Why is this circuit not working properly?!?

[Korxi]

Hi,

I'm currently trying to install some plant light in my terrarium using 5mm LEDs to get the exact wavelengths needed - plus some white light so it doesn't look like the 70's

So now I just tried to put a circuit of 19 5mm LEDs together to try and see how the plants would react if I went full scale. However I must have done something wrong (btw. I'm far from an electronics expert ) because those white LEDs are definetly not putting out the 11.000 mcd they should. I tested one individually and it is putting out ALOT more light than the ones in the circuit.
The red ones seem very bright, and actually dominate, even though they are fewer and less bright..

Here's the circuit:

Can anyone tell me what's wrong?? I thought that the resistors would make every one of the parallel series have a voltage drop of 12V and hence the current through each should be 20ma.. But as I said I'm far from an expert so I'm not even sure I make sence here

Hope someone can help!

Christian

Edit: Btw. the power supply is regulated, and rated at 500mA

 

[Illum]

However I must have done something wrong (btw. I'm far from an electronics expert ) because those white LEDs are definetly not putting out the 11.000 mcd they should.

got a link as to where you found the specs from?

The red ones seem very bright, and actually dominate, even though they are fewer and less bright..

using the naked eye to judge the intensity of different color LEDs can be tricky.

A guess would probably be that your resistors are too small in terms of wattage. Using a resistor with a wattage value too small will deprive your LEDs with the current they need to illuminate in full brightness. have you checked your circuit using an DMM to see how much current is getting to each LED?
resistors come in from 1/8W, 1/4W, 1/2W, 1W and so on...all the way up to 10+ watts

what type of resistors are you working with?

 

[macforsale]

Calculation only get you to a starting point. Check the current and voltage drops of each leg. The Vf is only a nominal value and will vary from manufactures lot to lot production. Now that you got it up and running it just fine tuning the resistor value to get 20ma in each leg.

 

[evan9162]

The Vf of your white LEDs is probably higher than you think they are. They really should be in the same range as the blue LEDs, since white LEDs are just blue LEDs covered with a phosphor.

Measure the Vf of your white LEDs individually with a proper constant current circuit. My bet is that they are actually closer to 3.6V than the 3.2V that you've indicated. That would mean that your whites are only getting 10mA vs. the 20 that you've picked the resistor for.

Measure the voltage across the resistor for the white LED strings, then divide by 120 - that will tell you how much current is actually flowing through the white strings.

 

[Geogecko]

Even is spot on, on this one. The white should be close, if not the same Vf as the blue ones (may be slightly different between manufacturers).

You should be okay on the wattage of your resistors, if you are using 1/4 W resistors. The one consuming the highest power would be the 220 ohm resistor, at around 88mW. (P=V*I or P=I^2 * R) It is a good rule of thumb to use a resistor with a power rating double of what it will actually consume, so for that particular one, a 1/4 watt would be best, although an 1/8 watt would work. Generally, what you find at Radio Shack will be 1/4 watt, in the through-hole package.

This is a neat little project that you can build yourself, if you think you'll be doing a lot of LED projects. You don't really need the PCB, if you have a breadboard, or you could use perfboard material to make it, since it's such a simple circuit.

http://www.robotroom.com/LEDTester.html

The reason it is nice, is because it is a constant current source, so if you set it up for 20mA, no matter what the Vf of the LED is, it will always drive it with 20mA. That way, you could then measure across the LED to find out what the "actual" Vf of the LED is, especially if you have a ton of LEDs that you do not have the data sheets for anymore.

Good luck, and have fun with it...

 

[Korxi]

Thank you guys for the advice, it's greatly appreciated.

I got the specs. from the vendor's site, there was no datasheet actually, just a description.. Howver for the Nichia LEDs there was, and they have a Vf og 3,6..
Sadly I don't have any measuring instruments availible, so I think I'll just try to buy some 60 ohm resistors and see how that goes (they are all 1/4W).

