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Why is this LED not 20 Watts?

Bluejets

Oct 5, 2014
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That would be "input" readings I assume?
.....in that case I'd say .....solved
 

Colin Mitchell

Aug 31, 2014
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You have bought the wrong driver.

The driver you are using will only deliver 20 watts when the voltage across the string of LEDs is about 33v. If each LED is say 3 to 4 volts, you need 10 LEDs.
 

Colin Mitchell

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What is the characteristic voltage across the 20 watt LED you are using?

The characteristic voltage is the voltage that naturally develops across the "chip" or "chips" that are incorporated in the LED device being illuminated.
Sometimes there is one chip or 2 chips or 3 chips.
You have to know this before doing anything else.
I am surprised that no-one has mentioned this important fact.
This is where you start.
 

Colin Mitchell

Aug 31, 2014
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I am surprised that no-one has mentioned this important fact.
We've only mentioned it three or so times.
Show me where you have asked him to find out the characteristic voltage of his 20 watt device.
 

(*steve*)

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Show me where you have asked him to find out the characteristic voltage of his 20 watt device.

Start by reading every one of my posts in tho thread (except one) where I implore him to measure the voltage across the LED (and the current through it) when connected to this driver.
 

Colin Mitchell

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If you take the reading already provided, the 9.1 watts @ 600mA represents 15v across the LED for 100% efficiency. If we take 80% efficiency, the LED is actually a 12v device with internal resistor.
This is basically the wrong type of LED to connect to a constant current supply however if the 12v LED is 20 watts, the current it requires is 1600mA and you should be looking at a constant current supply of 1500mA to 1600mA capability. It must also be capable of delivering slightly over 12v (13v to 15v).
 

Colin Mitchell

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(*Steve*) Can you measure the current drawn by your LED when connected to this driver and the voltage across it?

Why did you ask this question when you know the driver is a constant current device that delivers 600mA.
 

(*steve*)

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Because I want to check an alternative explanation. Perhaps its really a 300mA source. Perhaps the LED has a Vf above the max voltage for this driver and it is incapable of supplying 600mA at this voltage.

Although I assume I know the cause, the current reading will confirm it, and if it is one of the other issues, the current reading will also be valuable.

It was noted that the current would vary with input voltage. I'm discounting that as a probable cause, but I'd prefer to have actual information than guesses and conjecture.
 

BobK

Jan 5, 2010
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Would somebody please close this thread unless the thread starter will give us the specs of the LED he is using so that we can reccommend a proper driver?

Bob
 

Al Slitter

Nov 4, 2016
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I decided to go at this different from me testing the driver and LED with my metering system.
What I did was use my multimeter to test the voltage and current at the LED.
As you are aware a 20 Watt Led needs between 32 and 35 volts DC to operate properly and a current of 600 mA.
While the voltage is important it is the amperage that really makes the LED work at peak performance.
The results that I got using my multimeter was a voltage around 30 volts DC not perfect but some what acceptable.
However on the current side is was .295 of an Amp well below the 600mA required to run a 20 Watt Led.
If we do the math on working out the Wattage we take the voltage 30 volts and multiple it by the amperage .295mA
we get a result of 9.145 Watts. Which is what my testing meter showed.

The bottom line is that the driver was never designed to drive a 20 watt Led as the current is more in line with what is required by a 10 watt LED.

I am satisfied that both my metering system and my multimeter results are in line with each other.

Case Closed
 

Al Slitter

Nov 4, 2016
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I check the link out and while this is not the seller they are it seems using the same manufacture for the drivers.
I would like to point out that the LED's shown here have a full aluminum backing which makes soldering the wires on to it very problematic due to heat dissipation. For me I will stay away from these types of chips and stay with the 10 chip in series version that have the positive and negative solder points on wings off the side.
I hope you follow my description.
I have up loaded a chip design that makes soldering much easier.

Winged Chip.jpg
 

(*steve*)

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I suggest that the driver may be a 10W driver relabelled as 20W. It seems to be delivering 300 mA which I'd what one might expect.

Also Al, your understanding is a little poor in some respects.

A 10W LED could be 12V 600mA or 24V 300mA. The actual voltage and current depend on the way the individual LED elements are connected. When operating correctly the voltage times current equals the rated power.
 

Al Slitter

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I suggest that the driver may be a 10W driver relabelled as 20W. It seems to be delivering 300 mA which I'd what one might expect.

Also Al, your understanding is a little poor in some respects.

