mrpackethead
Full time elf
Hi Folks, theres a lot of discussion going on about RGB LED Pixel Strings at the moment. And when that happens its sometimes hard to distinguish fact from fiction.
So i've put up some basic questions that you should ask before you consider investing in some. Later this week, i'm going to post some technically focused diagrams and discussions for those who want to dive in a bit deeper.
(1) Constant Current or Constant Voltage
The light output of LED's is largely controlled by the amount of current flowing through them, and not the voltage applied across them. In order to make all the LEDS in the string glow with equal intensity its important that the current flowing in each led is as close as possible. This is achieved with a constant current controller, but not with a constant voltage control, where the current is set with a current limiting resistor. No string is exempt from voltage drop, ( ohms law always applies ). Measurements of the light output in a string of constant current leds typically show show variations of less than 5%. We have seen variations of more than 40% in constant voltage controlled strings. Why doe sit matter? Well, the big reason is that when you go to mix your colours, different leds will behave differently from each other, ie, some leds might look "pink", and some "purple" when told to do the same thing. ( and don't' be fooled about arguments with higher supply voltages, ohms law still applys! )
THe supplier may not be able to tell you this ( they may not know ), but find out what controller IC the strings using, and post it back here; most IC's are well known now and we can help you figure it out.
(2) What is the light output?
A technique that has been employed by some manufacturers to get over the voltage drop issues in some strings, is to simply reduce the current flowing in the strings.. ( halving the current will half the voltage drop ).. However lower currents result less light coming out of your pixels.. In effect what might seem like a bargain isn't because you'll need twice as many leds to get the same light levels..
(3) How energy efficient is the set up?
This is a major differentiator among led strings that i'm seeing. Wasted energy is 99% lost as heat. And heat and electronics is typically bad. Every time something heats up it expands, and when it cools it contracts. Expand Contract Expand Contract, over time that results in thermal stressing, and thats the major cause of failures in electronics.. Every led pixel system is to some extent going to waste some energy.. However the more waste heat the worse things are.. It doesn't end there though.. Less efficient led strings mean you need bigger DC power supplies.. ( and do you want to buy bigger more expensive power supplies simply to run your little led heater? At least for me i'm building LIGHTING displays not HEATING displays..
I can be a little hard to establish the system efficiency, without some technical data or measurements.. What you want to know is how much energy is being used in total, and how much energy is being used to just drive the leds.. The difference is "waste" because its doing no useful work, and ratio between the two is the efficiency. A simple way to spot potentially inefficient systems is this.. (and i know i said i wasn't got to go tech, but )… For single LED pixels, you want a supply voltage that is close to the Forward voltage of the LED itself.. ( thats 1.8V for Red, 3.2 for Blue and Green ).. The reality is you need a bit more voltage than that to keep everything working properly.. but not lots more.. Generally speaking the waste energy in the system is the ( supply voltage - led forward voltage ) x operating current of the led. Good string design requires a number of factors to be considered. Simply optimizing a design for one parameter is likely to result in poor performance overall. ( please note that there are advanced controllers, that can use higher voltage supplies, to drive leds, which don't' waste all this energy. these are typically used for high power leds ( >1W ). These use inductors and capacitors and complex switching arrangements to achieve this.. Typically low power led pixel strings ( like the ones we commonly pop up ) are just a simple transistor based current control or transistor switch. )
(4) Build quality?
Theres some major differences between the way different strings are being manufactured. You really need to establish how well your strings are going to last. Also the quality of the components in the actual pixel varies widely. There are several chipsets that commonly being used which are manufactured in factories that have very very poor Quality Control, and the failure rates are high. The actual manufacturing process's vary between suppliers.. Some manufactures will claim waterproof to IP68, but the first shower of rain will kill them. My suggestion is before you dive into spending hundreds or thousands of dollars, get your self a sample of their product and establish for yourself the build quality.. If your supplier won't supply you a sample, then you've got to ask why don't you?
(5) Warranty, Support, Backup and Spares?
The answer to this will depend entirely on how much risk you are prepared to take, and how confident you are of sorting things out for your self. The old saying is true " you get what you pay for "
Disclosure: I do this stuff commercial for large-scale displays. The number of led strings that were out on display in 2010 was many more than you can count on your fingers and toes. Our target market is the prosumer and commercial markets, with plug and play solutions. In no way am i trying to discourage or scare the hobbyist, in fact i'm encouraging you to jump in big time. In posting this, i hope you are better equipped to ask your suppliers some key questions about their products so you can make a better informed choice.
