Help understanding forward current in 3W RGB LED

Thread Starter

Sfantry

Joined Apr 25, 2020
16
Hi,

I am working on a project where I will use multiple LEDs like the 4-pin 3W RGB LEDs that I purchased on eBay (https://www.ebay.com/itm/124011607823). The specifications claim the following forward voltages: Red: 2.0-2.2v Green: 3.2-3.4v Blue: 3.2-3.4v. Each LED has a forward current rating of 700 mA.

i have been thinking of using an LM1084IS-3.3 voltage regulator (5A) for this project. I want to use 9 of these LEDs, which would require almost 19 amps!

i decided to test these numbers and I connected 1 LED (red only) to my bench power supply, set it at 2.1 V, connected my multimeter in series, and measured about 34 mA. The LED was full brightness (Blinding). I then tested the Green at 3.3 V, and measure 116 mA.

Can this possibly be correct? The datasheet states 700 mA but if it's only 116, that seriously changes the power requirements. Thanks for your help.
 

Papabravo

Joined Feb 24, 2006
13,714
Hi,

I am working on a project where I will use multiple LEDs like the 4-pin 3W RGB LEDs that I purchased on eBay (https://www.ebay.com/itm/124011607823). The specifications claim the following forward voltages: Red: 2.0-2.2v Green: 3.2-3.4v Blue: 3.2-3.4v. Each LED has a forward current rating of 700 mA.

i have been thinking of using an LM1084IS-3.3 voltage regulator (5A) for this project. I want to use 9 of these LEDs, which would require almost 19 amps!

i decided to test these numbers and I connected 1 LED (red only) to my bench power supply, set it at 2.1 V, connected my multimeter in series, and measured about 34 mA. The LED was full brightness (Blinding). I then tested the Green at 3.3 V, and measure 116 mA.

Can this possibly be correct? The datasheet states 700 mA but if it's only 116, that seriously changes the power requirements. Thanks for your help.
Your power supply has some internal resistance which is limiting the current at the voltage you have chosen. You can run the LEDs from a higher voltage with a resistance that will allow more current without popping them. Try putting a resistor in series with the LED and set the supply output to 12@V, but limit the current to say 100 mA and see what happens. You know how to do that calculation -- right?
 

Papabravo

Joined Feb 24, 2006
13,714
Your power supply has some internal resistance which is limiting the current at the voltage you have chosen. You can run the LEDs from a higher voltage with a resistance that will allow more current without popping them. Try putting a resistor in series with the LED and set the supply output to 12@V, but limit the current to say 100 mA and see what happens. You know how to do that calculation -- right?

The noise level on the EBay site is way too much to be helpful.
 

ronsimpson

Joined Oct 7, 2019
677
Red: 2.0-2.2v ………………….. 34 mA
You are thinking of a LED as being like a light bulb. It is not. A LED is a current device not a voltage device.
If you have a current limit on the power supply, set it to 170mA or what ever the rated current is. With rated current going through the LED the voltage will be 2 to 2.2 volts depending on temperature and many other factors.
It is not good to put a voltage on a LED.
You can use a 5.0V power supply and add resistors to limit the current. Example: 5V-(about 2.1v)=voltage across the resistor. Now do the math to get the right resistor value. 2.9V on the resistor, current =170mA what resistor do you need?
--------------
In your case you used 2.1V and got 32mA and 3.3V and got 116mA (two different colors). Next time you with a different LED from a different batch you might get 50mA and 2A. It is too unpredictable.
 

Thread Starter

Sfantry

Joined Apr 25, 2020
16
Welcome to AAC!

Please post the datasheet.
Hi thanks for welcoming me.

I wish I could post the datasheet but unfortunately the eBay seller can't get me one. I knew I would be on my own buying cheap eBay stuff. The best I can do is reference the link above to the listing where they have some technical info. Sorry
 

dl324

Joined Mar 30, 2015
10,733
I wish I could post the datasheet but unfortunately the eBay seller can't get me one.
It's best to ask questions before buying.

You can test the LEDs at the rated current by using the nominal voltage and a resistor.

Using a 3.3V regulator without resistors isn't a good idea. What are your supply voltage options?
 

Thread Starter

Sfantry

Joined Apr 25, 2020
16
Thanks PapaBravo. I will try that tomorrow at 12V. I can calculate the resistor value for it. R = Vsource - Vf / If. So then if I plug in 12V - 2.1V / 100 mA = about 100R.

I have been setting my (home made from a PC) PSU to the Vf of the LED without using a resistor. So now my question is this, if I use about a 15R resistor and get about 700 mA, would the LED be significantly brighter than with the 100 mA of current?

Thanks so much!
 

Thread Starter

Sfantry

Joined Apr 25, 2020
16
It's best to ask questions before buying.

You can test the LEDs at the rated current by using the nominal voltage and a resistor.

Using a 3.3V regulator without resistors isn't a good idea. What are your supply voltage options?
Hi dl324,

I don't currently have any power options selected. I'm looking for inexpensive, as it's just for a hobby. The project is to make a DMX controlled light like they have in clubs with 2 motors to control movement and the light head with 3 rows of 3 LEDs per row. I will base my power on the requirements and right now I'm trying to determine the LED power requirements out. I can use a 110v to 12V DC power supply, but I was trying to keep the size down to fit in the housing. I was planning on using LDO linear regulators to power the LEDs like the LM1084IS-ADJ which I already have a few of. It has a Max output of 29V. I am going to play with some higher voltages to test upping the current with resistors.
 

