I have an SMD LED which is 3mmx2.7mm which I cannot find, can you help?

[TT_Vert]

So after some trial and error I think I got to where I want to be.  I noticed the original array drew about 140mA total.  I played w/ resistors to that current draw w/ 24V input.  I ended up at 22 . If I understood more about calculating the values based on the series/parallel configuration I'd do the math. I'm still going to try but the way these are wired and the effect that has on resistance has me confused.   Just as having one of the 3 parallel portions of the original wiring scheme w/o any current limiting resistor at all.  It's not exactly pretty but it'll do the job.  Now I just need to better understand why what I did is working.

Dave

[TT_Vert]

I see what you saw before w/ that hot wire.  I moved it over to the other side of the resistor so I could test the board.  It was easier to connect my meter to a fixed wire rather than probe.  Anyway I did get it working after some trial and error.  I am trying to understand how these series/parallel configurations affect resistor calculations.  I saw the original 12V array was drawing about 140mA.  I ended up getting there at 24V w/ 22ohm resistor on each of the two arrays I have.  I guess these two arrays are 2P3S4P if I'm understanding this correctly.  Is there some easy way to calculate the resistance of these types of configurations?  Do you calculate each group individually and then add them?

Thanks for all the help.  I've included a pic before I cleaned it up some, it's not pretty but it'll do the job.

[Brumby]

The topography you have worked out is exactly what I had sketched up for 24V.

The resistor in the middle was a neat move.

[TT_Vert]

Thanks again, few questions to try to better educate myself.

  Is there some easy way to calculate the resistance of these types of configurations?  Do you calculate each group individually and then combine them? 

Dave

[Brumby]

You need to work out the voltage across the LED matrix and the current they will draw - in total.  You then subtract the LED matrix voltage from the supply voltage to give you the resistor voltage.  You then take the total current and the resistor voltage and plug these into Ohm's Law to give you the resistance.

Let's say we have LEDs with an operating forward voltage drop of 3.5V and a current of 65mA

With the labels I used above:

* LED 1 has a voltage of 3.5V and a current of 65mA
* LED 1 and 2 in parallel (group A) have a voltage drop of 3.5V and a combined current of 65mA + 65mA = 130mA
* Groups A, B & C in series have a voltage drop of 3.5V + 3.5V + 3.5V = 10.5V and a combined current of 130mA.  (The current going through group A is the same current that goes though group B and then group C)
* There are 8 sets of 10.5V, 130mA in parallel which gives 10.5V and 8 x 130mA = 1040mA = 1.04A

So, with a 12V supply, the resistor has to drop 12 - 10.5 = 1.5V with a current of 1.04A.  In this example, the resistor would be 1.44 ohms.


If the LEDs ran at 3.2V and 20mA, the LED numbers would be: 9.6V @ 320mA.  For a 12V supply, that works out as 12V - 9.6V = 2.4V @ 320mA which gives 7.5 ohms.

(I was drawing up a sketch to illustrate - but the program crashed when I saved it and corrupted the file.)

[Brumby]

The symmetry of this LED matrix makes calculation easy - but remember that direct paralleling of LEDs (as has been done here) can result in varying brightness across the LEDs due to unequal current sharing.  This risk is much lower when they are all manufactured together.

[Kjelt]

direct paralleling of LEDs (as has been done here) can result in varying brightness across the LEDs due to unequal current sharing.  This risk is much lower when they are all manufactured together.

I was worried for this a few years back and asked a specialist who insured me nothing could go wrong. The manufacturing process is so good at the moment that the differences in Vf for the individual leds are marginal but true what you wrote, use leds from the same bin.
What is important however is that you should actually use a current source for these kind of setups.

[Brumby]

What is important however is that you should actually use a current source for these kind of setups.

Ideally, yes - but a resistor is cheap, easy and good enough for a lot of applications .... even if it is a bit power wasteful.

[TT_Vert]

Thanks guys.   I want to do this math but given i don't have a datasheet I'm trying to figure these specs out but I may be doing something wrong.  I had read that you can measure your Vf by powering your LED at 12V w/ a 500 ohm resistor to get a 20mA current limit and measuring across the LED leads..  When I do this i get 2V.  However when I put it in my diode tester on my meter It shows .875V.  I then checked the diode testing specs of my meter and it states a forward DC current of ~1mA and reverse DC voltage of ~1.5V,  Is it possible my meter is reading low because of the low 1mA current?  Moving forward I decided to check the Vf of the SMD LED that I had been using for this project and I cannot even get a reading on my meter.   Anyone care to chime in on what I'm doing wrong here, if anything?

Thanks much,

Dave

[Brumby]

Is it possible my meter is reading low because of the low 1mA current?

You are on the right track.   

The actual voltage that drops across the LED when forward biased (V_f) changes according to the current flowing through it.  This relationship also varies with temperature.

If you look up the datasheet for any given LED, it should show you a chart with current (I) vs forward voltage drop (V_f) ... and it will look something like this:



This image shows a comparison of two different LEDs, so you can see these graphs can vary ... and they do.  (The three curves for each LED will be for 3 different temperatures.)

