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!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)?

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  • \$\begingroup\$ Note: this is for a hobby project not high tech stuff :) \$\endgroup\$ – Ultralisk May 4 '17 at 21:16
  • \$\begingroup\$ Leds are current controlled. If the voltage is too low, no light. If the voltage is too high without a current limiter the it will glow brightly for a very short time and then burn out. You cannot "under volt" an led. You must reduce the current. \$\endgroup\$ – JRE May 4 '17 at 21:48
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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.


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.

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  • \$\begingroup\$ 3. That's what I meant. \$\endgroup\$ – Ultralisk May 4 '17 at 22:00
  • \$\begingroup\$ ok. I ma going to order a LED+driver kit. \$\endgroup\$ – Ultralisk May 4 '17 at 22:01
  • \$\begingroup\$ "an led operating at just over its forward voltage will draw just a little current" - That's what I meant! An 'undervolted' LED will draw a minimal amount of current, WAY WAY WAY under the critical value. Simply put a "3V" LED at exactly 3V without any other resistor or limiting circuit and it will work forever. All those cheap Chinese LED key chains lamps are working this way. So, in a way your no. 3 contradicts no. 1 :) \$\endgroup\$ – Ultralisk May 5 '17 at 18:24
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    \$\begingroup\$ The cheap chinese lights have voltage higher than the forward voltage of the LEDs. They depend on the internal resistance to keep from burning out the LEDs. \$\endgroup\$ – JRE May 11 '17 at 18:42
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    \$\begingroup\$ My last comment was about the key chain lamps. Don't try to apply that to a 30W light - that will get you in a world of hurt. \$\endgroup\$ – JRE May 12 '17 at 13:00
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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 Vf. The output voltage is a function of the CoB's Vf.

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.

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  • \$\begingroup\$ why 18v? why 18 and not 24? \$\endgroup\$ – Ultralisk May 9 '17 at 11:11
  • \$\begingroup\$ The supply needs to be higher than the LED's forward voltage. 24V is the typical voltage at the test current and temperature. In real life, LEDs are not typical. As current increases so does forward voltage. I have one CoB that is spec'd at 34V but the real life Vf is 39V. \$\endgroup\$ – Misunderstood May 9 '17 at 11:21
  • \$\begingroup\$ you mean to use LED rated 18V with my 24V power supply? \$\endgroup\$ – Ultralisk May 9 '17 at 11:23
  • \$\begingroup\$ Yes, see my update \$\endgroup\$ – Misunderstood May 9 '17 at 12:24

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