I think I do understand the calculation leading to the result that only 10mA is currently passing through the white LEDs, however I don't understand how to interpret this table from the nichia datasheet:

Shouldn't Vf actually be around 3,2V if the LED is run at 10mA?? Or what does this curve mean??
On another note, wouldn't the temperature rise in the LED lead to differences in Vf? The LEDs are run 12 hours a day non stop, so I think their temp is aprox. 20 degrees above ambient temp.

Aaaand lastly: How does the 'rule' "the current allways chooses the path of least resistance" fit in here?? Since the white LEDs currently are run with a to large resistor in front, wouldn't that then mean that the red and/or blue are running on a to high current? or am I just confused here??
(I think I remember reading a reply somwhere saying something like this..)

Please forgive the newbie questions.. I'm just starting to dip my toes here

Christian

 

[Geogecko]

You are correct about what the Vf is, if it is run at 10mA. That chart tells you what the Vf of the LED will be for what current you want to run it at. If you run it at 10mA, it will be around 3.3V, if you run it at 20mA, it will be around 3.6V.

The temp rise in an LED being driven at 20mA is minimal. In this case, at 3.6V, that is 72mW (P=VI), so it's very low power. Running it at this current 12 hours a day, or 24/7 should provide no noticeable temperature rise. When you start getting into the 1W and greater LEDs, you'll see that they have to be heat sinked, as in practically all high power LED flashlights.

As far as the rule you mentioned, it is true, however, you have to take it in context.

Think of it if you took the LEDs out of the picture, and just connected 6 resistors from + to -. In your case, say it was the resistors you originally chose. 3 - 120 ohms, 2 - 220 ohms, and 1 - 30 ohms.

Calculate the current for each resistor.

120 ohms
I = V / R = 12/120 = 100mA

220 ohms
I = V / R = 12/220 = 55mA

30 ohms
I = V / R = 12/30 = 400mA

Total current supplied by power supply = 100mA (x3) + 55mA (x2) + 400mA = 810mA.

So, which path did the current take?

Since your circuits are in parallel, the other branches do not effect each other, except to effect what the power supply has to supply. In other words, you could setup each branch to be different, 5mA, 10mA, or 20mA in any combination, and they never effect each other, only the total that must be supplied by the power supply, assuming the supply is regulated.

I think you're starting to understand things.

 

[Korxi]

Thank you for clearing that up!

However there is still one thing I can't really wrap my brain around:

When you first calculated that my white LEDs were running at 10mA by saying: 12v-(3*3,6V)/120ohms = 10mA

Why is it the Vf of 3,6V that is used to calculate the current to be 10mA, and not the 3,3V (Vf @ 10mA) as stated on the graph?

I think I lack some fundamental knowledge of circuits here How exactly is it the resistor works? Does it just "eat" any residual voltage not used by the LEDs and then force the current to be some value defined by the size of the resistor or what? Like in the equation above the LEDs consume 10,8V, leaving 1,2V which then equals to 10mA @ 120 ohms?

Christian

p.s.
when I touch the back of the PCB I feel that the components are really hot, and the plastic casing on the LEDs get hot as well.
The resistors should get hot though right?
So it would make sense if they transfered some of their heat to the LEDs making them hot - if let's say my lack of soldering skills resulted in a direct connection between the resistor and the LED in solder - transferring the heat. But that should only make the first LED in each parallel series hot, and that's not the case - they are all hot.

The 19 LEDs are setup right next to eachother on a round homemade PCB, and then encased in an old halogen bulb, could the enclosed space be the reason why the heat is not dissapated?

 

[Geogecko]

Just lost about a page long post...

Anyway, to make a long story short, as I have to go now...

The first calculation at 10mA must have been wrong. In order to solve the equation, you need 2 of the 3 unknown variables. In this case, during the design, you would know what current you wanted to drive them at (20mA), and knowing that and having the chart, you would know what Vf the LED was going to be.

