Maybe a silly electrical question
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R = V / I where V is the voltage across the R. If I is in AMPS (A) then R will be in OHMS. If I is in MILLIAMPS (mA) then R is KILOHMS.
Example: Lets assume you want to run the LED at 20 mA, the forward voltage drop of the LED is 2 volts and the power source is 12 volts. The voltage across the resistor will need to be 10 volts (12 - 2). Do the math to determine the value of the resistor: 10 volts / 20 mA = 0.5 kilohms or 500 ohms. The nearest standard value will be 510 ohms.
You can get a thing called a "current regulator diode" which will (over a certain range of conditions) pass a fixed current.
Here is one rated at 10 mA
http://www.mouser.com/ProductDetail/Semitec/E-103/?qs=wgO0AD0o1vvf%2bAlP9/JcaQ==
But it costs a good deal more then a simple resistor.
If you don't have the LED specs you can determine a usable forward voltage by doing a little testing.
To do so setup a power source and a resistor to run 20 mA through the LED and measure the voltage across the LED.
20 mA is fairly safe value for most LEDs. Different current levels will yield somewhat different forward voltage drop.
If the LED is too bright at 20 mA try lower values until satisfied and then measure the voltage across the LED.
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wombat457
I'm the one
Dale, I solder my LED's by twisting the wire around the LED lead, then solder it OR solder them to the sides but solder (my normal way) all the way around the LED lead. I have always had a minimum of a 1/4" of solderable wire. So far I haven't had a problem with bad contacts.
A universal "socket or connector" would be ideal.
Not a silly question either by the way. When I was learning to do that I was told to solder them end to end using solid wire. More a pain in the butt than it was worth.
A universal "socket or connector" would be ideal.
Not a silly question either by the way. When I was learning to do that I was told to solder them end to end using solid wire. More a pain in the butt than it was worth.
Lynnb
Well-Known Member
I don't have much experience in this area but a few years back I wanted to try connecting red and green led's to my tortoise switch so when the turnout was thrown it would show green or red and all I did was tin the wire and as neat as I could wrap the wire around the lead of the led and solder away.
Keep in mind if your dealing with leds you can also add a resistor in series if you got to much volts going to it or what ever the term is.
Keep in mind if your dealing with leds you can also add a resistor in series if you got to much volts going to it or what ever the term is.
Iron Horseman
Well-Known Member
I usually twist a wire on, then solder, then slide a heat shrink tube over it so it is insulated all the way up to the LED casing.
LED's are polarity sensitive. There will be a flat spot on the plastic housing indicating the negative side. I have not found out how to determine the resistor required yet. LED's can range from 1.5 volts to 5 volts but they all look the same. Parallel and series starts to get confusing especially when you try to remember what you did later down the road. I'm still searching for one way to do them so everything on the layout is the same.
R = V / I where V is the voltage across the R. If I is in AMPS (A) then R will be in OHMS. If I is in MILLIAMPS (mA) then R is KILOHMS.
Example: Lets assume you want to run the LED at 20 mA, the forward voltage drop of the LED is 2 volts and the power source is 12 volts. The voltage across the resistor will need to be 10 volts (12 - 2). Do the math to determine the value of the resistor: 10 volts / 20 mA = 0.5 kilohms or 500 ohms. The nearest standard value will be 510 ohms.
You can get a thing called a "current regulator diode" which will (over a certain range of conditions) pass a fixed current.
Here is one rated at 10 mA
http://www.mouser.com/ProductDetail/Semitec/E-103/?qs=wgO0AD0o1vvf%2bAlP9/JcaQ==
But it costs a good deal more then a simple resistor.
If you don't have the LED specs you can determine a usable forward voltage by doing a little testing.
To do so setup a power source and a resistor to run 20 mA through the LED and measure the voltage across the LED.
20 mA is fairly safe value for most LEDs. Different current levels will yield somewhat different forward voltage drop.
If the LED is too bright at 20 mA try lower values until satisfied and then measure the voltage across the LED.
