Hello All,

I bought some small LED PCB's from AliExpress. I'm trying to figure out what LED's they are using. The board has three resistors in parallel, connected to 6 leds in parallel. It's designed to be powered from 5v.

I can see from the resistor that they are 3x12 ohm in parallel resulting in 4 ohm. I assume they are doing this to ensure proper dissipation.

Now my issue - I can't figure out the forward voltage / milliamps of the LEDs. I asked and they said 0.5w, 3v and 120-150milliamp. I just can't make those number work with 4 ohm resistance.

If i use something like: https://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator the resistance comes out much lower - like 2.7 to 3 ohm (depending if i use 120 or 150 mA).

To get something around 4 ohm I have to drop the mA of each led down to around 80.

Are they just under powering the LED's or are the numbers I'm getting from them wrong? Or - maybe more likely - am I misunderstanding something?

Any input is much appreciated.

Best,
Olav

 

Are you sure the resistors are all in parallel...the math works if there is 1 resistor per 2 LEDs.

The forward voltage across the LED is likely to be above 3...probably more like 3.2 to 3.5.

 

Welcome to AAC!

I bought some small LED PCB's from AliExpress. I'm trying to figure out what LED's they are using. The board has three resistors in parallel, connected to 6 leds in parallel. It's designed to be powered from 5v.

A schematic or picture of the board and LED color would be helpful.

Operating from a 5V supply, 3 parallel strings of a 12 ohm resistor and 2 LEDs would make the most sense. If that's the case, the resistors are a higher wattage than they need to be.

 

Whether the LEDs and resistors are wired up all in parallel or 3 sets...you are looking at around 75 mA per LED.

So, your assumption that they are under powering the LED is correct, but that is a good thing.

So, the total LED wattage for the board should come in at about 1.5 watts. if I did the math correct.

 

Thank you for the input everyone! I really appreciate you taking the time! And sorry for not including a photo initially.

Yes I would say I'm pretty sure all resistors are in parallel. It's not the best photo below but you can still make out the traces. As far as I can see the outer ring is positive and connected to the + pad through all the 3 resistors.

They sell these as 3 watt, but I found that a bit hard to believe. If the linked calculator is to be believed, powering the LEDs at 150mA each the resistors would have to dissipate 1.8 watt. As I understand watt rating above 0.5 is not that common and these are cheap Chinese pcb's (ofc they could also just be taking the chance but..). I did notice however that manufacturers label their 3v / 150 mA LED's as 0.5w - so I'm assuming thats how they came to the 3w.

At 75 mA each it comes out to what sounds like a more resonable 0.9 watt to dissipate. Makes me think ElectricSpidey is probably correct. Just out of curiosity sir, could you elaborate on "but thats a good thing"?

For context, I'm designing my own board with some additional components. I'm just trying to ensure I match the max brightness which I'm pretty happy with.

 

I say it's a good thing because derating an LED increases its lifespan and reliability.

 

If the LEDs are rated at .5W each, that is 3W total.

Now my issue - I can't figure out the forward voltage / milliamps of the LEDs

Of course you can’t. But you could measure it.

 

Just to consider another option as three resistors in parallel is unusual.

Three sets of two LEDs in series with one resistor and then the three sets are wired in parallel makes more sense to me.

 

Just to consider another option as three resistors in parallel is unusual.

Three sets of two LEDs in series with one resistor and then the three sets are wired in parallel makes more sense to me.

But the picture makes it pretty clear that the three resistors are in parallel and that they, as a set, are in series with a set of six LEDs that are all in parallel with each other.

But it should be easy enough to verify both the connectivity and the values of the components, as well as the Vf of the LEDs and the total current drawn by the circuit as a whole via direct measurement with a cheap DMM.

If the LEDs have a Vf of about 3 V, then that is putting the total current at about 500 mA total, making total power dissipation about 2.5 W with about 1 W dissipated in the three resistors (so spreading that out among three resistors makes sense, though perhaps spreading it out a bit more would make better sense, but I don't know what physical size those resistors are. That leaves about 0.25 W per LED, which is half of the rated power, which is a very common rule of thumb.

Seems would seem to be working out reasonably.

@Olavko Designers seldom run components close to their rated specs. A common rule of thumb for many applications is that you don't ask a component to actually dissipate more than about half of the power it is rated for. By doing this, the life expectancy of the component is generally greatly increased.

 

Thank you the additional replies!

From roughly measuring by overlaying the photo with a ruler and going by the 26mm diameter of the board, the resistors seem to be 1812 package (or perhaps 2010).

Just for extra context my initial though was that the LED's might be 100mA or 60mA. And i thought if that was the case and I went for 150 ones, they might end up being dimmer (with the 4 ohm resistor set). Honestly I'm not even sure if it works like this. I just assumed a 60mA LED would be brighter at 60mA than a 150mA LED would be at 60mA.

100mA LEDs seem less common so my thought was thats probably not what they are using for a cheap Chinese PCB.

In any case I will get out a multimeter and measure as suggested. Reason for not doing it before is - besides being inexperienced - that I thought it wouldn't really get me to the mA rating of the LED's. But would still be nice to know the numbers. I will report back.

 

Sorry that would be 0805 resistors - i messed up with the metric vs imperial

 

Hello Again,

In case anyone is still interested I plugged the board into a power brick with an amp readout and it draws around 0,45-0,5 amp. So no chance the leds are putting out 3w. However to me it lines up pretty well with the LED's getting round 75mA each. It doesn't really do much to answer my original question about what the LED's are rated at - but probably it's just normal 150mA ones that are being undersupplied.

Best,
Olav

 

yes... i have some LEDs to test. they also show rated current of 150mA and Vf of about 3.1V but at that current they need a heatsink. current limiting can be done by either changing value of series resistor or PWM.
at current down to 30mA or so, heat is practically undetectable. the nice thing about LEDs is that they still output nice white light even when current is really low.in such case incandescent bulbs would change color to yellow, orange or red.

 

Ah thats a great input! Thank you for that! I was exactly worried that the light would not be that bright.

 

brightness does go down too but the light stays white. and even then it is still pretty bright.
this is set of 18 LEDs running only 20mA and it is actually bright enough that it is painful look at them directly.
this is 24V 0.060A (three LED strings are in parallel). i could put this on a heatsink and crank it up but there is no need.
if anything, i would rather add more LEDs, they are cheap
HL-A-2835D46W-S1-08-HR3 is only $0.02 a piece and that is in small quantities. when buying more, price goes down.