Going backwards is a lot harder, as you only know the resistance value, and not the Vf or I. If you had test equipment, you could measure the Vf as it is operating, and then calculated I ( I = V / R).

You are right, the resistors just dissipate the "left over" voltage as heat. Since a resistor resists current flow, it allows you to set up the amount of current through the resistor, based on the voltage across it. Since the voltage across the LEDs, for a given current, should never change (theoretically), then the left over voltage can be used to determine a current limit.

As for heat, yep, those LEDs and resistors in that combination will get a little warm, but shouldn't get overly warm to where you cannot touch it (or something else is wrong).

Remember, for all 19 LEDs, you are consuming 1.44 watts.

P = V * I = 12 * (0.02A * 6) = 1.44W

6 comes from the 6 branches of the circuit, each consuming 20mA.

 

[Korxi]

Ok, so the calculation of the 10mA is not completely accurate, but serves to illustrate that the white LEDs was not operating at the correct current?

I think I'm beginning to understand a bit more now - as long as I have the correct specs. on the components to start with I guess I will be ok. Otherwise I should get a multimeter and build that constant current LED tester

As for the heat bit, It's not like you can't touch it, but I guess it's above 50 degrees celcius, which has had a noticable effect on the output of the LEDs, already, and they've only been running for a month (I comparred with some new ones not in a cirtcuit, but under the same resistance and voltage)

Again thank you for your help! And sorry to hear about that long post, I would have liked to read about this in more detail

Christian

 

[Geogecko]

Yes, that is correct. The 10mA was just to illustrate.

The constant current LED tester works great. I just spent a few minutes building one on a breadboard here at work so that I can check high brightness LEDs for a project I'm working on, without having to calculate a new resistor for every different LED that comes in.

I just tried a bi-color LED, red/green, which has two different Vfs, and measured one to set it up for 20mA, and then reversed the LED (opposing diodes, so it's only two pins), and then checked the current, and sure enough, it was at 20mA, at a different Vf. Pretty cool.

50C seems a bit high for what you have there. You may need to check your circuit once again, and see if there are any wiring errors or something. I've had this green LED running for close to an hour now, and it is barely able to be noticed, heat wise, of course, I don't have a dropping resistor either.

Actually, 50C is only 122F, so that is not that far above body temperature, although I wouldn't expect it to even be that high...but I guess with 19 LEDs, and 6 resistors, it could be getting a bit toasty.

I wouldn't think it would effect the LEDs, as long as you are running them within specifications. Eventually, the LEDs will become dim, as that is how they normally die out...in fact, most are rated to their half-brightness level, so when you see 50,000 hours, it doesn't usually mean it just goes out, it usually means it will be at it's half-brightness level (half as bright as it was when it was new, at the same current).

 

[Korxi]

Ok, so finally got my act together and bought a cheap multimeter and made the LED tester.. I think I put it alle together correctly, since I measure values close to the specs. of the LEDs!

So I wanted to test those white LEDs - however I found out that I haven't got anymore "fresh" ones - so I decided to test the reds, since I can see they also are dimmer comparred to the ones if put on the tester.

Here's what I gotfrom the LED tester:

Vf @ 20mA = 1,79V (the spec said 1,9V..)

Then I measured the actual output of my 12V powersupply, it was 12,5V

This is when I accidentaly short circuited something, and ALL the LEDs just died....
ANYWAY, I had another circuit going on the same power supply, with 17 LEDs instead of 19..
There's only 5 red leds in this circuit..
Using the Vf from the specs. (1,9V) and my assumed 12V powersupply I had calculated the needed resistor to be 125ohms, but put on a 130ohm, since I had one lying around..

Anyways, now that I found out the Vf was infact not 1,9 I calculated what the resistor "should" have been for the 5 red LEDs:

(12,5V - (5 * 1,79V))/0,02A = 177,5 ohms or 177 ohms I guess?

And that's all fine.