Thanks FCWILT: I don't what to jack this thread but I think it is in line with the original posters question. My current thought is to use 3 5 volt leds in series off of 12 volts with no resistors. Yes, they will be dimmer but easier to wire and remember. My purpose would be for building lights and maybe a street light or five. As I stated before, I have no way of knowing the voltage of the LED except when it comes out of the package. I don't want to have to guess at what I did, I would like a standard way of doing lighting that is easy to remember with all the same voltage bulbs. Not sure if I made this understandable and sorry for butting in.
Lynnb
Well-Known Member
Wholy crap I now feel edumacated. Good post.
Iron Horseman
Well-Known Member
Unfortunately that won't work as you are thinking. As has been repeatedly stated an LED is not a lamp, it is first and foremost a diode. Diodes are creatures of current not voltage. Resistance and inductance loads (lamp, resistor, motor) will consume only what current they need. The rest of the current remains as potential energy. A diode on the other hand, once it starts conducting it will attempt to allow as much current as is available to pass through. So if you have say a 5 amp supply the diode will attempt to allow all 5 amps through. Unless the diode is rated for 5 amps it will burn out.
What Iron means is that unlike a light globe which has it's own internal resistance and hence limits the amount of current that it will draw from a power source, an LED has nothing internally to limit the amount of current that may pass through it.
This is why you should always put a series resistor in line with the LED to limit the current to a safe value.
In Iron's example, he was talking about using a 12V power supply capable of supplying up to 5A of current.
Even if you wire your LED's in series, if you do not use a series limiting resistor, there is a very good chance that the LEDs will try and draw whatever current the power supply is capable of delivering - in this example 5A. Your Red, Green, White, Amber - whatever colour LEDs you choose will probably be a uniform shade of black.
Using LEDs without the series resistor is like playing with fire. What you can do is wire two of your 5V LEDs in series to use on a 12V power supply but add a series resistor to drop the remaining 2V and calculate it's value to limit the current to around 20mA by using the formula listed earlier in the thread.
The remaining thing to be wary of is the power rating required for the resistor. This can be calulated by the formula
P = V x I where P is the power in Watts, V is the voltage dropped across the resistor in Volts and I is the current flowing through the resistor in Amps.
e.g. if you are using the two 5V LEDs discussed above in series with a resistor to allow connection to a 12V power supply.
The resistor will need to drop 2V and we want to limit the current to say 20mA,
The value for the resistor will be :
R = V/I R is in ohms, V is in volts, I is in Amps
R = 2/0.02
R = 100 ohms
The power that will be dropped across the resistor will be
P = V x I P is in Watts, V is in volts, I is in Amps
P = 2 x 0.02
P = 0.04 Watts (40mW)
Most small sized resistors are rated for at least 1/8 Watt (0.125 W) or 1/4 Watt (0.25 W) so either of these would suffice. Whatever your resistor wattage calculates out to be - make sure the rating of the resistor is at least equal to the power it will be experiencing. The preference is to use a resistor with a higher power rating than required to ensure it does not overheat
e.g If your LED circuit calculates out that your limiting resistor will have to handle 0.4 Watts of power, you would use a resistor with a power rating of 0.5 Watts or higher. You would not use one rated at 0.25 Watts
Also bear in mind that if the amount of power being dissipated by the resistor is close to the rated value for the resistor, it will get physically quite hot. Anything heat sensitive (such as a plastic shell) could melt or catch fire.
Experiment first on the bench. Make sure you check the temperature of the resistor with the circuit running.
Hope this helps
This is why you should always put a series resistor in line with the LED to limit the current to a safe value.
In Iron's example, he was talking about using a 12V power supply capable of supplying up to 5A of current.
Even if you wire your LED's in series, if you do not use a series limiting resistor, there is a very good chance that the LEDs will try and draw whatever current the power supply is capable of delivering - in this example 5A. Your Red, Green, White, Amber - whatever colour LEDs you choose will probably be a uniform shade of black.
Using LEDs without the series resistor is like playing with fire. What you can do is wire two of your 5V LEDs in series to use on a 12V power supply but add a series resistor to drop the remaining 2V and calculate it's value to limit the current to around 20mA by using the formula listed earlier in the thread.
