Uncategorized

# Why the COB (Chip On Board) LED’s Forward Current vs Forward Voltage characteristic, shows 30V+ at 1A?

Ask QuestionAsked todayActive todayViewed 64 times1Thanks for submitting an edit. It is only visible to you until it’s been approved by trusted community members

I have an LED project using 100 Citizen CLU048-1212C4 LEDs.

I need to run each LED at 70W, and I want to run every 5 LEDs on 1 power supply.

I was thinking of using the MEAN WELL LSR-350-35 (it takes a long time to show PDF)

Video review of the power supply.

This screenshot is from the LED datasheet

I saw in the power supply datasheet:

INPUT Voltage Range : 90 ~ 132VAC / 180 ~ 264VAC by switch 240 ~ 370VDC (switch on 230VAC)

This screenshot is from the power supply datasheet

Where I live, the electrical power 220V and 50Hz.

I think if I connect LEDs parallel it’ll be good the current of the power supply (9.7A) will be divided on LEDs count so every LED will take 9.7A/5 = 1.94A for each LED DC voltage 36V I think that means 1.94*36=69.84W ~ 70W (automatically.)

• Is that power supply good for this project?

I feel confused from this screenshot from the LED datasheet. I think I need to drive the LEDs at ~38V to reach 1.9A, I’m not sure about that.

This screenshot is from the from the LED datasheet

• Please note that you can’t directly connect bare LEDs to a power supply. Or at least not to a constant voltage power supply. Or you can, but most likely you would not be happy about the results. You are missing a key component between the power supply and LEDs, and that’s called a LED driver. – Justme 3 hours ago

The MEAN WELL LSR-350-36 (there is no LSR-250-35) is not a good match for your project.

• LEDs need a regulated current to operate properly. The LSR-350-36 is a constant voltage power supply.
• You can approximate a constant current power supply from a constant voltage power supply, but that requires resistors in series with the LEDs and a voltage source several volts higher than the LED forward voltage.
• LEDs in parallel without balancing the current is not a good idea. The simplest way is to put a resistor in series with each LED. Without the resistors, the current will go to whichever LED has the lowest forward voltage. It will get all of the available current and burn out. Then the LED with the next higher forward voltage gets all the current and burns out. Repeat until all LEDs are dead. All your LEDs have the same rated forward voltage, but real LEDs are never identical.
• The 36V output voltage of the LSR-350-36 is too close to the LED forward voltage for it to be used with current limiting resistors.

Resistors waste power as heat, but they are the simplest way to limit the current.

If you can use the LSR-350-48 (48 V output voltage,) then you could do this:

simulate this circuit – Schematic created using CircuitLab

The way that works is like this:

1. IfIf = 1.8A (forward current of the LEDs.)
2. VfVf = 38V (forward voltage of the LEDs at 1.8A from the datasheet chart.)
3. PLEDPLED = If×Vf=68.4WIf×Vf=68.4W (power to each LED.)
4. R=Vpowersupply−VfIf=5.4ohmsR=Vpowersupply−VfIf=5.4ohms (series resistor value. Use 5.6 ohm because it is a standard value.)
5. PR=(Vpowersupply−Vf)×If=18WPR=(Vpowersupply−Vf)×If=18W (18 watts wasted in each resistor.)
6. Ptotal=4×(PR+PLED)=4×86.4W=345WPtotal=4×(PR+PLED)=4×86.4W=345W (total power consumed is just under the rated 350W for the LSR-350-48.)

The resistors limit the current to about 1.8A. The resistors also balance the current between the LEDs – no one LED can take up more of the current than it is supposed to.

The resistors (in total) also waste as much power as one LED would consume.

That is not efficient, but it is simple. Each of the resistors will waste 18 watts of power as heat – you need big power resistors to make it work.

That wasted power is the reason you should use proper drivers for each LED. Scaled up to your target 100 LEDs, that would be 1800 watts of wasted power.

Total power is something you need to consider, anyway. 100 LEDs at 70 watts each is 7000 watts of power. The outlets where you are can probably supply a bit over 3000 watts each. You’ll need to get power from outlets on separate circuits in your house in order to properly drive all those LEDs without causing a circuit breaker to trip.