A 10W LED could be 12V 600mA or 24V 300mA. The actual voltage and current depend on the way the individual LED elements are connected. When operating correctly the voltage times current equals the rated power.
Thank you Steve for the additional information.
I was not aware that a 10Watt Led could operate at 24 volts DC.
Yes you are right about my understanding is a little poor in some respects.
It is for this reason that I am here now!

Thanks again!
 

(*steve*)

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I was not aware that a 10Watt Led could operate at 24 volts DC.

A lot depends on the construction of the LED. An individual element inside one of these compound devices, when operated properly will have about 3.5V to 4V across it (this is a very rough estimate).

If these are arranged in multiple strings of 3, the LED is nominally a 12V device, if in strings of 6, then it is nominally a 24V device. And clearly these are not the only options, so the nominal voltage can vary widely.

What is most important for correct operation is the current. It might be that each string of these elements is rated to operate at up to 50mA. If you have 6 of these in parallel, the array is rated for up to 300mA.

One major problem is that the voltage is not fixed. As the elements heat up, the voltage across them necessary to maintain the desired current falls (this is exactly the opposite to what a light bulb does). If LEDs are supplied with a constant voltage, this causes the current to increase as they heat up. Because of this increased current they heat up further, lowering the required voltage, thus increasing the current heting them more... and so on until they fail.

As a consequence of this behaviour, LEDs (especially high power LEDs) are driven from a constant current source. The effects of heating now result in slightly lower power, so the system becomes stable and nothing burns out.

Unfortunately, if you ever purchase a high power LED it will be rated with a power and a nominal voltage (if you're lucky). It's up you you to determine the current (divide the power by the nominal voltage) and get a suitable driver.

Because the voltage can vary, constant current sources (similar to your LED driver) will generally specify a nominal power, a current, and an output voltage range.

So, to match a LED with a driver:

  1. Determine the current required by the LED
  2. Choose a driver with the appropriate power rating delivering the current calculated above
  3. Check that the output voltage range for the driver includes the nominal voltage for the LED
Unfortunately, LEDs can be sold as something they're not, and so can drivers. However, in general you can probably argue with your supplier if the actual capabilities or specs are not what you were told they were when you bought them.

Another thing you may not have considered is that LEDs similar to the ones you're using require heatsinking to keep them cool. If they are allowed to operate without a heatsink at anywhere near their rated power, they will heat excessively and fail quite quickly.
 

Al Slitter

Nov 4, 2016
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A lot depends on the construction of the LED. An individual element inside one of these compound devices, when operated properly will have about 3.5V to 4V across it (this is a very rough estimate).

If these are arranged in multiple strings of 3, the LED is nominally a 12V device, if in strings of 6, then it is nominally a 24V device. And clearly these are not the only options, so the nominal voltage can vary widely.

What is most important for correct operation is the current. It might be that each string of these elements is rated to operate at up to 50mA. If you have 6 of these in parallel, the array is rated for up to 300mA.

One major problem is that the voltage is not fixed. As the elements heat up, the voltage across them necessary to maintain the desired current falls (this is exactly the opposite to what a light bulb does). If LEDs are supplied with a constant voltage, this causes the current to increase as they heat up. Because of this increased current they heat up further, lowering the required voltage, thus increasing the current heting them more... and so on until they fail.

As a consequence of this behaviour, LEDs (especially high power LEDs) are driven from a constant current source. The effects of heating now result in slightly lower power, so the system becomes stable and nothing burns out.

Unfortunately, if you ever purchase a high power LED it will be rated with a power and a nominal voltage (if you're lucky). It's up you you to determine the current (divide the power by the nominal voltage) and get a suitable driver.

Because the voltage can vary, constant current sources (similar to your LED driver) will generally specify a nominal power, a current, and an output voltage range.

So, to match a LED with a driver:

  1. Determine the current required by the LED
  2. Choose a driver with the appropriate power rating delivering the current calculated above
  3. Check that the output voltage range for the driver includes the nominal voltage for the LED
Unfortunately, LEDs can be sold as something they're not, and so can drivers. However, in general you can probably argue with your supplier if the actual capabilities or specs are not what you were told they were when you bought them.

Another thing you may not have considered is that LEDs similar to the ones you're using require heatsinking to keep them cool. If they are allowed to operate without a heatsink at anywhere near their rated power, they will heat excessively and fail quite quickly.
Thank you Steve for sharing this information it is much appreciated.
 
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