I am putting together a multi-lamp circuit with about 6-8 LED CoB chips, and I've managed to get the voltage and power requirements down to two -2.4VDC for one group and 3-3.4VDC for the other. Is it safe for all concerned to run everything off a 3volt power supply, or do I need to use two power supplies, one for the 2-to-2.4Volt group one for the 3-volt group? Note-the 2-2.4Volt group is about 6 LED chips, while the 3-3.4volt groups is only 1 or two chips. So can I safely run all of them at 3 volts?

 

Post a schematic or block diagram so we can see exactly what you're working with.

 

-2.4VDC or 2.4VDC? post a schematic.

 

Depending on the current that the LEDs will be drawing, series strings could make a lot of sense. AND, if you are using a series resistor to limit the current as the forward voltage changes due to heating, a series arrangement may be simpler.
Very sedom are LEDs directly driven from a voltage source, usually a resistor is in series.

 

When it says 3 to 3.4V, then it can be anywhere in that range, and it will also vary with temperature. It’s not you choice, it’s the LED’s choice. If it needs 3.2V and you only provide 3.0V, it’s not going to light up like you want it to.
[edit] edited for greater clarity re. @MisterBill2

 

We need to see a schematic in order for you to get the right advice. This is more efficient than answering 20 questions

 

I am putting together a multi-lamp circuit with about 6-8 LED CoB chips, and I've managed to get the voltage and power requirements down to two -2.4VDC for one group and 3-3.4VDC for the other. Is it safe for all concerned to run everything off a 3volt power supply, or do I need to use two power supplies, one for the 2-to-2.4Volt group one for the 3-volt group? Note-the 2-2.4Volt group is about 6 LED chips, while the 3-3.4volt groups is only 1 or two chips. So can I safely run all of them at 3 volts?

Really need to see a schematic, as your verbal description is pretty ambiguous.

How are these 6 LED chips in the first group arranged? If they are in series, your stated voltage is very dubious. If they are in parallel, how are you controlling the current sharing?

How are you limiting the current?

If your single LED in the other group needs 3.3 V, how do you expect it to work if you only supply 3 V?

How much current do each of the LEDs need (or how much are you planning to provide)?

How much current can your power supply provide?

A schematic would go a long way toward addressing several of these and better focusing the discussion for the rest.

 

When it says 3 to 3.4V, then it can be anywhere in that range, and it will also vary with temperature. It’s not you choice, it’s the LED’s choice. If it needs 3.2V and you only provide 3.0V, it’s not going to light up.

Wrong!! LEDs do light up at lower voltages than the ones that result in the specified current, but the brightness is less, sometimes quite a bit less. An LED is, after all, a diode, and diodes ARE NON-LINEAR. Certainly the intensity at lower currents is less, but they do start up with a faint glow at lower voltages and currents. And, at those lower levels, the non-linear relationship is different from at higher currents. This can be apparent even in series strings of "identical" LEDs. They can be very close at the specified operation point, but still be different at other levels.

 

Thank you all for your replies and questions. I'm still working out the design of the whole shebang, e.g., whether to use individual LEDs or CoB units. For what it's worth, reducing the brightness by varying the voltage and/or the amperage is just fine with me, as long as I can vary those parameters without damaging the device(s). I'm trying to build a miniature "stage" for doing oil and watercolor painting. The "stage" is basically a wooden box lying on its side, about 12" top-to-bottom, 18" side-to-side, and 12-15" deep, front to back. The "front" panel is not present, since the front opening serves as the proscenium, and makes it a stage. The LEDs or CoBs are to illuminate whatever is on the stage. So I would like to be able to control the amount of light from each source, the brightness, color and intensity, and by "source" I mean one LED, or a group of three, or a chip-on-board. What I can learn here will help me decide which devices, how many, where, etc. Suggestions?

 

Controlling the Intensity is the easy part,
Controlling the Color with LEDs is a total bucket of worms, that's already starting to smell kinda rotten.

Using small, 12-Volt, Quartz-Halogen Flood-Lights,
will provide the best CRI, ( Color-Rendering-Index ), hands-down,
but they should not be dimmed more than around ~20% or their Life-Expectancy will be severely shortened.
https://duckduckgo.com/?q="12-volt"...+lamp&t=brave&iar=images&iax=images&ia=images
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Controlling the Intensity is the easy part,
Controlling the Color with LEDs is a total bucket of worms, that's already starting to smell kinda rotten.

