First, given that you’re using the OSRAM LEDs mentioned earlier

Where was that mentioned?

 

the LEDs are rated for 1 amp,

What LEDs are you using exactly?

 

If you could power it from 15V you could put all four in series. (might need a little more, 14.4V out is required.)

If you must use one, just put a 4.7Ω 2W resistor in series with each. That should be pretty safe. I sized it to drop about half the difference between the 12V supply and the 7.2V needed.

 

First, given that you’re using the OSRAM LEDs mentioned earlier,

We still don't know what LEDs the OP is using.

He gave a voltage spread of 3.6-3.7V for the forward voltage and I was providing datasheet examples that showed that that wasn't realistic unless he had ones that had forward voltage matching.

Then he said he had LEDs that had been binned for forward voltage, wavelength, and brightness. Which is also unlikely. Binning by brightness is free. Binning by forward voltage would cost more and I've never heard of binning by wavelength. Of course, I hadn't heard about forward voltage binning until a decade ago or so. Maybe that came with the advent of so many inexpensive LED flashlights using LEDs in parallel with no ballast resistors.

 

We still don't know what LEDs the OP is using.

He gave a voltage spread of 3.6-3.7V for the forward voltage and I was providing datasheet examples that showed that that wasn't realistic unless he had ones that had forward voltage matching.

Then he said he had LEDs that had been binned for forward voltage, wavelength, and brightness. Which is also unlikely. Binning by brightness is free. Binning by forward voltage would cost more and I've never heard of binning by wavelength. Of course, I hadn't heard about forward voltage binning until a decade ago or so. Maybe that came with the advent of so many inexpensive LED flashlights using LEDs in parallel with no ballast resistors.

Sorry. I thought your example WAS the LEDs the TS was using.

 

If you don't care about variations in brightness, why are you using a driver chip in the first place?

What is it that you are doing with these LEDs? How much can your supply voltage vary by?

Your solution may be nothing more than a series resistor in series with one or two LEDs to make up a string and then enough of those strings in parallel to get the total light output you need.

 

If you don't care about variations in brightness, why are you using a driver chip in the first place?

What is it that you are doing with these LEDs? How much can your supply voltage vary by?

Your solution may be nothing more than a series resistor in series with one or two LEDs to make up a string and then enough of those strings in parallel to get the total light output you need.

i'm using high power LEDs rated for 1 amp.
these are used on vehicles with 12 volt battery systems and various charging systems.
supply voltage can vary from 11 to 15 volts. that's why i can't use resistors.

 

i'm using high power LEDs rated for 1 amp.
these are used on vehicles with 12 volt battery systems and various charging systems.

Connect the LEDs as you have them wired directly to your 12 volt power supply and see what the current reads?

 

Connect the LEDs as you have them wired directly to your 12 volt power supply and see what the current reads?


View attachment 300747

What do you base that expectation on?

Why would you expect LEDs rated for one ampere to only pull just over half that when connected like this?

I would expect at least two, if not three, failed LEDs in short order.

 

i'm using high power LEDs rated for 1 amp.
these are used on vehicles with 12 volt battery systems and various charging systems.
supply voltage can vary from 11 to 15 volts. that's why i can't use resistors.

Could you PLEASE give us a part number for the LEDs you are using?

 

Why would you expect LEDs rated for one ampere to only pull just over half that when connected like this?

I deleted that comment. They should pull 1 amp total but they won't fail if rated 1 amp at 12 volts.

 

I deleted that comment. They should pull 1 amp total but why would they fail if rated 1 amp at 12 volts?

Where did he say that they are rated to pull 1 A at 12 V?

All I've seen is where he has said that the forward voltage is 3.6 V to 3.7 V. (https://forum.allaboutcircuits.com/threads/led-current.195471/post-1842977)

He has also stated that they are going to be used in a vehicle with a 12 V electrical system. (https://forum.allaboutcircuits.com/threads/led-current.195471/post-1843124)

I do so wish he would give us a part number for these LEDs so that we could look at what they actually are and what the specs actually say. Are these bare LEDs? Do they have current-limiting built into them?

 

Where did he say that they are rated to pull 1 A at 12 V?

I stand corrected. Misread the post.

 

all 4 LEDs are very close to same forward voltage.
they're the same part coming on same reel.
forward voltage is 3.6 to 3.7 volts.
is this close enough that i don't need the resistor?
i don't know how to indicate matched LEDs.
bottom line is - if LEDs are matched and driver chip is producing 1 amp, does each string get 0.5 amp?
thanks

Being next to each other on the same reel does not necessarily mean that they are from the same die or even the same lot of wafers -- but it is pretty likely.

I don't know about your particular LEDs, but if they are not operating in a mode where they have significant linear resistance effects, a change of a one or two hundred millivolts can equate to an order of magnitude difference in current. In addition, unless they are mounted so that they are strongly thermally coupled, the Vf will change significantly if one heats up more than the others.

Bottom line is -- any design that counts on them sharing current equally is a very poor design.

 

I stand corrected. Misread the post.

Happens to all of us.

 

these are used on vehicles with 12 volt battery systems and various charging systems.
supply voltage can vary from 11 to 15 volts. that's why i can't use resistors.

Why not? I have some LED bulbs in 194 form factor. They don't draw an amp, but they have a resistor on them.

 

What do you base that expectation on?

Why would you expect LEDs rated for one ampere to only pull just over half that when connected like this?

I would expect at least two, if not three, failed LEDs in short order.

i understood the driver chip limits or sets the current regardless of forward voltage.

 

LEDs, 555s, Flashers, and Light Chasers

 

i understood the driver chip limits or sets the current regardless of forward voltage.

Note that the post I was responding to was specifically talking about just the four LEDs connected as shown directly across a 12 V battery, so no driver chip at all.

And, once again, please provide the part number of the LEDs you are using.

 

looks like you guys were right.
with the line tying the 2 strings together, 1 side draws .6 amp and the other draws .25 amp.
without the tying line, 1 side draws .51 amp, the other draws .3 amp.
guess i'll be using 2 drivers. i hate to because this requires even more power and cost.
thanks to everyone.