SMD LED - trying to identify/determine equivalent in another color

Thread Starter

Taymar

Joined Jan 13, 2017
19
Hello all,

I have a set of small white SMD LEDs, used for the overhead map lights in a vehicle. I need to replace them with cooler color temp versions, but am having trouble identifying a suitable replacement. I can find plenty that match the dimensions, but none which are rated for the voltage these seem to be receiving.

I've attached a photo of the LED itself, and also the driver board.

Here's what else I've been able to determine so far:

- the white housing measures 5mm x 5mm x 1.5mm

- it receives approximately 8-9 volts at the pins, the driver board appears to step it down from the vehicle's 12v supply.

- I cannot see any markings on the LED itself, but could desolder it in case there is likely to be relevant info on the back.

- I would like to replace it with something as similar as possible in terms of specs, but with a color temperature of 5500k or higher if available.

Thank you very much for any assistance in identifying these LEDS.


ledresized.jpg driverboardresized.jpg
 

Thread Starter

Taymar

Joined Jan 13, 2017
19
Thank you very much, this is a big help. Am I correct in assuming that if I get a new LED that's rated for '3.4v absolute maximum' and in my application it's receiving 9v, that's not going to work correctly?

I was able to follow the circuit traces on the PCB, and here's a quick sketch of how it appears to be wired. My electronics knowledge is very basic, so apologies if this is a stupid question - Is the fact that they're wired in series why this unit can handle a 9-10v input load (as the voltage is split between three LEDS?)

I'm not clear on whether the max voltage on the datasheets refers to the whole LED unit, or each individual LED within the unit.

wiring.jpg
 
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bertus

Joined Apr 5, 2008
21,148
Hello,

It looks like that R3 (marked 1500 = 150 Ohms) is limiting the current to about (12-9 Volts)/150 Ohms = 0.02 Amp = 20 mAmp
You can see that the position for R4 is vacant.

Bertus
 

LesJones

Joined Jan 8, 2017
2,935
If the drive unit is designed correctly it should drive it with a current of about 20mA. You should not drve LEDs from a voltage source as they behave a bit like zener diodes. They will draw very little current for a voltage below about 2.5 volts but the current rises rapidly as the voltage is increased above about 3 volts. These values vary between samples of the same type of LED and also with temperature.
The voltage of 3.4 volts is the maximum voltage that is likely to be across the LED with a current of 20 mA passing through.
bertus, Well spotted. I did not follow the track to R3. I was assuming it would use a switch mode constant current source. As there was no likely looking IC or inductor on the visible side of the board I assumed they must be on the back. As the battery voltage in the car while being charged will be about 14 volts the LED current will be about 30 mA (Assuming 3.2 volts across each LED.)

Les
 
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Thread Starter

Taymar

Joined Jan 13, 2017
19
Thank you both again so much for helping me out with this. I am afraid a couple of these points went over my head a little though.

I think I followed the Ohm's Law calculation ok, and I believe I now understand the voltage rating in the spec sheet is for a given current.

I'm stuck on the current piece though. if I understood correctly, my circuit is supplying around 20ma to the LED. I see some of these other LEDS state 3x20ma, or 60ma on the datasheets. How do I tell which ones will be compatible please? Ideally I'd like the new LED to be as close in terms of current/voltage to the original one as possible.

In case it's relevant, here is a bit more info on the function of this circuit - The LED in question lights up automatically when a door is opened, but also functions as a map light via the push switch. When a door is opened, it 'overrides' the push switch and the light cannot be switched off. (I'm wondering if the route through D5 handles the door triggered switching and D7 handles the push switching?). There are no components on the back of the PCB save for the switch contacts and the wiring connector.
 

LesJones

Joined Jan 8, 2017
2,935
If the three LEDs in the packege are connected in parallel then the total current trough them will be 60mA (3 x 20mA) and the voltage across them about 3.2 volts. As yours are connected in series there will be the same 20 mA passing through them but in this case there will be 3 times the voltage that is across each individual LED (3.2 volts) in the package. So the total voltage across the package will be about 9.6 volts.The power will be the same as power = current times voltage.

Les.
 

Thread Starter

Taymar

Joined Jan 13, 2017
19
Thank you Les, I think I follow now. Do you think the ones you linked above (superbright LEDs) will be a good choice for this application then?

I really appreciate all the explanations guys, thank you again.
 

LesJones

Joined Jan 8, 2017
2,935
The data sheet seems to give conflicting information on the absolute maximum ratings for the device. It says the maximum current for each LED is 20 mA but the maximum disipation is 120 mW If we had a sample of LED that was at the high end of the forward voltage spec (3.4 volts) and the 30 mA that will be passing through the LEDs in your unit with the battery voltage at 14 volts. (This would be when the engine is running charging the battery.) then the power dissipation for each LED would be 102 mW These calculations are based on the assumption that R3 (150 ohms) is the only thing that likits the LED current. You could conform this by measuring the voltage across R3 with the engine running. You could also see if you can find another manufacturer of type 5050 LEDs using a search engine (Such as Google) and see if the information on their data sheet is any different.

