Yesterday I was reading the datasheet for that type of LEDs, an those 2 voltages -at least in that model- where for the typical voltage and maximum voltage drop; not the minimum and maximum.

So I was thinking, if 2.2v is the typical voltage, 12v for the 5 LEDs in series you had originally intended would come to 2.4v each; which shouldn't be too dim.

Have you given it a try yet?

No not as yet, they have to arrive which they will soon, but worried about what MikeML said:
Now, if you hook the four LEDs to a 12V high-current constant-voltage power supply, you will instantly vaporize them. You need to put a resistor between the 12V supply, and the four LEDs to prevent that!

Unfortunately, the specific value of resistance required to limit the current through the LEDs (think of ballast in the context of a florescent tube) is dependent on the exact total of the forward voltage drops of the LEDs. This is not the preferred way of doing this.

It is much easier to buy a LED driver constant-current type of power supply, which has a pot on it to set the max. allowed current for your specific LEDs, and the power supply voltage magically takes care of itself with no resistor required...

 

Yes, but that's because 4 LEDs in series with a 12v supply comes to 3v each; which is 0.3v over the maximum rated voltage. However, if I read correctly the rest of the thread, your problem was not having a high enough voltage to drive your initial 5 LEDs, not 4 LEDs (5 / 12 = 2.4v, which is 0.3v less than the maximum 2.7v).

 

4 Leds or 5Leds in series, the problem is that the supply is still not a constant-current supply that will drive 750mA through the LEDs. It is a constant-voltage supply that will drive 12V to what ever is connected to it, up to its maximum current capacity of 1.5A. It will burn up a series string of either four or five of the OP's Leds if he does not provide a means of limiting the maximum current to 750mA.

 

Agreed. That supply appears to be constant voltage for devices that need 12V. It's not an "LED driver" supplying a constant, settable current.

 

I see what you mean now. I just thought those would also limit the current by themselves like regular LEDs. I mean, before suggesting it I did a test: I took 3 LEDs (regular 3v blue LEDs) and arranged them in series, applied 9v without limiting the current, and measured almost 5mA.

Don't all LEDs do this?

 

I took 3 LEDs (regular 3v blue LEDs) and arranged them in series, applied 9v without limiting the current, and measured almost 5mA. Don't all LEDs do this?

They certainly don't. If the nominally '3V' LEDs had had an actual total forward voltage of, say, 8V they may well have fried.

 

Well, yes, I completely understand and agree with you about the risk of not limiting the current; it's not the best solution.

Said that... could there be any chance that 3 x 3v LEDs, which may have its maximum voltage rated at 3.4v or 3.5v, end up having a total forward voltage of just 8v in series, or get fried?

...

Edit: I'm not arguing, I'm asking because I suppose that would require an odd 3v LED having a 2v voltage drop, 2 of them having 2.5v, or all of them having 2.66v... would this be common?

 

LEDs forward voltages (Vf) do change with heat,etc...
A constant current power supply is ALWAYS recommended.
A resistor with any "high power" LEDs is never the right way to do it. Because a resistor cannot adapt on the fly the account for the changing Vf the way a constant current supply does.

Back to your 9V "test".. was this a 9V battery? If so batteries have internal resistances that in effect limit the current. Thats why cheap toys have LED attached to batteries with no external current limiting.. The batteries internal resistance does the job.

Now.. I build LED fixtures for reefs and algae scrubbers all the time.. from 3W to 200+W
I can tell you exactly how I would "properly" do it. (as I suggested already)
And thats with a meanwell constant current led driver like the LPF series (ac/dc constant current supply) or with a constant voltage power supply and a meanwell LDD (which is a dc/dc driver that turns the constant voltage supply into a constant current one).
You simply take the sum of the max Vf of all the LEDs in the string and find a LPF with a DC output voltage rating that covers that Vf and an output current to match the LEDs.. (again you don't add the currents.. You simply use a power supply that is equal to or less than the lowest current rating of each of the LEDs in the string... So for example if you had 5 leds in a string and 4 had a current rating of 1000ma and one was 750mA.. You get a driver that does 750mA.. and they all will be fed with 750mA automatically.

If you aren't sure of how many LEDs your best choice is a LDD driver with a constant voltage power supply as it has a much wider DC output voltage range.
If you know how many then either LDD+CV power supply or LPF is fine assuming there is an LPF that fits your requirements.

 

... could there be any chance...?

That's a statistical question that can only be answered by having detailed knowledge of the population of LEDs from which your 3 LED sample is drawn.

I think in your example scenario it would be quite rare to accidentally pick out 3 LEDs with such a large deviation from the mean Vf. (I think Alec_t was making a point, not suggesting that this was likely event.)

