[Graham N]

Hi,

I need to make up a large led circuit and tried using an online LED circuit planner. The LED's are rated 3.2v-3.4v (3v Typ) I have entered the figures into the planner but don't know what voltage to select. Should I select 3.3v which is in the middle of the first figure or should I just use 3v.

One version shows the LED's wired in series groups of 4 with a 1 ohm resistor and the other voltage uses series groups of 3 with 110 ohm resistors I think it was. The important thing is I just want to know which voltage to enter in the planner.

Thanks for any help

Graham

[S D I]

Any of the voltages will be fine.
Enter 3 volts in the interests of rounding the maths up easily

[Graham N]

Originally Posted by stevedilworth

Any of the voltages will be fine.
Enter 3 volts in the interests of rounding the maths up easily

Thanks Steve

[supraTTman]

12V LED strips use 3 LED's in series with a 150R-330R resistor to define and limit the current.
Don't use 4 LED's if a 12V supply is used.

[Graham N]

Originally Posted by supraTTman

12V LED strips use 3 LED's in series with a 150R-330R resistor to define and limit the current.
Don't use 4 LED's if a 12V supply is used.

I have just ordered the LED's based on this plan using 1 ohm resistors

[S D I]

Everything looks in order Graham.
Wasn't sure what a LED planner is until saw this.
You'll be fine.

[Graham N]

Originally Posted by stevedilworth

Everything looks in order Graham.
Wasn't sure what a LED planner is until saw this.
You'll be fine.

Yes, found two or three planners from different sites and they all came up with the same results, as they should I suppose

Thanks,

Graham

[Darren Blake]

As mentioned above, using four 3.3V LEDs in series with a 12V supply isn't a great idea.

For starters, you won't be running the LEDs at peak brightness as the maximum voltage drop across each one will only be 3V, not 3.3V (although to be fiar you probably won't even notice this).

A more serious issue is that because your current limiting resistor is so small, the power dissipated in it becomes massively sensitive to the voltage dropped across the LEDs.

With that amount of LEDs you will get one or two fail (my home made lounge lights use 160 LEDs and I regularly get failures). LEDs can fail open or closed circuit. If one fails open circuit then it behaves like a fuse and protects the rest of the circuit - all well and good. If it fails closed then you have to make sure that the increased current flow does not cause any secondary damage.

See the bit at the bottom of your calculator where it says that "each 1 ohm resitor dissipates 0.4mW" and "the wizard thinks that 1/4W resistors are fine for your application"? Well, if all LEDs are working then that's true, but if you have one LED fail closed-circuit then the current in that part of the circuit will rocket up to about 3A. This will put 9W into your current limiting resistor. If the resistor or one of the other LEDs doesn't blow open-circuit, and if you don't have a fuse somewhere then you could get all sorts of nasty things happening, including all the magic smoke coming out . 3A is a LOT of current.

The maths in the online calculator is correct, but unfortunately its not good circuit design.

Using 3 LEDs in series and 150ohm resistors would enable you to be safe even if you had one LED failing closed circuit in each branch of the array, even with 1/4W resistors (but I would use 1/2W to be extra safe.

[S D I]

Bit pessimistic Darren.
While your maths obviously correct, your experience of LED failure I don't recognise.
Maybe you should have used the LED planner!
ONLY JOKING LOL
Be interesting to see what happens with Graham.

[Graham N]

Thanks Darren and Steve,

I take on board what you have said and although my LEDS are on the way with 1 ohm resistors I will probably buy the necessary half watt resistors and wire them in threes as you suggest. I have ordered 100 LEDS and holders so will probably design the circuit with 90 (30x3)) to allow for failures.

I want the lighting to be as bright as possible without stressing them, so what voltage should I enter into the planner for this?

Regards,

Graham

[supraTTman]

Originally Posted by stevedilworth

Bit pessimistic Darren.

NO NO NO. Very accurate Darren and eloquently described.
There is no way that the 1R resistor will define or limit the current through the 4 LED's. It is very important to ensure that all LED's are running within their forward current ratings - usually 20-30mA. Any increase in supply voltage (e.g. by using a non-regulated psu working beneth its rated load) will dramatically increase the current unpredictably causing early failures.
Just learn from the LED strip suppliers and use 3 LED's + 150R-ish.

