View Question
Q: LED Lamp Circuit ( Answered ,   1 Comment )
 Question
 Subject: LED Lamp Circuit Category: Science > Technology Asked by: carl5219-ga List Price: \$10.00 Posted: 24 May 2003 13:43 PDT Expires: 23 Jun 2003 13:43 PDT Question ID: 208170
 ```What voltage are most hobbyist LED lamps rated for (is there a range of voltages)? When wiring up an LED lamp I seem to remember something about "of course, adding a (resistor? capacitor?) to the circuit" in a discussion somewhere. Which, what values, and in parallel, series? Is there a website out there somewhere with information about simple LED circuits?```
 ```Hi there, Carl5219! For a basic LED circuit, you will need a resistor to limit the amount of current flowing through to prevent a short circuit. I found an excellent web page describing this very well which contains the following quotes: "It turns out that normal wire has very low resistance. Often, its in the neighborhood of .01 ohms, or even less. If you use a 6 volt battery, and connect a plain wire across its terminals, then the amount of theoretical current flow is Current = Voltage / Resistance, or Current = 6 / .01 which equals 600 amps. Now, it turns out that most batteries will only hold around 2 or 3 amps. This means the battery will drain almost immediately, which is not good. (...) you should never allow electricity to flow from one end of the battery to the other without using some sort of resistance. If this occurs, its called a short circuit, and will usually start to burn or at least smoke badly." So basically, the resistor is there to prevent the huge current flowing straight from the positive terminal (+) on the battery to the negative (-) which will cause a short circuit. "(...) resistance is extremely important in our circuit. First, it insures that we haven't created an electrical short. Second, it turns out that the LED has some really small wires inside it, and cannot be allowed to carry very much current. In fact, most LED's will burn up if you allow more than 30mA to pass through them." So the resistor will have two roles here. One, to limit the current to prevent a short circuit and two, to prevent the LED from overheating due to excessive current. The LED has very little in the way of resistance by itself and because the wire inside is so small, it will overheat in the same way as a lightbulb and it will not lower the current going back to the negative terminal on the battery. Resistors are usually wired in series. All quotes taken from: http://www.nwlink.com/~kevinro/guide/electronics.html A basic circuit can be seen here, taken from the above page: http://www.nwlink.com/~kevinro/guide/images/elect2.gif =========== There are several ways of calculating resistance and current (amps) in a circuit. Voltage = Current * Resistance Current = Voltage / Resistance Resistance = Voltage / Current (For more on these, visit Ohmslaw.com: http://ohmslaw.com/ohmslaw.htm ) So, for a 9 volt battery we can work out the following: Current = 9 / 330 ohms which is .027 amps or 27mA. So knowing that a typical LED will not handle more than 30mA, a 330 ohm resistor would be almost at the limit of a typical LED. I also found a very handy resistor calculator for LEDs: http://colourclassicfaq.com/general/led.html =========== For example, have a look at the LED here: http://www.allelectronics.com/spec/LED-94.pdf (PDF document) It has a 2.4v max voltage with an average 30mA current rating (max 50mA). If we use the resistor calculator I mentioned above, we need a 120 ohm resistor. But what if you only have a 1.5v battery? Simple, using the Resistance method: 1.5v / .050 amps (50mA) = 30 ohms resistor needed. In general, most LEDs will be rated for a max of 5v but you should check first, if possible. I hope this helps! Kind regards, errol-ga. Related Google searches: "led voltage" ://www.google.co.uk/search?q=led+voltage "led guide" ://www.google.co.uk/search?q=led+guide "led circuit guide" ://www.google.co.uk/search?q=led+circuit+guide``` Request for Answer Clarification by carl5219-ga on 25 May 2003 19:47 PDT ```Great answer... if I might ask something for clarification... with such a low/almost no, resistance it sounds as if the LEDs could be wired in series just as well as parallel (in a multi-LED circuit) with no decrease in output, unlike incandescent bulbs. Is this right?``` Clarification of Answer by errol-ga on 25 May 2003 21:22 PDT ```Hi! There will be a slight decrease in output due to the fact that each LED will consume *some* power but it won't really be noticeable unless it's a huge array of them. They only consume a tiny amount of milliamps each and have virtually no resistance properties. Try wiring up ten or more in series to see the effects. errol-ga.```
 carl5219-ga rated this answer: and gave an additional tip of: \$5.00 `Great, just what I needed!`
 ```Many thanks for your generosity, Carl5219! Answering this question was a nice refresher course for my electronics skills so I am very grateful for your question. errol-ga.```