The remaining thing to be wary of is the power rating required for the resistor. This can be calulated by the formula
P = V x I where P is the power in Watts, V is the voltage dropped across the resistor in Volts and I is the current flowing through the resistor in Amps.
e.g. if you are using the two 5V LEDs discussed above in series with a resistor to allow connection to a 12V power supply.
The resistor will need to drop 2V and we want to limit the current to say 20mA,
The value for the resistor will be :
R = V/I R is in ohms, V is in volts, I is in Amps
R = 2/0.02
R = 100 ohms
The power that will be dropped across the resistor will be
P = V x I P is in Watts, V is in volts, I is in Amps
P = 2 x 0.02
P = 0.04 Watts (40mW)
Most small sized resistors are rated for at least 1/8 Watt (0.125 W) or 1/4 Watt (0.25 W) so either of these would suffice. Whatever your resistor wattage calculates out to be - make sure the rating of the resistor is at least equal to the power it will be experiencing. The preference is to use a resistor with a higher power rating than required to ensure it does not overheat
e.g If your LED circuit calculates out that your limiting resistor will have to handle 0.4 Watts of power, you would use a resistor with a power rating of 0.5 Watts or higher. You would not use one rated at 0.25 Watts
Also bear in mind that if the amount of power being dissipated by the resistor is close to the rated value for the resistor, it will get physically quite hot. Anything heat sensitive (such as a plastic shell) could melt or catch fire.
Experiment first on the bench. Make sure you check the temperature of the resistor with the circuit running.
Hope this helps
The only LEDs rated at 5 volts that I am aware of are ones with a built-in resistor.
Example:
http://www.avagotech.com/pages/en/l...ugh_hole_lamps/resistor_leds/hlmp-1640-b00a1/
So if this is the case the concept of connecting 3 in series to a 12 volt power source is feasible.
Example:
http://www.avagotech.com/pages/en/l...ugh_hole_lamps/resistor_leds/hlmp-1640-b00a1/
So if this is the case the concept of connecting 3 in series to a 12 volt power source is feasible.
wombat457
I'm the one
Hey Dale, aren't you glad you asked your question? 
Those guys know their stuff - too well me thinks. As I have absolutely no idea what they are all talking about, I'll throw in my completely ignorant (about electrics) thought:
When I put my LED's in, I use a 1000 ohm, 1500 ohm or 2000 ohm resistor, depending on the brightness of the lighting I want. I have found that a 1000 ohm or 1500 ohm resistor works pretty well for structure lighting.
Those guys know their stuff - too well me thinks. As I have absolutely no idea what they are all talking about, I'll throw in my completely ignorant (about electrics) thought:
When I put my LED's in, I use a 1000 ohm, 1500 ohm or 2000 ohm resistor, depending on the brightness of the lighting I want. I have found that a 1000 ohm or 1500 ohm resistor works pretty well for structure lighting.
kjd
Go make something!
RE needing a resistor in series.
How are strings of Christmas LEDs wired? I don't remember any resistor, just a big long string of them in series. They are obviously running on AC because they strobe. It's not evident to our eyes but can create some cool effects in a photo if motion is involved. In this case there could be 15 or 20Amps in the circuit.
How are strings of Christmas LEDs wired? I don't remember any resistor, just a big long string of them in series. They are obviously running on AC because they strobe. It's not evident to our eyes but can create some cool effects in a photo if motion is involved. In this case there could be 15 or 20Amps in the circuit.
Thanks guys for all the info, I printed the thread and I am beginning to see the light but I will continue to read what you said. I'm sure it will sink in.
Dale: check this link for a wire wrap tool. http://en.wikipedia.org/wiki/Wire_wrap
They work well and the ones I have will do 20 ga wire. It will also wrap the resistor lead to the LED. No need to solder but you can if you want to. They are used by the phone company and make a great contact.
here is a youtube link: https://www.youtube.com/watch?v=8SSOKppwpAE
Dale: check this link for a wire wrap tool. http://en.wikipedia.org/wiki/Wire_wrap
They work well and the ones I have will do 20 ga wire. It will also wrap the resistor lead to the LED. No need to solder but you can if you want to. They are used by the phone company and make a great contact.
here is a youtube link: https://www.youtube.com/watch?v=8SSOKppwpAE
CSX Robert
Member
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