JoakimFlorence
Newly Enlightened
- Joined
- Jun 4, 2016
- Messages
- 137
There have been so many repeated questions about this that I thought I would start a definitive thread explaining it once and for all.
You have your LED, how do you power it?
The first thing you need to know about LEDs are that they are not like regular (incandescent) light bulbs. LEDs, while they have a very long lifespan, are also very sensitive. It only takes a split second of too much power to overload them and cause a burnout. You can't just hook up an LED to a battery. If there's nothing to limit the current, an LED will allow more power through than it can handle.
That's where a constant current driver comes in.
How do constant current drivers work? Well actually they regulate the voltage, but that ends up regulating the amount of current indirectly. A constant current driver will increase the voltage until a certain amount of current flows through the circuit. We normally don't think of an LED as having any resistance, but actually it does (within its narrow operational range of voltages). If the voltage potential across the LED falls just a little bit, the amount of current able to flow through will decrease. A little more voltage and more current will be able to flow through. It would be very easy for too much current to flow through and the LED to suddenly burn out.
Of course drivers can't regulate the voltage to just any level. They're usually custom designed with a certain voltage range. The driver might say 8-12v. That means the minimum amount it can drop the voltage down to is 8 volts, while it also can't go above 12 volts. It's important to select the right driver voltage range depending on how many LEDs you plan to be running in a string (in series in the circuit). Oftentimes the driver will only indicate a single voltage rather than a range. In that case it's usually safe to assume the driver cannot drop the voltage more than 3 or 4 volts below the indicated voltage.
You usually do not want to go above the rated voltage on the LED. Remember, when LEDs are connected in series (in a string) the voltages all add up. Connect them in parallel and it's the current (mA) that add up, not the voltage.
Of course, if you do not have enough voltage, no current will be able to flow through the LEDs at all. (This is another way that LEDs are different from incandescent bulbs)
Being a semiconductor, an LED does need a certain minimum voltage threshold to conduct current.
So if you have three 3.2v LEDs wired in a string, a driver rated for 8-12v would be an appropriate choice.
---O---O---O---
The second thing is the amount of current that the driver is rated for. You usually want the driver current (mA) to match up with the rated LED current (mA). It's okay to use a little less current. For example, if the LED is rated for 350mA it's okay to use a 300mA driver, but be aware that the power will be a little bit lower and the LEDs might not be quite as bright.
Another thing that needs to be covered, when connecting two or more LEDs in parallel it is usually good to supply a little lower current that the sum of what all the LEDs are rated for.
You see, unless the voltage being supplied is at the lower end of the LEDs operational range, there is a tendency for all the current to start flowing through just one of the LEDs, rather than being spread out evenly between the LEDs, and then there will be a burnout. A 3.2v 350mA rated LED may be able to handle 3.4v 371mA, but try connecting three of these LEDs in parallel, naturally with a driver supply designed for 3 times the current which would be 1050mA, and there is a fair chance there could be a burnout. Lowering the power a little bit could avoid this. (The reason for this problem is because once the voltage gets on the high end of the operational voltage range of the LED, the amount of current that can flow through the individual LED quickly increases exponentially. The driver dropping the voltage at a given amount of current through the circuit isn't going to do any good because the circuit in this case is designed for more current than any single LED can handle.) To summarize, when powering multiple LEDs in parallel (or several different strings of LEDs in parallel) be conservative in estimating the amount of power they can handle. If you have two strings of 350mA, using a 600mA driver instead of 700mA wouldn't be a bad idea.
Oftentimes the LED specifications might not indicate an operational current (mA). In that case it's easy. Blue and white color LEDs are generally 3.2 volts. Take the power rating (Watts) and divide by 3.2 to find the current. For example, 1 Watt divided by 3.2 = 0.312 Amps, or 312 mA. (In this case most of these "1 Watt" LEDs are actually rated for 350mA)
The amazing thing about constant current drivers, if you had a driver with a wide enough functional voltage range, you could string 3 LEDs into a string or 5 LEDs into a string, and the amount of power supplied by the driver would be entirely proportional to how many LEDs there are. Remember, the driver is going to change the voltage until a certain current amount is flowing through.
