I have been experimenting with some cool white LEDS from a Christmas light set. Much of the information I read says I needs resistors with each LED. While this is certainly true if I wire individual LEDS to a 12 volt power supply or wire parallel, I don't find it true if I hook up LEDS in series as long as the voltage to each LED is 3 volts or less.
For example: I find I can wire 4 to 8 LEDS in series to 12 volts depending on how bright I want the lights without blowing them. Or I can hook individual LEDS to two 1.5 volt batteries producing 3 volts without resistors also.
Question: What is the advantage of using resistors and wiring the LEDS in parallel to a 12 volt transformer over wiring them in series and not using resistors?
eaglescout ....Much of the information I read says I needs resistors with each LED. While this is certainly true if I wire individual LEDS to a 12 volt power supply or wire parallel, I don't find it true if I hook up LEDS in series as long as the voltage to each LED is 3 volts or less..... Question: What is the advantage of using resistors and wiring the LEDS in parallel to a 12 volt transformer over wiring them in series and not using resistors?
....Much of the information I read says I needs resistors with each LED. While this is certainly true if I wire individual LEDS to a 12 volt power supply or wire parallel, I don't find it true if I hook up LEDS in series as long as the voltage to each LED is 3 volts or less.....
The advantage of using individual resistors and parallel wiring is 1) control over the brightness of each LED, and 2) not losing the whole string if one LED should fail.
In series, the same current flows through all the LEDs in the string. Different LEDs will have different brightness for the same current (within the usable current range). This is particularly true for different color LEDs. An example is using LEDs in signals on our layouts. To get equivalent brightness levels requires different resistor sizes and parallel wiring.
Different color and types of LEDs also have different voltage drops when forward biased. Some are as low as 2.5 volts, some are more like 3.5 volts. Battery power is usually safe by using LEDs with an appropriate voltage drop. Battery voltage is nominal - most don't put out their full nominal voltage even when new. And battery voltage eventually falls off as the battery is drained. OTOH, most low voltage power supplies put out more than the nominal voltage rating on light loads (LEDs).
Can you do what you want? Yes, but the advantage goes to the parallel/individual resistor scheme.
Fred W
Yes, the LION tried that. It works too. But I was told by electrical engineers that I should not do that. That is to say that you should have at least one resistor in your circuit. Even as Christmas lights (which is of course what a cheap LION would use) do have resistors hidden within their circuitry.
But the LION bought 1000 1KΩ resistors, and it really is no problem to wire them into the circuit. In my stations I may have 30 or 40 lamps, they all have resistors (now). I also use them for signals, there I tie the cathodes together and ground them through a 1KΩ resistor to the system ground. (Anode or Cathode--the LION does not care--him is dyslexic anyway), the other side (either + or - 12 dc--whichever lights your lamp) is then routed through whatever hoops are necessary to display the correct aspect. Indeed, for simple red/green signals I will put the anode of one and the cathode of the other together and tie them to ground via the 1KΩ resistor, and the the polarity of the circuit (also feeding the switch machine) controls both the position of the turnout and the aspect of the signals.
For interior car lighting, I am using a full wave rectifier followed by a 5 volt regulator so that lights will work correctly regardless of the direction of travel, and these are connected to the LEDs with individual 300-500Ω resistors depending on desired brightness. They also have a 0.33 Farad capacitor across the circuit to keep the lights on even in the stations. (the regulator is there to protect the capacitor(s) which have a 5.5 volt limit) All of these parts are very cheap at All Electronics (an MR advertiser).
(ROAR)
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Using current limiting resistors in series with each LED will give you a longer lasting more robust setup. LED's have so little resistance that we model them as zero resistance devices. If you make a circuit from power supplies (batteries) and LED's there is nothing to stabilize the current. Ohms law says current is equal to V/R. When R goes to zero, the term V/R goes to infinity. Then a LED blows out (takes about 10 milliseconds, faster than the eye can see) and the current goes to zero.
It is possible to carefully balance the driving voltage against the forward biased voltage drop of a LED and come up with a circuit that lights the LED without blowing it for quite some time. But all it takes is a slight change anywhere in the circuit, and poof. Your LED will become a Darkness Emitting Diode (DED). The right way to bias LED's is to put a resistor in series with the LED to limit the current thru the LED to 20 mA. In fact better is to limit the current to 10 mA, which will make the LED glow plenty bright enough, and you have some margin in case the resistor is a little low or the voltage is a little high.
By the way, you don't want to reverse bias a LED. Although a LED is a rectifying diode, it isn't very strong. It only takes 5 or 6 volts reverse bias to blow one out.
David Starr www.newsnorthwoods.blogspot.com
David did a good enough job with the technical details, so I'll spare you the re-hash. If you want to string them together like you did, and you have a source with a high enough output impedance, you can get away with what you're doing, because the high impedance source provides the resistor already
However, the more amperage the supply can source means lower source impedance, and you're going to blow them out. I'd put some kind of resistor in the loop to limit the current, say 50-100 ohms, for 4-5 LEDs in series.
I used to be clueless, but i've turned that around 360 degrees.
eaglescoutQuestion: What is the advantage of using resistors and wiring the LEDS in parallel to a 12 volt transformer over wiring them in series and not using resistors?
Just in general, I would never choose series wiring over parallel wiring just for the versitility aspect.