The important things to note are:

• Low currents produce low V_f
• Currents can skyrocket when you get past a certain point
• Different LEDs have different curves
• Temperature affects the curves

We use constant current sources or current limiting resistors to prevent excess current from flowing through LEDs ... because left alone, they will try and draw destructive currents if the supply voltage is high enough.

[TT_Vert]

Yes I did notice that when playing around. I paid attention to current draws at a given voltage and actually wrote them down for my own personal knowledge.  This particular datasheet shows a forward current of 2.8V even at 1mA.  I don't know the specs of said LEDs I'm testing but I do know what my meter is saying which is confusing me.

Red LED i have.  Meter shows .875V when on diode testing.  Meter shows 2.0V when light is illuminated.  Any idea the discrepancy? Could the curve be that steep below a certain current threshold (Say 1mV in this example?_
White SMD LED, I get nothing on meter w/a  known good LED.  Could this be because it's a white LED and a higher forward voltage?  \\

I'm just trying to make heads or tails of what I'm seeing here so I can apply it moving foward.

Dave

[Brumby]

White SMD LED, I get nothing on meter w/a  known good LED.  Could this be because it's a white LED and a higher forward voltage? 

Exactly right!  This is one of the reasons why Dave has a 15V diode test range on the 121GW

[Brumby]

Red LED i have.  Meter shows .875V when on diode testing.  Meter shows 2.0V when light is illuminated.  Any idea the discrepancy?

It comes down to the current used in each situation.

A meter will do a diode test at the lowest current possible.  It is only trying to determine that the diode works as a diode (remember, LEDs are only one type of diode) - and it will give the voltage it sees when doing that test.  When you actually drive an LED for it to light up, you are putting a significantly larger current through it.

Interestingly, LEDs can actually emit light at voltages far below those on the charts - but the level can be incredibly low.  If I remember correctly, Dave did a video where he used an ultra sensitive device that could actually count individual photons - and I'm thinking he put an LED in the chamber and passed a spectacularly small current through it to get a really small - but measurable - amount of light out of it.


LEDs are not linear devices.  Not by a long chalk.

[TT_Vert]

so in essence the only accurate way to get Vf of an LED is in use if you don't have a datasheet?  And then just guess on an If (Or start at 20mA).  Or buy Dave's $250 meter which while I'd love it I just can't justify the cost at my current level.

Dave

[drussell]

so in essence the only accurate way to get Vf of an LED is in use if you don't have a datasheet?  And then just guess on an If (Or start at 20mA).

You should always test and characterize your components...

Perhaps, you seem to forget that the LEDs are diodes?  They behave as diodes.  For best practice, you really need some kind of way of doing some sort of external current limiting rather than relying solely on the precise characteristics of each diode to set your operating parameters.  Even a bit of resistance definitely helps on that...

[Brumby]

Picking the right current is not always easy, but it's not all that difficult to get a V_f.

Let's say you have a 12V DC supply and you want to test some LEDs.  Lets use a series resistor - but what value?

Pick a current.        I'll go with your 20mA for this exercise.

Now, if you know the colour of your LED, you can look around and see what the typical V_f is for that colour and use that - but for a general figure to use in this exercise, let's use 2.5V.

Now, the resistor will have to drop 9.5V  (12 - 2.5)  when carrying 20mA

V = I x R
9.5 = 0.02 x R
R = 9.5/0.02
R = 475 ohms

So, grab a 470 ohm resistor and go for it.

Note: This will give you "pretty close" voltages, because the current will likely not be exactly the nominated value, but the numbers you get will be quite useful.  You could also measure the voltage across the resistor and work out the exact current.


The other approach - and perhaps the best - is to get yourself a constant current circuit.  With this, once you've set it for a particular current, your measurements will be more "correct".

[Brumby]

Just looking back at that ... and I realised determining the maximum current is not easily done without some idea of its characteristics.

I had some 1W and 3w LEDs ... and working out the current for each was not staightforward.  I worked backwards, slowly increasing the voltage, measuring it and the current to get the power they were rated for.

[TT_Vert]

but how do you really know what they are "rated for"?  I do understand that basic math of calculating a resistor for an LED given supply and Vf but I don't know when you've got too much current.   My power supply does show me current draw and voltage so i can watch these realtime but I don't know when I've gone too far aside from monitoring LED temp and brightness.

Dave

[Brumby]

Good question.

Datasheets are the primary reference here, but if you don't have one - or at least some idea of the characteristics of the LED - then it becomes much harder.

I can't say I have the experience to guide you on this - perhaps someone better informed can offer something useful - but I would try doing the following:

1.  I would leave brightness out of my initial investigation - unless you had a brightness figure from some source you could use.
2.  I would plot my own V_f vs I curve and compare that with other LEDs
3.  I would watch the temperature of the LED like a hawk.  Letting the chip overheat and release the magic smoke wouldn't be helpful.

[TT_Vert]

great thanks much

Dave