Running COB LED without driver (undervolted)

Question

!Hobby project!

I have a power supply form a printer (actually I have 3 :) ). The PSU is 24V, 1.25A. I want to connect some COB LEDs to this PSU without additional electronics. The idea is that the LEDs requires no driver if they are under-volted (am I right?).

I have found some cheap 10W rated 32-34V. I use 3 of them in parallel.

1. Will they light up at 24V? (I know, it is more like a "guess" question but no other documentation is provided with the LED)
2. There are COB LEDs out there that are "natively" running at 26V or 32V is the minimum voltage for a high power LED?

If question 1 and 2 fails, how to drive the LEDs with minimal electronics (using the PSU I have)?

Answer 1

Oh for pete's sake:

1. You can't "under volt" an led. Too little voltage and it won't turn on at all.
2. If you try to put leds in parallel, then you must provide each parallel line with its own current limiting.
3. You must use current limiting to drive your leds. An led operating at just over its forward voltage will draw just a little current. At a slightly higher voltage it will draw much more current - enough to burn out.
4. Leds of that type are typically operated with a constant current power supply. The voltage range you listed is the range in which the power supply can maintain the desired current.

Leds are not like light bulbs, despite the fact that both produce light. Read up on them before you destroy a bunch by treating them like incandescent light bulbs.

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You can't operate a 32Volt led on 24 Volts. You must get the voltage up to the minimum forward voltage, and then limit the current.

The simplest way would be to get a proper power supply.

To use your current power supply, you must use a boost regulator to generate a higher voltage. You then either use a series resistor to limit the current, or you use an active current limiting circuit - which a proper power supply will already have.

Answer 2

You can use a boost step up constant current driver. Something like a $5 Mean Well LDD module would do the trick. You would need one for each CoB.

Or you could use 18V CoBs. There is not many CoBs available between 18 and 24 volts.

UPDATE

You can use a supply with a voltage higher than 24v. You have to look at the datasheet for worst case maximum forward voltage which is probably around 27v.

You then may need a few volts headroom for the LED driver. If you don't care about efficiency or inconsistent brightness from one to another, you can use a current limiting resistor, instead of a constant current driver, and that will add a little voltage.

To do it correctly, you use a constant current driver for each CoB or get as driver that with a high enough voltage (e.g. 80v or more) to drive them all in series with one driver. A constant current driver output voltage can be greater than the CoB V_f. The output voltage is a function of the CoB's V_f.

Your supply is a constant voltage supply. They do not work well with LEDs. They can be used to power inexpensive ($2-$5) constant current DC/DC converters.