So i've put up some basic questions that you should ask before you consider investing in some. Later this week, i'm going to post some technically focused diagrams and discussions for those who want to dive in a bit deeper.
(1) Constant Current or Constant Voltage
The light output of LED's is largely controlled by the amount of current flowing through them, and not the voltage applied across them. In order to make all the LEDS in the string glow with equal intensity its important that the current flowing in each led is as close as possible. This is achieved with a constant current controller, but not with a constant voltage control, where the current is set with a current limiting resistor. No string is exempt from voltage drop, ( ohms law always applies ). Measurements of the light output in a string of constant current leds typically show show variations of less than 5%. We have seen variations of more than 40% in constant voltage controlled strings. Why doe sit matter? Well, the big reason is that when you go to mix your colours, different leds will behave differently from each other, ie, some leds might look "pink", and some "purple" when told to do the same thing. ( and don't' be fooled about arguments with higher supply voltages, ohms law still applys! )
THe supplier may not be able to tell you this ( they may not know ), but find out what controller IC the strings using, and post it back here; most IC's are well known now and we can help you figure it out.
(2) What is the light output?
A technique that has been employed by some manufacturers to get over the voltage drop issues in some strings, is to simply reduce the current flowing in the strings.. ( halving the current will half the voltage drop ).. However lower currents result less light coming out of your pixels.. In effect what might seem like a bargain isn't because you'll need twice as many leds to get the same light levels..
(3) How energy efficient is the set up?
This is a major differentiator among led strings that i'm seeing. Wasted energy is 99% lost as heat. And heat and electronics is typically bad. Every time something heats up it expands, and when it cools it contracts. Expand Contract Expand Contract, over time that results in thermal stressing, and thats the major cause of failures in electronics.. Every led pixel system is to some extent going to waste some energy.. However the more waste heat the worse things are.. It doesn't end there though.. Less efficient led strings mean you need bigger DC power supplies.. ( and do you want to buy bigger more expensive power supplies simply to run your little led heater? At least for me i'm building LIGHTING displays not HEATING displays..
I can be a little hard to establish the system efficiency, without some technical data or measurements.. What you want to know is how much energy is being used in total, and how much energy is being used to just drive the leds.. The difference is "waste" because its doing no useful work, and ratio between the two is the efficiency. A simple way to spot potentially inefficient systems is this.. (and i know i said i wasn't got to go tech, but )… For single LED pixels, you want a supply voltage that is close to the Forward voltage of the LED itself.. ( thats 1.8V for Red, 3.2 for Blue and Green ).. The reality is you need a bit more voltage than that to keep everything working properly.. but not lots more.. Generally speaking the waste energy in the system is the ( supply voltage - led forward voltage ) x operating current of the led. Good string design requires a number of factors to be considered. Simply optimizing a design for one parameter is likely to result in poor performance overall. ( please note that there are advanced controllers, that can use higher voltage supplies, to drive leds, which don't' waste all this energy. these are typically used for high power leds ( >1W ). These use inductors and capacitors and complex switching arrangements to achieve this.. Typically low power led pixel strings ( like the ones we commonly pop up ) are just a simple transistor based current control or transistor switch. )
(4) Build quality?
Theres some major differences between the way different strings are being manufactured. You really need to establish how well your strings are going to last. Also the quality of the components in the actual pixel varies widely. There are several chipsets that commonly being used which are manufactured in factories that have very very poor Quality Control, and the failure rates are high. The actual manufacturing process's vary between suppliers.. Some manufactures will claim waterproof to IP68, but the first shower of rain will kill them. My suggestion is before you dive into spending hundreds or thousands of dollars, get your self a sample of their product and establish for yourself the build quality.. If your supplier won't supply you a sample, then you've got to ask why don't you?
(5) Warranty, Support, Backup and Spares?
The answer to this will depend entirely on how much risk you are prepared to take, and how confident you are of sorting things out for your self. The old saying is true " you get what you pay for "
Disclosure: I do this stuff commercial for large-scale displays. The number of led strings that were out on display in 2010 was many more than you can count on your fingers and toes. Our target market is the prosumer and commercial markets, with plug and play solutions. In no way am i trying to discourage or scare the hobbyist, in fact i'm encouraging you to jump in big time. In posting this, i hope you are better equipped to ask your suppliers some key questions about their products so you can make a better informed choice.
