I bought some led's at radio shack and must have bought the wrong resistors for them. they dont work :( the led's are yellow 5mm 2.1 volt 20mA 720mcd. I bought A470 resistors and they dont work can anyone help me with this ?
E. T. Atkins spake thus:
Don't work? We need a bit more info there:
Assuming 12V supply, with 470R, the LED voltage will be 2.6V Is this within the LED max voltage? The high brightness LEDs I have are rated at 1.8V nominal and 2.4V maximum. You would probably be better off with 560R
Test them with a 1.5V torch battery to make sure that you didn't let the smoke out. Although my experience is that if you supply too much voltage they tend to explode.
LED's are diodes, did you try them in both current directions? What was your voltage source?
Is that 470 OHMS? or what???
Do you have a multimeter? Will the LED's light up if you use an AAA battery on them directly?
I'm running Podogy advance dcc system ho 12 volt. the led package says .... Forward supply voltage 2.1 typ. 3.0 max. forward current 20mA typ. 30mA max. a470 ohm's resistor
E. T. Atk> I bought some led's at radio shack and must have bought the wrong
Nope, polarity doesnt matter for resistors, better to make sure it is rated high enough to do the task in wattage. Unless you are shedding a lot of electrons, 1/4 watt resistor is normally good enough, if in doubt use 1/2 watt. And the value of the resistor is not critical - I have seen anything used from 220 ohms to 1000 ohms depending on the preference of the installer.
You could also try just one resistor in the blue (common) wire, rather than the feed to each LED assuming we are only talking front and rear lights.
Steve Newcastle NSW Aust
E. T. Atkins spake thus:
Resistors don't have polarity; LEDs do (they're diodes). If the LED is connected backwards, it won't light. Try it the other way (won't hurt it).
Scott
E. T. Atk> I'm running Podogy advance dcc system ho 12 volt.
With his stated supply voltage and resistor (470 Ohm), current flow would be 21 mA, well under the 30 mA max specified for the LED. This is assuming his supply is 12 Volts DC...if it's 12 Volts AC, then the peak current would be around 32 mA.
Scott
Robert Small wrote:
"E. T. Atkins" skriver:
DCC normally gives up to 16 volt on a lamp output.
When I hook up LEDs I normally use 1Kohm resistors. There is no need for full power unless you want to use them as a torch.
Be aware of the polarity of the LED and make sure that the lamp output is on...
Klaus
Here is a handy calculator for finding LED resistance values:
"video guy -
Actually it is lacking important information..
When you use LED's it is often not neccesary to use it at full current consumption. My experiance shows that reducing from 20 to 10mA doesn't half the amount of light from the LED. It stays allmost the same.
Most moderne LED's give sufficent light from 5mA, eveten though it can consume 20mA.
Klaus
Yepp... The LEDs could be the wrong way round. They WILL pass current in BOTH directions, but will only light in ONE.
The choice of a 470 ohm resistor is correct. Current = Voltage divided by Resistance.
Are you sure they are 470 Ohms, and not 470k Ohms?
To calculate, assuming a 'rough' LED Voltage of 2.0 and a
12 Volt supply, the voltage across the resistor will be 10V.(12 - 2 = 10).
The current flow through a 470 Ohm resistor will be
10/470 (Amps), which is approx. 21.3 milliamps (0.0213).If the LEDs have not been damaged, they will work OK with the 470 Ohm resistors.
The resistor may get too hot if it is not big enough to handle the power-loss, so it needs to be rated at more than 1/4 Watt. A 1/2 Watt resistor would be the best choice, so that it will not get too hot!
The power used in the resistor is equal to Volts times Current.
Taking the above figures, 10(V) * 0.0213(A) = 0.213 Watts.
Hope this confirms your design is OK.
David.
Robert and everybody else: please note that LEDs are current devices, not voltage devices. The thing that kills them is too much current when in the forward direction, not the voltage across them. In the reverse direction, voltage will kill them but that is just overloading the capabilities of the LED to resist the voltage that is across them like any diode will do. A LED will allow the voltage increase across the terminals until the device starts conducting. At that point, any increase in current will provide only a slight increase in voltage until you get to the overload point where the device starts overheating (anytime you drop a voltage across a resistance, you get heat!) and soon melts something inside, sometimes the wire connection, sometimes the device itself, and the diode stops emitting light. I say this stuff because a lot of you think, from the way that you are talking of the devices, that they work by putting a voltage across them. This is the way to disaster - a good stiff power supply can provide a lot of current to one and kill it in no time at all. Batteries are a good stiff supply and can provide more than enough current into the LED to destroy it. Been there and done that! LEDs are current devices so provide them a way to self-regulate their voltage by providing a resistor in series with the power at all times.
-- Yeppie, Bush is such an idiot that He usually outwits everybody else. How dumb!
David French spake thus:
Nope: they only pass current in one direction. They're *diodes*, remember?
The ones I use are probably reverse-protected, as they pass current in both directions. When alight, they measure around
2 Volts, and in reverse they measure 0.8 Volts.David.
Probably an internal diode to limit the reverse voltage applied to the LED if hooked up backward or if AC is applied to it...
Scott
David French wrote:
There is a great joke to be had in this thread:
Question: How many r.m.r. "electronics experts" does it take to enlighten an electronic newbie about how to hook up an LED (and bias it correctly)?
Answer: to be determined as the answers keep coming in at a furious pace.
:-) I'm quite amused by all the responses and by the amount of information (correct and not-so-correct).
Gotta love r.m.r.
Peteski
David, I've been "playing with LEDs for about 30 years.
I'm quite curious about the LEDs you describe. Could you give me more info on it (like where you bought it, the manufacturer and a part number)?
While I'm not doubting you, I've never seen any standard LEDs with built-in protection diodes.
LED Lasers have protection diodes but I doubt that you are using one for illuminating your model trains.
Thanks, Peteski
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