So why don't we just use a resistor with an LED to limit current? This works very well for very low power indicator light LEDs, but when we're talking about more power the inefficiency of using a resistor starts becoming a significant factor. (It would basically require twice as much power)
You have your LED, how do you power it?
The first thing you need to know about LEDs are that they are not like regular (incandescent) light bulbs. LEDs, while they have a very long lifespan, are also very sensitive. It only takes a split second of too much power to overload them and cause a burnout. You can't just hook up an LED to a battery. If there's nothing to limit the current, an LED will allow more power through than it can handle.
That's where a constant current driver comes in.
How do constant current drivers work? Well actually they regulate the voltage, but that ends up regulating the amount of current indirectly. A constant current driver will increase the voltage until a certain amount of current flows through the circuit. We normally don't think of an LED as having any resistance, but actually it does (within its narrow operational range of voltages). If the voltage potential across the LED falls just a little bit, the amount of current able to flow through will decrease. A little more voltage and more current will be able to flow through. It would be very easy for too much current to flow through and the LED to suddenly burn out.
Of course drivers can't regulate the voltage to just any level. They're usually custom designed with a certain voltage range. The driver might say 8-12v. That means the minimum amount it can drop the voltage down to is 8 volts, while it also can't go above 12 volts. It's important to select the right driver voltage range depending on how many LEDs you plan to be running in a string (in series in the circuit). Oftentimes the driver will only indicate a single voltage rather than a range. In that case it's usually safe to assume the driver cannot drop the voltage more than 3 or 4 volts below the indicated voltage.
You usually do not want to go above the rated voltage on the LED. Remember, when LEDs are connected in series (in a string) the voltages all add up. Connect them in parallel and it's the current (mA) that add up, not the voltage.
Of course, if you do not have enough voltage, no current will be able to flow through the LEDs at all. (This is another way that LEDs are different from incandescent bulbs)
Being a semiconductor, an LED does need a certain minimum voltage threshold to conduct current.
So if you have three 3.2v LEDs wired in a string, a driver rated for 8-12v would be an appropriate choice.
---O---O---O---
The second thing is the amount of current that the driver is rated for. You usually want the driver current (mA) to match up with the rated LED current (mA). It's okay to use a little less current. For example, if the LED is rated for 350mA it's okay to use a 300mA driver, but be aware that the power will be a little bit lower and the LEDs might not be quite as bright.
Another thing that needs to be covered, when connecting two or more LEDs in parallel it is usually good to supply a little lower current that the sum of what all the LEDs are rated for.
You see, unless the voltage being supplied is at the lower end of the LEDs operational range, there is a tendency for all the current to start flowing through just one of the LEDs, rather than being spread out evenly between the LEDs, and then there will be a burnout. A 3.2v 350mA rated LED may be able to handle 3.4v 371mA, but try connecting three of these LEDs in parallel, naturally with a driver supply designed for 3 times the current which would be 1050mA, and there is a fair chance there could be a burnout. Lowering the power a little bit could avoid this. (The reason for this problem is because once the voltage gets on the high end of the operational voltage range of the LED, the amount of current that can flow through the individual LED quickly increases exponentially. The driver dropping the voltage at a given amount of current through the circuit isn't going to do any good because the circuit in this case is designed for more current than any single LED can handle.) To summarize, when powering multiple LEDs in parallel (or several different strings of LEDs in parallel) be conservative in estimating the amount of power they can handle. If you have two strings of 350mA, using a 600mA driver instead of 700mA wouldn't be a bad idea.
Oftentimes the LED specifications might not indicate an operational current (mA). In that case it's easy. Blue and white color LEDs are generally 3.2 volts. Take the power rating (Watts) and divide by 3.2 to find the current. For example, 1 Watt divided by 3.2 = 0.312 Amps, or 312 mA. (In this case most of these "1 Watt" LEDs are actually rated for 350mA)
The amazing thing about constant current drivers, if you had a driver with a wide enough functional voltage range, you could string 3 LEDs into a string or 5 LEDs into a string, and the amount of power supplied by the driver would be entirely proportional to how many LEDs there are. Remember, the driver is going to change the voltage until a certain current amount is flowing through.
So why don't we just use a resistor with an LED to limit current? This works very well for very low power indicator light LEDs, but when we're talking about more power the inefficiency of using a resistor starts becoming a significant factor. (It would basically require twice as much power)
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