parallel driver circuit???

i want to drive 2 strings of 3 LEDs in each string from a single pwm driver with 12volt power source. the driver current was set so that each string gets half of the total current. i want to be able to turn 1 LED off in 1 string. when i've tried this by shorting across 1 LED, this whole string turns off and the other string still works(i think at the higher current setting used for both strings). is there a simple way to do this or is what i'm trying to do impossible with 1 driver? thanks

You are asking for trouble. In parallel strings, the forward voltage across each string is the same, regardless of the needs of the LEDs (this is one reason they aren't generally recommended). If you short across one LED effectively removing it from the string, the total voltage across the remaining LEDs stays the same. Lets say for the sake of argument that each LED has a vf of 3v, totaling 9v across all three LEDs. Short across one LED, and now the remaining two are forced to take 4.5v each. This will send current through the roof and kill the remaining LEDs in short order.

You can do this with individual drivers for each string though, as the driver will compensate for the LED that was removed from the string. You do need to be careful that when you remove one LED, the total forward voltage for the remaining LEDs is above the minimum voltage threshold of the driver.

i know you couldn't do this with resistor based current limit but, i thought the pwm chip would still divide total current going to each string? the current going to all LEDs would go up a little but i'm running them far below their maximum rating? why does the whole string go out when i short 1 LED but yet the other string still runs? i'm not arguing, just trying to learn.

Are the two strings connected together at each end, or do you have a dual-output driver?

If they were connected in parallel and you shorted one LED, I would expect that the other two in that string should light with twice the current, and the other string to go out, or nearly so.

This is because the driver should have a fixed current output, not a fixed voltage. Let's say your driver is set to 1A, and you expect 0.5A in each string. The LEDs will have a voltage vs. current curve, which might go through 2.2V at 5 mA, 3.0V at 500 mA, and 3.3V at 1000 mA. This is actually pretty close to realistic.

So if you have two strings of three, the total voltage would be 9V. If you had only two strings of two, the voltage would be 6V. But if you had only one string of 2, it would be 6.6V, because the LED voltage would be 3.3V at 1A. Now add a string of three in parallel with the string of 2. 6.6/2 = 2.2V, so the LEDs in the string of 3 would draw only 5 mA. They'd be so dim you wouldn't see them next to the other string. This is exactly what you have when you short out one of the LEDs.

Of course, if the driver has 2 separate outputs, then everything changes, as it's really like 2 separate drivers running 2 separate strings of LEDs. In that case, it's possible the driver doesn't like the string of 2, and shuts itself off to protect itself from overheating.

single output, ZLED7020. i'm running 700mA total so each string gets 350mA and LEDs are rated for 700mA each.
connected in parallel and you are correct. when i shorted 1 of the LEDs in the first string, all of the LEDs in second string went out and remaining LEDs in string 1 were a little brighter. is this because current takes the least resistance route? the power supply current went down about 150ma when i shorted and second string was out. sounds like i need to bite the bullet and use 2 separate drivers???

Very much as expected. Not really a problem here, unless you say there is.

i was just trying to save the cost of a second driver chip, coil, caps, etc.

The driver has no idea that you have one string or multiple strings connected to it. All it knows is it needs to spit out X current at Y voltage. Unless it's a multi-channel driver, it has no way to guarantee that each parallel string is getting the current and voltage it needs. You will have to use separate drivers per string to do what you are looking to do without killing your LEDs.

No, the driver knows that it needs to provide X current. The voltage will be determined by the load, subject to max and min of the driver.

+1 on the rest.