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I have an amplitude modulation LED driver (a dimmable constant current reduction LED driver.) It can power an LED up to 30W, 750mA, 6-40V. The LED I am connecting is good for 750mA, 12V. Will it be too much voltage?

I hear people say too much current will kill the LED and way too much voltage will wear it out with excess heat over time, but no one seems to detail out when it is too much. How much voltage is too much? What does it mean when the LED driver has a flexible range of voltage?

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3 Answers 3

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LEDs are generally designed for a specific current, you are advised to not exceed that current.

What is important to know is that the voltage-current relation of LEDs is such that it is difficult to know what voltage to apply to get a specific current. In addition to that, a small change in voltage can result in a large change in the current.

This means that if we apply a voltage to the LED which is slightly too high, a lot more current than we want could flow.

Also, the current - voltage relations is dependent on temperature and it is dependend on the individual LED. LEDs that are made in the same "batch" are often very similar but there can be significant differences between batches of LEDs even from the same manufacturer.

To deal with these uncertainties and risking overdriving and destroying the LEDs we should not apply a voltage to the LEDs, instead we set the current.

That means that the voltage will vary over temperature and between LEDs but that's OK as long as the current is kept under control. That current doesn't need to be accurate as long as it stays below the maximum current for the LED.

Your LED driver is actually OK for this LED. Your driver delivers 750 mA (same as the LED) and supports voltages between 6 and 40 V. Your LED has a nominal voltage of 12 V which falls within that range.

So with this LED the driver will deliver 750 mA (which is good) and the voltage will be determined by the LED at around 12 V (also good).

As this driver supports up to 40 V that means you could even power up to 3 of these LEDs in series with this driver. In series, the voltages add up so 2 LEDs = 2 x 12 V = 24 V, 3 LEDs = 3 x 12 V = 36 V and that is still below the 40 V that the driver supports. The current will stay 750 mA which is what is needed.

Also very important: keep your LED cool, 750 mA at 12 V means about 9 Watt so the LED will get hot so a heatsink will be needed. I would consider a heatsink of around 10 cm x 10 cm. The cooler you keep the LED, the longer it will last!

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  • \$\begingroup\$ "(...) and the voltage will be determined by the LED at around 12 V (also good)." This part is the part that confuses me most. How is the voltage determined by the LED? Is it better for the LED to have a smaller driver (e.g. 15W, 750mA, 6-20V) or doesn't it matter? \$\endgroup\$
    – hfossli
    Jul 7, 2021 at 12:46
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    \$\begingroup\$ How is the voltage determined by the LED? Do you understand that with a resistor, if I apply a voltage, a current will flow with value I = V/R (that's Ohm's Law). Equally, if I force that same amount of current through the same resistor, a voltage will develop: V = I * R. The same is more or less true for an LED (although it does not follow Ohm's Law but just ignore that). If we force a voltage across the LED, a current will flow. Equally, if we force a current through the LED, a voltage develops across the LED to allow that current to flow.... \$\endgroup\$ Jul 7, 2021 at 12:51
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    \$\begingroup\$ The voltage will be the exact voltage that the LED needs to allow that current to flow. Just realize that voltage and current have a certain relation. You set one, the other follows. Is it better for the LED to have a smaller driver You're asking if the LED would benefit from the driver supporting a smaller voltage range. It makes no difference. The driver sets 750 mA and the LED develops a voltage (say 12 V). It is of no consequence to the LED if the driver can support a voltage up to 20 V or up to 100 V or even 900 V. The LED only cares about the current... \$\endgroup\$ Jul 7, 2021 at 12:52
  • \$\begingroup\$ Excellent! Thanks! \$\endgroup\$
    – hfossli
    Jul 7, 2021 at 13:01
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What does it mean when the LED driver has a flexible range of voltage?

It means exactly that - it has a flexible range of voltage. You have a constant current driver. This will always try to output exactly 750mA. It will adjust the voltage to make sure the current is 750mA. This one can't go lower than 6V or higher than 40V.

750mA happens to be the right current for your LED, so the voltage should also settle at the right voltage for your LED, about 12V.

You might be confused because in most projects (apart from LEDs) we use constant-voltage power supplies rather than constant-current ones. Those supplies will try to keep the voltage constant, and adjust the current to make sure the voltage stays the same. But this is a constant-current power supply and it does the opposite of that.

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The voltage is dependent on the LED junction, if the current is limited, either through a regulator or a resistor, the voltage at the LED will be whatever is the junction voltage.

If the current is not limited, and the voltage exceeds the junction, the you will have infinite current, which would destroy the LED, if it is below, the led won't light up.

This is a bit of a simplification because the junction voltage is dependent of the current as well, but in a general manner, LEDs are always current driven, either by limiting the current through a resistor or a regulator.

In your case, the regulator will adjust the proper voltage to deliver the current. 6-40V only refers the range it can go at, which will be fine for your led.

Note that at this power, the LED needs some proper heatsink.

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