Using small, 12-Volt, Quartz-Halogen Flood-Lights,
will provide the best CRI, ( Color-Rendering-Index ), hands-down,
but they should not be dimmed more than around ~20% or their Life-Expectancy will be severely shortened.
https://duckduckgo.com/?q="12-volt"+quartz-halogen+flood+lamp&t=brave&iar=images&iax=images&ia=images
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.
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A halogen lamp will give you CRI = 100.
The best LEDs will manage about 90.
Those designed for street lighting will manage about 60. Greens are a real problem, because they are between the blue LED and the yellow phosphor. Your paintings will end up like those of L.S.Lowry - let's hope you become as famous.

 

Controlling the voltage will also control the current. And the ig "advantage" of using the halogen lamps is all of the free heat that they produce. It will help dry the water colors.

 

Thanks for the notes about halogen and quartz-halogen lamps, but I'm committed to LEDs. I need to learn more about them, like what I can accomplish with varied voltage versus varied amperage, and series v. parallel. I'll start with batteries, so as to fry as few of us as possible, and for all I know, batteries may suffice for the finished product. But I have some low-voltage variable power supplies as well. At this point, nothing is definite except the use of LEDs.

 

No problem ......... if you're seriously loaded with cash and don't know what to do with it all.

The Photography-Industry is alive and well, and has some really nice LED-Lighting-Equipment available.

Bring your financier associates with You, you'll need their approvals.
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Thanks for the notes about halogen and quartz-halogen lamps, but I'm committed to LEDs. I need to learn more about them, like what I can accomplish with varied voltage versus varied amperage, and series v. parallel. I'll start with batteries, so as to fry as few of us as possible, and for all I know, batteries may suffice for the finished product. But I have some low-voltage variable power supplies as well. At this point, nothing is definite except the use of LEDs.

LED's work at a specified voltage within a varaible current range.

 

Thank you all for your replies and questions. I'm still working out the design of the whole shebang, e.g., whether to use individual LEDs or CoB units. For what it's worth, reducing the brightness by varying the voltage and/or the amperage is just fine with me, as long as I can vary those parameters without damaging the device(s). I'm trying to build a miniature "stage" for doing oil and watercolor painting. The "stage" is basically a wooden box lying on its side, about 12" top-to-bottom, 18" side-to-side, and 12-15" deep, front to back. The "front" panel is not present, since the front opening serves as the proscenium, and makes it a stage. The LEDs or CoBs are to illuminate whatever is on the stage. So I would like to be able to control the amount of light from each source, the brightness, color and intensity, and by "source" I mean one LED, or a group of three, or a chip-on-board. What I can learn here will help me decide which devices, how many, where, etc. Suggestions?

would these lightings come from behind camera or from above the stage?

 

My comment about the halogen lamps was supposed to be a"funny," joke sort of remark. Waste heat is seldom a benefit.
LED lighting is usually better and always uses much less power, and mostly lasts much longer.

 

How do you expect to control the current through the LEDs?

Relying on the published VI curves is dangerous because the published curves are typical and since current is an exponential function of voltage there is likely to be a large variation in current. A large variation in current is likely to result in a large variation in power and therefore rate of degradation. This problem is ecacerbated

It is usually safest to drive the LEDs as a string with constant current. Alternatively driving with a resistor in series with each LED or string of LEDs will still be much, much better than trying to drive with a voltage source.

When using resistors to control current the more voltage you drop across the resistors the more the power source looks like a constant current source.

 

How do you expect to control the current through the LEDs?

Relying on the published VI curves is dangerous because the published curves are typical and since current is an exponential function of voltage there is likely to be a large variation in current. A large variation in current is likely to result in a large variation in power and therefore rate of degradation. This problem is ecacerbated

It is usually safest to drive the LEDs as a string with constant current. Alternatively driving with a resistor in series with each LED or string of LEDs will still be much, much better than trying to drive with a voltage source.

When using resistors to control current the more voltage you drop across the resistors the more the power source looks like a constant current source.

Certainly D.C. has provided what I agree is the correct information as to the BEST ways to power LEDs. Certainly there are other schemes that may work, possibly, for some very specific application. And probably after analysis those "other schemes" will be quite similar to what DC has listed already.
The only real exception is the very cheap schemes where low cost is the sole variable to be maximized, and LED life orperformance are not considerations.