3W 12V COB LED with 12V Campervan system

Question

I have an LED downlight in the camper we just bought but unfortunately the module inside is dead. I would like to replace it with another LED module/chip for cheap.

The light we have: http://www.pro-car.de/en/products/led-lights/surface-mounted-power-led/surface-mountedbuilt-in-power-led/

Dead module:

I have purchased this.

The camper circuit is run from a 12 V "leisure" battery.

It seems the light is on an independent switched circuit.

My logic has lead me to purchase the 12V COB LED commented with the hope to just directly connect it to the circuit where the previous module has been removed.

After further research I am confused with a few questions...

1. Does the LED require a resistor and/or LED driver to safely operate OR
   • a) does it already have this built into the chip.
   • b) it doesn't require either as it is rated to 12V.
2. What happens when the camper is plugged into a site and we have full mains power?

Any suggestions to cheap suitable drivers or alternative replacement LED module/chip are welcome.

Answer 1

Tom, your first photo shows a complete lighting circuit with a heatspreader printed board circuit and the COB LED and its boost converter/current regulator on it, while you second photo shows only a COB LED. No heatspreader board, no circuit. And the form factor doesn't even match so you cannot put this on the original board.

You cannot connect this second LED to your car. It will not work/blow up in an instant for two reasons:

• Missing boost converter/current regulator - without this a COB LED won't even light up on 14V (board voltage is 14V in a car!), most reach their nominal current at more than 30V due to the series connection of the individual LEDs on the lattice.

• Missing heat spreader and heatsink - you need this. Without a heatsink and spreader, the LED will heat up itself to destruction within a second.

Answer 2

This looks like it doesn't have any resistor built in – which really means it can be fine if driven with a constant 12 V (or lower) and if the individual LEDs (this is a multi-LED chip) are well-balanced and meet their specs exactly.

If that's not the case, you can get into trouble. LEDs are semiconductor devices. When they heat up, they conduct better, leading to more current, leading to more heat, leading to thermal runaway.

So, it would be a wise idea to actually limit the current through that LED to what the specs say for a maximum: 350 mA (the fact that the spec sheet says "mAh" doesn't inspire any confidence in the seller's abilities at understanding what he's selling).

I'll add a bit of safety margin and go for: the max current should be 300 mA.

Now, let's look at what means you'd have to limit current:

• Linear voltage/current controllers: These are nice and easy, but they'll need to drop some 0.7 V to 3 V, just to be able to regulate. That way, you'd never be able to run the LED at the spec'ed 12 V (only at V_bat-V_drop, i.e. max 11.3+ V for a fully charged battery)
• Switch-Mode Buck/boost converters: much more complicated, but your broken module seems to have something like that, so it's quite possible to use such circuitry. I'll rule this out here – if you wanted to go that route, you'd much rather have bought a 3V / 1.2A module rather than a 12 V / 300 mA module.
• since linear regulation doesn't work (see above), we might just as well use simple resistors. So, let's say that we guess (we have to guess, since what you've bought doesn't come with a proper datasheet, please don't buy stuff without data sheets) that at 10.8 V the LEDs would be happy to draw 180 mA (lower value from the product page) and at 14 V (what I'd assume is the max value to expect from a slightly overcharged battery) max 300 mA. So, from that upper value, we get that over the resistor would be a voltage drop of 14 V - U_LED = 2 V, and divide that by the 300 mA current, we get R_min = (2/0.3) Ω = 6.67 Ω. At the 180 mA, that resistor would still have a drop of 1.2 V, which feels "tolerable" for a 12 V LED. Again, this is all total guesswork, because you've bought something without a datasheet.

Don't buy stuff without datasheets. There's enough European (I think you are in Germany, Denmark, Sweden or Benelux) distributors that can sell you stuff at reasonable prices with a datasheet that has the least necessary information, such as current/voltage curves and thermal properties. You'd get something that a distributor is willing to sell you without risking their reputation. That makes a lot of difference. _{Reichelt,Farnell,Mouser,Transfer Multisort Europe, and many more}

Answer 3

1. With those components, you pretty much never know what you're going to get, so either:

   • It already has the necessary resistor built into the chip, and will work fine for many months/years, or
   • It doesn't have an adequate resistor built-in, and may overcurrent, dimm, then burst in a matter of seconds to minutes after first being powered-on. Since it's so inexpensive, I'd personally probably buy 2, install 1, then if the first one blows, install the 2nd with an 82ohm series resistor.

2. When you plug your RV into mains power, it has an internal converter that downconverts to the 12VDC to run all of your low-power stuff (lights, vent fans, radio, etc.), so that shouldn't have any noteworthy effect on this problem.