It IS possible to run a series string of LEDs with a precise, constant voltage supply. Current and thus brightness will vary somewhat with temperature, though, even with the precision voltage supply. It's not the right tool for the job.

 

Back to your 9V "test".. was this a 9V battery? If so batteries have internal resistances that in effect limit the current. Thats why cheap toys have LED attached to batteries with no external current limiting.. The batteries internal resistance does the job.

I was going to use as an example one of those lighters that have an LED directly connected to 2 batteries, but then I remembered studying about the internal resistance of batteries.

I did the test with a bench power supply at 9v and with the current limit at maximum (2A).

 

That's a statistical question that can only be answered by having detailed knowledge of the population of LEDs from which your 3 LED sample is drawn.

I think in your example scenario it would be quite rare to accidentally pick out 3 LEDs with such a large deviation from the mean Vf. (I think Alec_t was making a point, not suggesting that this was likely event.)

It IS possible to run a series string of LEDs with a precise, constant voltage supply. Current and thus brightness will vary somewhat with temperature, though, even with the precision voltage supply. It's not the right tool for the job.

The point I was trying to make with my initial post was that if the first figure for voltages given in the LED's specs is the typical voltage, and not the minimum voltage, then 12v might be good enough for 5 LEDs in terms of brightness. It was not my intention to suggest that he shouldn't limit or regulate the current.

 

...12v might be good enough for 5 LEDs in terms of brightness. It was not my intention to suggest that he shouldn't limit or regulate the current.

Agreed on both points.

 

thanks all so much for your input especially Mr Macgyver. I now understand about having a constant current equal to the smallest consuming LED, having trouble finding one to match, so was wondering if I bought a constant voltage device with spec to exceed requirements (or constant current ), would this little device help as it has two trimming pots?
http://www.ebay.co.uk/itm/DC-DC-100..._Measurement_Equipment_ET&hash=item3ce49e9fd2

 

That would work. My (mild) concern would be that, since it is rated to 10A, it might be touchy to adjust the current in the 600-700mA range.

 

That would work. My (mild) concern would be that, since it is rated to 10A, it might be touchy to adjust the current in the 600-700mA range.

And thats cheap junk that will more than likely fail.
Personally I stay away from "that price is really just too cheap" electronic components from China.. Its 100% guaranteed fake components (or factory rejects)/combined with horrible engineering, a possible fire hazard, etc... I value my safety more than that. Think how much would it cost you to ship anything to China.. More then a few dollars easily I'd guess unless you already live there. So how are they making any money unless something shady is happening. Don't support shady.. Unless its Eminem

As I stated above get a meanwell LDD. The current is set for you and the price is virtually identical and the company/product is proven. Then use a constant voltage supply that has an output voltage 3-5 volts or more higher than the sum of the Vf's of the LEDs in the string.
example.. if 5 LEDs with a max Vf of 3V (3 x 5 = 15 +3V = 18V) so a 19V laptop supply would work or a 24V supply

You can get a meanwell LDD-700HW for as little as $5.00 USD plus shipping. Assuming LED string current is 750mA or 1000mA.

 

And thats cheap junk that will more than likely fail.
Personally I stay away from "that price is really just too cheap" electronic components from China.. Its 100% guaranteed fake components (or factory rejects)/combined with horrible engineering, a possible fire hazard, etc... I value my safety more than that. Think how much would it cost you to ship anything to China.. More then a few dollars easily I'd guess unless you already live there. So how are they making any money unless something shady is happening. Don't support shady.. Unless its Eminem

As I stated above get a meanwell LDD. The current is set for you and the price is virtually identical and the company/product is proven. Then use a constant voltage supply that has an output voltage 3-5 volts or more higher than the sum of the Vf's of the LEDs in the string.
example.. if 5 LEDs with a max Vf of 3V (3 x 5 = 15 +3V = 18V) so a 19V laptop supply would work or a 24V supply

You can get a meanwell LDD-700HW for as little as $5.00 USD plus shipping. Assuming LED string current is 750mA or 1000mA.

Thank you for yet another detailed reply, I'm assuming you know I'm in the UK?
If so perhaps you'd be so kind to recommend where to buy.
Once again many thanks.

 

No you don't, I assumed it was on the profile when I signed up, but isn't.

 

Would this do the job on its own? Can't seem to get meanwell easily here. PCD 25-700

 

there are plenty of places in the UK to buy a meanwell LDD-700hw
Just google it.
mouser uk
ebay uk,etc...

 

there are plenty of places in the UK to buy a meanwell LDD-700hw
Just google it.
mouser uk
ebay uk,etc...

Did