Les.
 

EM Fields

Joined Jun 8, 2016
583
Thank you very much, this is a big help. Am I correct in assuming that if I get a new LED that's rated for '3.4v absolute maximum' and in my application it's receiving 9v, that's not going to work correctly?

I was able to follow the circuit traces on the PCB, and here's a quick sketch of how it appears to be wired. My electronics knowledge is very basic, so apologies if this is a stupid question - Is the fact that they're wired in series why this unit can handle a 9-10v input load (as the voltage is split between three LEDS?)

I'm not clear on whether the max voltage on the datasheets refers to the whole LED unit, or each individual LED within the unit.

View attachment 118626
LEDs are diodes, and the specifications defined by the data sheet for the voltage across the LED and the current through the LED don't say that with a certain voltage across the LED the current through it will be the current specified. Rather, the specifications state that with a certain current through the LED the voltage dropped across it will be within the range specified.
 

Thread Starter

Taymar

Joined Jan 13, 2017
19
I measured the voltage across R3 with some interesting results. With the engine running and the door OPEN, the led was on and I measured 0v across R3. I pressed the light button for this LED, door still open, and the light remained on at the same brightness but I now measured just over 3v across R3.

I am also finding wild variation in the datasheets online. Here's one I found (which is a PLCC6): http://www.qt-brightek.com/datasheet/QBLP679-IWK-XX.pdf

and another that is a 5050, but rated at 18v/220MA: http://www.luminus.com/products/Luminus_MP5050_6100_Datasheet.pdf

this second 5050 seems to only have two pins versus the three diodes.

Thanks EM Fields for the diode explanation, that does clear things up a little for me though I'm still a bit confused - does the LED draw that current or is the current simply what's available in the power source?

Sorry I'm not grasping this as well as I'd hoped.
 

LesJones

Joined Jan 8, 2017
2,935
Your tests on the voltage across R3 suggests that in some situations R3 provides the current to the LEDs (3 volts across R3 will give a current of 20 mA which is correct.) We would need to trace out the schematic of the board to understand what is happening. The information in your first link is the same as the link I found. It is just presented in a different way. It gives the current (60 mA)of the three LEDs connected in parallel instead of 20 mA for each LED. Your second link is a much higher power device which consists of 6 LEDs connected in series. I think that device needs heatsinking. The suggested large pads shown for the printed circuit board would help to remove heat. (It would probably need a PCB with aluminium substrate to disipate the heat. It looks like the 5050 part number is just the package dimensions in mm.

Les.
 

EM Fields

Joined Jun 8, 2016
583
I measured the voltage across R3 with some interesting results. With the engine running and the door OPEN, the led was on and I measured 0v across R3. I pressed the light button for this LED, door still open, and the light remained on at the same brightness but I now measured just over 3v across R3.

I am also finding wild variation in the datasheets online. Here's one I found (which is a PLCC6): http://www.qt-brightek.com/datasheet/QBLP679-IWK-XX.pdf

and another that is a 5050, but rated at 18v/220MA: http://www.luminus.com/products/Luminus_MP5050_6100_Datasheet.pdf

this second 5050 seems to only have two pins versus the three diodes.

Thanks EM Fields for the diode explanation, that does clear things up a little for me though I'm still a bit confused - does the LED draw that current or is the current simply what's available in the power source?
Sorry I'm not grasping this as well as I'd hoped.
The current available from the power source - the battery- is several hundreds of amperes, and that capacity is used to start the car, so there's much more current available there than is needed to run the map-light / door-open LED array.

That array will draw whatever current it's allowed to, and that current will be dictated by the battery voltage and be limited by the dynamic resistance of the LEDs and the fixed resistance of the series-connected ballast.

Can you get behind that, or do you need more help?
 

Thread Starter

Taymar

Joined Jan 13, 2017
19
Thank you both again. Les - I will do my best to trace the board or at least this section of it. Glad to hear that at least in that scenario the voltage was as expected.

EM Fields - that explanation certainly helps with my understanding of how the LEDs operate, thank you. Where I think I'm still missing the leap in logic sadly, is knowing how to take the known values of my circuit and compare to the datasheets to determine whether an LED I find is suitable or not.

Some more measurements I was able to take while bench testing on a DC power supply @14v:

- resistance across push switch when in 'on' position: 400 ohms
- voltage drop across each of the three individual LED pins: 2.66v
- voltage drop across R2 (equivalent of R3 but for the other side I believe): 4.75v
- voltage measured at pins into LED package:11.4v

Something doesn't seem to be adding up with the voltages I'm measuring, I think. I would have assumed the voltage drop across the total LED package pins would equal that of the three internal components, but it's higher.

