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Topic: Running high power led's without drivers. (Read 7 times) previous topic - next topic

carl1864

Well, the driver is designed for 18650 batteries, and says 4.2 input voltage.  So input voltage should be good.  Don't have a direct link, but its the same type typically used in flashlights to drive the 10w cree xml-t6 led's.

The battery does have protection on it, but when I experimented by hooking it directly up to a xml-t6 led, it pulled 1.6A and i'm guessing it can pull more, so its definetly capable of delivering more than the 700ma-1A that I was getting with the other led's.

Someone mentioned one of the weak led's taking all the current from the others.  However if they are all wired in parallel, I don't see how this could happen, each one should get as much current as it wants right?

Am I really doing this the wrong way?  Do they need to be in series instead?  And if so, can anyone explain why they would need to be in series and parallel doesn't work?  I don't exactly see why it would make a difference as long as they are being fed their correct voltage (for example say there are 4, 3.5 volt led's, either feed 3.5 volts to them all in parallel, or feed 14v to them in series, why would there be a difference).

retrolefty

#31
Oct 08, 2012, 11:07 pm Last Edit: Oct 08, 2012, 11:09 pm by retrolefty Reason: 1
Am I really doing this the wrong way?  Do they need to be in series instead?

Yes, by having only one constant current driver, you then have no defined control mechanism for equal current flow to each parallel LED, your just assuming somehow that each will take its equal share, and while that may be your wish and desire that doesn't make it so. Kirchhoff has defined how current and voltage works in series and parallel circuits and is worth a review: http://physics.about.com/od/electromagnetics/f/KirchhoffRule.htm

And if so, can anyone explain why they would need to be in series and parallel doesn't work?

Well according to one of Kirchhoff's rules in a series circuit the current flow is equal at all points in the circuit. So if three leds are wired in series and 700ma is the circuit current flow then of course 700ma is flowing into and out of each led, so they all operate at the same current. But of course as the desired LED current is 700ma for your LEDs, your constant current driver needs to run at 700ma output only, you can't use one that outputs a constant 2800ma output.

 I don't exactly see why it would make a difference as long as they are being fed their correct voltage (for example say there are 4, 3.5 volt led's, either feed 3.5 volts to them all in parallel, or feed 14v to them in series, why would there be a difference).

As other have said, you need to put LED forward voltage drop specifications into the background of your thinking, you need to keep controlling the current to a desired value in the forefront of your thinking. You don't operate or control a led by manipulating the circuit's voltage supply, you control the current by whatever method you deem to use, be it a constant current driver (which manipulates applied voltage to maintain a desired current) or with a series current limiting resistor (which just sets a max current that can flow given a constant applied voltage) which is not recommended for high power LEDs unless you run them less then their max rated current spec.


 And if so, can anyone explain why they would need to be in series and parallel doesn't work?  I don't exactly see why it would make a difference as long as they are being fed their correct voltage (for example say there are 4, 3.5 volt led's, either feed 3.5 volts to them all in parallel, or feed 14v to them in series, why would there be a difference).

If you wish to run LEDs in parallel then each LED needs it's own constant current driver rated at the leds operating current, 700ma in your case. If you run the LEDs in series then a single constant current driver will work with only the caveat that the driver must have a maximum output voltage capability of at least the combined forward voltage drop sum of the number of leds in the series string.
Lefty

dhenry

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Someone mentioned one of the weak led's taking all the current from the others.


Whether a led is "weak" doesn't matter (how do you define "weak" anyway?). What matters is their Vfwd. The led with the lowest Vfwd will begin to light up, until its Vfwd reaches the level to light up the led with the 2nd lowest Vfwd, ...

So paralleling different leds have the disadvantage of uneven current. But it provides better reliability than serially lighting the leds.

What people usually do, with large number of leds, is 1) drive them serially, with individual drivers: each led string has its own drivers. or 2) parallel led string but with resistors to even out large current unevenness.

The 1st approach is the best but more complexity and cost; The 2nd approach offers a good compromise.

Grumpy_Mike

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But it provides better reliability than serially lighting the leds.

Care to explain your thinking on that point?

winner10920

Reliability in terms of if one led in the chain failed they all would stop lighting, basically saying you can do it three different ways
you can light 100 leds in parallel (alot of current at a lower. Voltage + most expensive in term of material and wire but whichever one fails doesn't affect any others)
you can light 100 leds in series ( higher voltage but less current, cheapest probably dependig on ease of getting the higher voltage, drawback is that if one fails all 100 go out)
you can light ten in series and ten of those in parallel, now this is the compromise, one fails only ten go out but its not as expensive as all in parallel

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