 5 amps at 5 volts and if your device uses only 2 amps, then that's what it will use. Volts is akin to the water pressure in the pipe. Too much water pressure and you can burst the pipes in your home. Again, if the device only requires 5 volts, but you attach a 10 volt power supply, you'll break the device. What the group is discussing (in general) is the voltage being pushed through. An LED only wants 2 volts, but you're trying to shove 5 volts through it. The resistor reduces the "pressure" so you don't break it. Amps is part of the equation, but seeing as how we have plenty of it, it's generally disregarded. Try not to depend on that water analogy for everything though. It's what confused me for years before I discovered I was looking at the whole thing wrong. If I'm way off on my explanation, please correct me. I don't want to be confused anymore ;D |
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