[Darren Blake]

Can you get the manufacturer's data sheet for the LEDs you are buying from the vendor? If not then the manufacturer's name and product code? Even if you can't get it from your supplier, datasheets are readily available from websites like Farnell and RS.

The datasheet will almost certainly have a graph showing brightness against different forward voltages and currents so you can choose what you want to run it at.

[Graham N]

Right, if I use the wizard at 3 or 3.1 volts it gives 150 ohm resistor value. 3.2 and 3.3v gives 120 ohm and 3.4v gives 100 ohm. This is with 90 LED's, supply voltage 12v and 20ma current.

The wizard says to use 3.3 as the suggested voltage for a white LED. unless you guys suggest otherwise I am tempted to order the 120 ohm resistors. As I said before the rating for the LED's are 3.2v-3.4v (3.0v Typ) Do you think this is ok or should I stick with 150 ohms. What difference do you think these two options will make to the reliability and brightness,


Thanks again,

Graham

[supraTTman]

Sorry I didn't have time on my first reply to explain a bit more - Darren did all the hard work. As he said, if you have a data sheet for the devices that you are buying, then if you post this on the Forum we can be a bit more scientific and run through the calculations regarding maximum & recommended forward current/voltage!!!

All the 12V warm white & cool cool white LED strips that I have used have 3 LED's in series with 150R.
The dual-white strips I discussed on another thread (using cool white & warm white alternately down the strip) use 130R (not easy to find).

Without any technical data, if you want reliability, go for 150R, if you want to run the LED's closer to their peak output go for 120R. 100R is definitely pushing it.

Low-power white LED characteristics are quite similar whether they are surface-mount devices (on an LED strip) or leaded devices that you solder to.

Note that high power 1W/3W devices used in MR16/GU10 ceiling lamps have very different characteristics.

[Darren Blake]

I've attached a SAMPLE datasheet. If you look through you will see that for each type of LED there are two relevant charts. One is the relationship between Vf and If (forward voltage and current) and the other is the relationship between luminous intensity and If.

So in your case you woud need to find the value of If that gives you the maximum lumious intensity and then find the corresponding value for Vf. As you can see from he datasheet attached these are unlikely to be the "typical" values.

[Graham N]

Thanks again guys.

To Darren, I am reading this on my iPad and I can't see any attachments? Did you forget to attach your data

Graham

[Graham N]

To Supra,

This is a link to the LED'S I have bought on eBay. 110845448505

Graham

[supraTTman]

Using typical data on your ebay link:
3V each diode @ 20mA
So 3 diodes = 9v
So voltage to drop =12-9 = 3V
R=V/I = 3/0.02 = 150R

I rest my case.

[Darren Blake]

Originally Posted by Graham N

Thanks again guys.

To Darren, I am reading this on my iPad and I can't see any attachments? Did you forget to attach your data

Graham

Its definately there. Must be an iPad thing.

I've had a look at the eBay ad. Nothing that would hint at the values you need there - typical values only - unless the seller can get you a datasheet. Might be worth asking him?

Might be able to find a datasheet for a similar item on Farnell if push comes to shove.

[Graham N]

Originally Posted by supraTTman

Using typical data on your ebay link:
3V each diode @ 20mA
So 3 diodes = 9v
So voltage to drop =12-9 = 3V
R=V/I = 3/0.02 = 150R

I rest my case.

Hi supra,

Thanks for that, I was just trying to convince myself it was ok to go go 120R, I think I will stick with the 150r

Graham

[Graham N]

Originally Posted by Darren Blake

Its definately there. Must be an iPad thing.

I've had a look at the eBay ad. Nothing that would hint at the values you need there - typical values only - unless the seller can get you a datasheet. Might be worth asking him?

Might be able to find a datasheet for a similar item on Farnell if push comes to shove.

Hi Darren,

Yes, I usually use the forum app. I checked via Safari and I can now see your attachment,

Thanks very much will take a look at it

Regards

Graham