There is one other product I found that I'd like to run by you guys please, though it seems to present it's data in a different format (some charts vs. tables): http://www.bivar.com/portals/0/products/SMTL6-UWDC.pdf
 

LesJones

Joined Jan 8, 2017
2,935
The LED package in your link #15 looks similar to the link I posted and the first link in your post #12 (20 mA per LED) One interesting point is the internal schematic shows back to back zener diodes connected in parallel with each LED but it does not mention them in the text. I asume these are to prevent a long string of them failing if one LED fails open circuit. If you can find a place to buy them at a reasonable price I would be tempted to use any of the ones with 3 x 20 mA LEDs in the package. I could not find any on Ebay other than made up into LED strips. Can you post pictures of the other side of both PCBs ?

Les.
 

Thread Starter

Taymar

Joined Jan 13, 2017
19
Thanks Les - I believe you're right on the zener diodes - I think I recall reading that on the page that led to the datasheet.

I have attached photos of the back of both boards - the two switches have very large backs and get rather warm, so there may be more in there than simply a switch contact.

A couple of other notes:

- there are two tiny leds mounted to the component side of the driver board. These illuminate the switch buttons. They are NOT always on.
Here's what the wires go to, left to right when looking at the back side of the board:

LED's 1 and 2 are the two SMD ones we're looking at.

1. LED 1 negative
2. LED 1 positive
3. regular incandescent dome light bulb negative
4. (marked PWM) regular incandescent dome light bulb positive
5. Yellow lead (marked Black) - unknown, may receive door open or vehicle lights on trigger.
6. Blue lead (marked PWM) - unknown, may receive door open or vehicle lights on trigger.
7. led 2 negative
8. led 2 positive

I am more than happy to take any additional photos, get any other kind of measurements etc. that may help. I have got some of the schematic mapped out by my multimeter does not seem to be wonderful for continuity tests. Have a new one ordered.

Thanks again so much for the help!

ledback.jpg driverboardback.jpg
 

EM Fields

Joined Jun 8, 2016
583
Thank you both again. Les - I will do my best to trace the board or at least this section of it. Glad to hear that at least in that scenario the voltage was as expected.

EM Fields - that explanation certainly helps with my understanding of how the LEDs operate, thank you. Where I think I'm still missing the leap in logic sadly, is knowing how to take the known values of my circuit and compare to the datasheets to determine whether an LED I find is suitable or not.

Some more measurements I was able to take while bench testing on a DC power supply @14v:

- resistance across push switch when in 'on' position: 400 ohms
Interesting, but with the 14 volt power supply energizing the circuit when you made the measurement, how did you manage to determine the resistance of the switch?


- voltage drop across each of the three individual LED pins: 2.66v
- voltage drop across R2 (equivalent of R3 but for the other side I believe): 4.75v
Is the load resistance the same on both sides?

voltage measured at pins into LED package:11.4v
Something doesn't seem to be adding up with the voltages I'm measuring, I think. I would have assumed the voltage drop across the total LED package pins would equal that of the three internal components, but it's igher.
There is one other product I found that I'd like to run by you guys please, though it seems to present it's data in a different format (some charts vs. tables): http://www.bivar.com/portals/0/products/SMTL6-UWDC.pdf
And you expect what, of us?
 

Thread Starter

Taymar

Joined Jan 13, 2017
19
Ah, I'm sorry for the confusion there. I did indeed measure the resistance of the switch without power running through the circuit.

I will check the resistance for both the left and right LEDs asap.

With the data sheet I was hoping the more experienced eyes here could let me know if that component looks like a suitable replacement. If I'm following correctly, it sounds like the measurements I've taken from my circuit calculate out to values (current specifically?) Which are outside of those ratings in the data sheets we are finding please.
 

EM Fields

Joined Jun 8, 2016
583
Ah, I'm sorry for the confusion there. I did indeed measure the resistance of the switch without power running through the circuit.

I will check the resistance for both the left and right LEDs asap.
And how will you do that?

With the data sheet I was hoping the more experienced eyes here could let me know if that component looks like a suitable replacement. If I'm following correctly, it sounds like the measurements I've taken from my circuit calculate out to values (current specifically?) Which are outside of those ratings in the data sheets we are finding please.
I'm thoroughly confused.
Your measurements make no sense, and you allude to the data sheet for an LED you'd like to substitute for what's in your kit, now, yet you don't seem to have a clue about what's in there, now.

Unless you can clearly identify what you want to do in terms of "before and after" and you can provide accurate documentation for "before", I fail to see how we (me, anyway) can help you get to "after".

But, continuing along with this merry chase, this seems to reflect what you've told us about what you have, so far:
Chrysler map lamps.png
If you could annotate the drawing with the various currents and voltages you've found, so far, with the switch open and closed, that would help.

Also, how do the PWM inputs affect the circuitry?
 
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