Given the size of the project and the cost of the LEDs (I figure close to \$3000 just for the LEDs,) and the complexity of building power supplies, I’d suggest you look into purchasing proper constant current power supplies rather than trying to bodge things together.

Mean Well makes many other power supplies that would be better suited for your project.

The simplest (though maybe not the cheapest) solution would be to use individual 75 watt power supplies for each LED. You can adjust them to get the proper power output for your LEDs.

The ELG-85-42 would be the correct model. It will put out 1.8A of current at up to 42V. The voltage is determined by the current – it won’t destroy your LEDs.

The downside is that each power supply costs as much as the LED it drives. The upside is that your LEDs will operate properly and won’t waste a lot of power.

You’ve picked a very large project to start with. 7000 watts is a serious amount of power.

• You can buy components and get on with reaching the goals of your project, but it will cost you financially and you won’t learn much about the basics of LEDs.
• You can try to build your own power supplies to save money, but I think you will in the end spend more to replace damaged LEDs and power supply parts than you will save – and it will delay your project while you are learning.

No matter how you power your LEDs, keep in mind that each of them is going to need a heat sink to keep it cool.

This is a 100 watt LED work light that I use around the house:

The housing is basically one enormous heat sink. It is made of aluminum, with the LEDs solidly attached to the back plate to carry off the waste heat from the LEDs.ShareCiteEditFollowFlagedited 1 hour agoanswered 2 hours agoJRE47.2k88 gold badges7474 silver badges128128 bronze badgesAdd a comment1

7000W of LEDs? Are you lighting a stadium?

Well, since this project will make you blind, it wouldn’t be complete without the risk of electrocution, so let’s wire the LEDs in series.

As you noticed, the voltage across your COBs varies with current (and also temperature, and between each COB). So you can’t drive them with a constant voltage supply, it has to be constant current. You can, perhaps, parallel them, if they are well matched, but that’s pretty hazardous because when one COB fails open, then the driver will send the whole current to the others, so they will also fail in cascade.

So you want to run the COBs at around 2 amps, let’s look for a constant current driver that will do that…

Meanwell ELG-240-C2100 looks like it, it can output 120V 2A so it will power 3 COBs in series.

There are of course many more options regarding output voltage: higher means more COBs in series, and more dangerous.ShareCiteEditFollowFlaganswered 1 hour agobobflux43.2k33 gold badges5555 silver badges127127 bronze badgesAdd a comment0

1. About COB LEDs’ V-I Characteristics

Well you LED module is COB (Chip On Board), which means the many LEDs are serially squeezed within one module. You might like to read this tutorial The Basics of Chip on Board (COB) LEDs – Rich Miron, Digi-Key Electronics 2016aug03 for more details.

Usually one LED taking 1A has a voltage of approx 3V. So your module has approx 36V at 1A, implies there are 12 LEDs in series inside the module.

2. About COB LED’s input voltage

What’s the voltage used to power LEDs inside an LED light bulb? Asked 2 months ago Active 2 months ago Viewed 170 times

My answer explains that special AC mains to DC conversion uses the “Capacitive Dropper” accepts AC input in a range of 185 to 150VAC, like many smart phone chargers accepting 110VAC or 230VAC.

Update 2021apr13hkt2018

3. Using a CCS (Constant Current Source)

You are asking two questions, one on AC mains input. which I said it can be a wide range.

You other question is related to the DC voltage of the COB LED module (about 30V DC for a series of 12 LEDs at 1A).

Suppose now you want to use a DC voltage source to power the LED module of 12 LEDs, the standard trick is to use a CCS (Constant Current Source) of 1A, and the voltage source can be, a couple of Volts higher than 30V, perhaps 36V to even 48V. This is a bit confusing. You might like to get a rough idea by reading my answer to the following Q&A on how to use a CCS:

How can Rpi Python PWM GPIO pins control and dim LED strips of 1W 350mA?

This is also a good newbie friendly tutorial on power LED.

1W LED Tutorial (On why use a CCS) – Components 101