But now I just thought I'd do some measurements on the working cirucit to see what the differences would be.. Measureing from one leg to another on one of the red LEDs I got:
V = 1,79
I = 38,8 mA

And this is what I don't get.. The resistor is 130 ohms and the Vf is also 1,79V in the circuit.. but calculating the current, that should be:
(12,5V - (5 * 1,79V))/130ohms = 27,3mA

Why is it not 38,8 mA as it "should" be?? Is it because the other components of the circuit interferes with the measurement when I try to measure the current across the to legs of just one LED or what?

This is the short version - lost the entire thing to! please tell me if I don't make sense!

 

[evan9162]

When you're measuring current, does the LED that you're measuring at go out?

If so, you're bypassing that LED with your current meter, essentially removing one from the circuit. Now there are only 4 LEDs in series, not 5. So naturally, current will increase.

A meter in current mode looks like a short (i.e. piece of wire). You need to take that in account when measuring current. You cannot measure current without inserting the meter in series with the entire circuit. You were pobably putting the meter in parallel with one LED, shorting it out, and effectively removing it from the circuit.

Let's work the math here, but now with 4 LEDs, and with a slight increase in Vf due to the extra current flow:

(12.5 - (4 * 1.9V))/130 = 37.6mA

Of course, this is a touch off from what you measured due to Vf differences, etc, but it works out and shows you that calculating with 4 LEDs and a slight increase in Vf due to higher current matches the results you got more closely than a calculation with 5 LEDs and a lower Vf due to the lower current.

See? The way you were measuring, you changed the circuit so there were only 4 LEDs in series, not 5. If you want to measure current, you must put the meter in series with the circuit, not in parallel with one component. I would put the meter in series between the power supply and the circuit, since that's probably the easiest place for it.

When measuring current, you must imagine that your meter is a piece of wire. Only connect the meter (in current mode) where it would make sense to connect a plain piece of wire. So you're planning on connecting your meter a certian way. Step back and imagine that you took a piece of wire and were going to connect it the same way....Is it going to short something out? (like a power supply or important component in the circuit). If so, then you shouldn't connect your meter that way.

 

[Korxi]

Ah I see! But in the LED tester, since there's only one LED, I can actually measure the current by just shorting across it right? (or just shorting the circuit prior to fitting an LED to it..)

The measurements i get of the LED tester they should be the "correct" ones right? Are they "good" enogu to calculate resistors for designing future circuits?

Christian

 

[evan9162]

You can only do that with the LED tester because it's a constant current supply. If you did that with any other kind of power supply (or battery even), you risk destroying the supply, your meter, or both.

The measurements you get from the LED tester should be accurate - it uses a constant current supply to drive the LEDs. You should use the values you get from the tester, since you're actually measuring the individual LEDs you're going to use, rather than relying on the datasheet from the manufacturer. Remember, the datasheet only specifies typical values - which can only be relied on to give you a general idea of the values you will be working with - individual samples will vary.

 

[Geogecko]

Evan is correct on all points. Your LED's probably died, because standard 5mm LED's are rated at an absolute maximum of 30mA, which you exceeded when you shorted one of the 5 LED's out.

I'm not sure there is an easy way to remember this, but voltage is always across, and current is always through. This means when using your volt meter, when measuring voltage, you should measure across the component, when measuring current, you should have the current pass through the meter (should be added in series).

If you take a closer look at the schematic of the LED tester, you'll see that the two pin jumper allows you to put the meter in series (jumper removed). That makes a series circuit. When you measure the voltage, you'd measure across the LED, like you were doing.

Electronics can be confusing at first, but once you understand the basics, it becomes a lot easier, well, in most cases.

I've been using the LED tester at work for a couple weeks now, and it's just great having that thing. I even used it to test a 1W blue LED for a project, and it worked great. The LM317 got quite warm, but that's because I was using an 18V battery pack for the supply...

 

[Korxi]

Thank you guys very much! I really appreciate the help I'm getting here - I actually think I'm beginning to understand

Christian