Sixty INDIVIDUALLY on/off switchable COB LEDs connected in parallel: how to drive?

Question

For an atmospheric custom ambient lighting project, consisting of 60 individually on/off switchable COB LED's, each rated 2 Watts, 12~14V, max 200mA, all connected in PARALLEL, i'm looking to solve two questions.

First I'll sketch the situation. Often times only 1 (out of 60!) Led will be switched on. Other times between 3 to 6 leds will be switched on. Rarely will there be more than 18 led's switched on and never all 60 at once.

Q1. As a single LED driver, will a 35W, 3A driver suffice for my project?
Q2. Will that tiny resistor on the cob pcb protect each led from high currents of the driver? Or will I have to manually solder resistors in series with each LED cob? If so, what Ohms / Power value am I looking for?

Answer 1

As the others said you will need a constant voltage power supply of 12-14V (your choice) capable of outputting enough current, which depends on how many LEDs you want to light at the same time. Since 12V supplies are quite cheap this shouldn't be a problem.

Note that normally "led drivers" are constant current, this is not what you need here, since the current will depend on the number of LEDs which are ON.

However manufacturers have discovered that when people who want to use "12V LEDs" (which already include a current limiting resistor and need a constant voltage supply) will only buy it if the label says "LED driver".

Confusion is the result. If it has "LED driver" written on the label, now you got to check if it's constant voltage (it will say "12V 3A") or constant current (it will say "3A 8V-18V", indicating the output voltage will adjust inside the range mentioned to keep the current constant). If you're really lucky it will say what it actually is on the label...

Now you will need individual switches to control your LEDs, considering the number, the easiest would be MOSFETs driven by switch registers or i2c IO expanders. You can use SMD dual FETs in SO-8 packages for small size. There are also power shift registers.

If you want to use PWM at high frequency to avoid blinking, make sure you can clock the shift registers fast enough.

Answer 2

Q1. As a single LED driver, will a 35W, 3A driver suffice for my project?

Maybe RdsOn should be < 0.1 Ohm for low V drop. of 20mV which reduces brightess by 6mA out of 200mA.

If I assume COB is designed the way it is rated...200mA @ 12~14V Rs=3.3 Ohms and RdsOn if = 0.1 Ohm is 3% drop or 20mV .

Your mileage may vary with RdsOn and thus will affect conduction losses and exact output current. Hopefully negligible.

Q2. Will that tiny resistor on the cob pcb protect each led from high currents of the driver?

YES

If the tiny R is 3.3 Ohms the current and brightness will be controlled by 12~14V, by the difference between V+ and low current Vf of COB I=deltaV/Rs It will be dim at 11.34V and thus using 12.0V, 0.66/3.3R= 200mA but if COB has high ESR, then more voltage may be needed thus loose spec ~14V in order to get 200mA.

This means process control of Vf may be good or not so 12~14V range is your choice to get max rated brightness or not.

COB should get warm 50'C but not burning hot.

Of course you need a supply > 30A such as a PC PSU 500W more or less depending on rating for 12V

Answer 3

1. I would use a 50W supply as 18*2=36W. But to be on the safe side, I would recommand 120W just in case all the 60 leds lit on together by mistake, while you are away and come back three days later. You never know. Depends on you budget. Choose an adjustable power supply and play with it. Constant voltage.

2. R = (Vsupply - Vled) / I ,so R = (12-3)/0.2, R = 45ohms. If I is 0.15 R = 60ohms. I would recomand 60 ohms. Check if the resistance are between these two values. Note that there are two resistors, and, as I imagine, two parallel circuits on the COB. Each should be rated 1W. They may be strong enough to sustain 1W, yet barely enough. If the these resistors are much less than 45 ohms, then you'll have to add resistors for each COB. But I don't think so. You can also increase the existing resistance by adding resistors to make the leds more stable and long lasting albeit less powerful. Say, from 45 to 60ohms.

Answer 4

Q1. As a single LED driver, will a 35W, 3A driver suffice for my project?

It would suffice. A 12v, 60 watt, 5A (25 LEDs) would do better. 3 Amp will works well for 15 of these LEDs. If the power supply is insufficient, likely the only drawback will be the LEDs will not be as bright. 200 lumens is a very bright LED. In most cases the human eye would not perceive the dimming associated with insufficient power.

My concern would be the amount of time an LED will be on continuously. A 2W LED will get very hot. You may need a heatsink. CoBs are usually designed to be mounted directly to a heatsink.

Heatsink USA 2.425" wide @ 50¢ per inch

Q2. Will that tiny resistor on the cob pcb protect each led from high currents of the driver?

Yes, if powered with a 12v power supply. If you want anything other than a 200 lumen LED, do not buy these LEDs, buy some that work for your project.

These LEDs are for lighting, not to be viewed directly.