Audioguru again

Joined Oct 21, 2019
1,475
An LED is NEVER fed a voltage (from a power supply), instead it is fed a calculated current through a current-limiting circuit or resistor. Without a datasheet, you do not know the forward voltage at different currents and at different temperatures. The sales sheet says 3.2V-3.4V but does not say at what current or temperature. 3W and 700mA are probably the absolute maximum allowed and only if the LED is cooled properly because it will get very hot. 3.4V x 700mA= only 2.38W so at 3W the forward voltage will be higher.
 

Thread Starter

Sfantry

Joined Apr 25, 2020
16
An LED is NEVER fed a voltage (from a power supply), instead it is fed a calculated current through a current-limiting circuit or resistor. Without a datasheet, you do not know the forward voltage at different currents and at different temperatures. The sales sheet says 3.2V-3.4V but does not say at what current or temperature. 3W and 700mA are probably the absolute maximum allowed and only if the LED is cooled properly because it will get very hot. 3.4V x 700mA= only 2.38W so at 3W the forward voltage will be higher.
Thanks, I knew going into this that the information was going to be limited being as I purchased it from China on eBay. I now understand that I need to do the calculations for the current and not the Voltage. I will use a 12V source and start out with about 100 ohms resistor to get about 100mA through the LED and then start increasing the current by decreasing the resistor. At about 15 ohms, I should be getting about 700mA through the Red LED with 12V.
 

Papabravo

Joined Feb 24, 2006
13,714
I hope you don't manage to let the magic smoke out before that happens. I would have a fan and a fire extinguisher handy if I were you.
We once had a technician nicknamed "Fast Eddie" who touched both terminals of a Big Blue Capacitor with a Craftsman screwdriver and couldn't pull it away fast enough to prevent welding the screwdriver to the terminals and creating a large degree of consternation besides making a mess. Don't follow in his footsteps.
 

Thread Starter

Sfantry

Joined Apr 25, 2020
16
I hope you don't manage to let the magic smoke out before that happens. I would have a fan and a fire extinguisher handy if I were you.
We once had a technician nicknamed "Fast Eddie" who touched both terminals of a Big Blue Capacitor with a Craftsman screwdriver and couldn't pull it away fast enough to prevent welding the screwdriver to the terminals and creating a large degree of consternation besides making a mess. Don't follow in his footsteps.
Hahahaha Fast Eddie wasn't fast enough. I did manage to make some magic smoke just a few minutes ago. I don't have any resistors rated for anything more than 1/4w but I wanted to try anyway just to see if I could make it light with 700 mA. The PSU read about 200 mA before the resistor disentigrated. I was prepared for it though and was careful. I have a fire extinguisher near the bench, luckily I didn't need it. I guess I'll get some heavy duty resistors on order so I can complete trials. Thanks for all your help.
 

WBahn

Joined Mar 31, 2012
25,750
Thanks, I knew going into this that the information was going to be limited being as I purchased it from China on eBay. I now understand that I need to do the calculations for the current and not the Voltage. I will use a 12V source and start out with about 100 ohms resistor to get about 100mA through the LED and then start increasing the current by decreasing the resistor. At about 15 ohms, I should be getting about 700mA through the Red LED with 12V.
Be sure to use resistors rated for the power you are asking them to dissipate!

You will have about 10 V across 100 Ω, so about your 100 mA. Fine so far. But how much power is that?

That's 1 W of power. That's actually a surprisingly large amount of heat and if you just put a 1/8 W or 1/4 W resistor there you will likely smoke it very quickly. You want to use at least a 2 W resistor and you want it positioned so that the heat can dissipate safely.

If all you have is 1/4 W resistors, then you can build a composite resistor that can handle that power for testing purposes. To get the ability to handle 2 W, you need 8 resistors that all share the current pretty evenly. Step up to 9, because that's a perfect square, and make three strings of three resistors (300 Ω each string) and then put the three strings in parallel (back to 100 Ω). Mount these resistors so that they have plenty of free air around them and so that they are not one above another. If you have a small fan, have it blow on them, though this probably won't be needed.
 

Thread Starter

Sfantry

Joined Apr 25, 2020
16
Be sure to use resistors rated for the power you are asking them to dissipate!

You will have about 10 V across 100 Ω, so about your 100 mA. Fine so far. But how much power is that?

That's 1 W of power. That's actually a surprisingly large amount of heat and if you just put a 1/8 W or 1/4 W resistor there you will likely smoke it very quickly. You want to use at least a 2 W resistor and you want it positioned so that the heat can dissipate safely.

If all you have is 1/4 W resistors, then you can build a composite resistor that can handle that power for testing purposes. To get the ability to handle 2 W, you need 8 resistors that all share the current pretty evenly. Step up to 9, because that's a perfect square, and make three strings of three resistors (300 Ω each string) and then put the three strings in parallel (back to 100 Ω). Mount these resistors so that they have plenty of free air around them and so that they are not one above another. If you have a small fan, have it blow on them, though this probably won't be needed.
Thanks for the idea, I will try that tomorrow while I wait for some 3W resistors to show up in the next few weeks.
 

WBahn

Joined Mar 31, 2012
25,750
Thanks for the idea, I will try that tomorrow while I wait for some 3W resistors to show up in the next few weeks.
You also need to heat sink those LEDs!

If 200 mA through 100 Ω (4 W) smoked a resistor, think of how much more easily you will damage those LEDs.
 

Thread Starter

Sfantry

Joined Apr 25, 2020
16
You also need to heat sink those LEDs!

If 200 mA through 100 Ω (4 W) smoked a resistor, think of how much more easily you will damage those LEDs.
I haven't figured that out yet. I don't know if a copper layer on the PCB with vias will work.
 

WBahn

Joined Mar 31, 2012
25,750
Track down information about similar LED modules and read up on what they have to say about proper thermal management.
 
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