Parallel LEDs by AMC7135 driver & Vf testing

[mastou]

I planning to mod a dive light (plastic body) with 4 XR-E driven 2and2 in parallel by two AMC7135-1050mA drivers (mode 1) and two AMC7135-700mA (mode 2), thus driving the LEDs at either ~500mA or ~350mA each (hope you can understand what I mean.

I know it safer to drive the LEDs in series or each with their own driver, but thought it should be okay since I'm way below max mA.

Here to my questions:

1) I was planning to test the Vf of each LED (I have 10) by hooking them up to a AMC7135-350mA driver powered by a 4.5V power supply. This should let me choose the ones with similar Vf (at least two and two) - does this make sense or do I need to take other things into account e.g. burn in of the LEDs?

2) Do you think I will be able to see any difference between 350mA and 500mA? (maybe I should go for 700mA each I just don't know if it is safe with a plastic body)

Kind regards Magnus

 

[AnAppleSnail]

I planning to mod a dive light (plastic body) with 4 XR-E driven 2and2 in parallel by two AMC7135-1050mA drivers (mode 1) and two AMC7135-700mA (mode 2), thus driving the LEDs at either ~500mA or ~350mA each (hope you can understand what I mean.

I know it safer to drive the LEDs in series or each with their own driver, but thought it should be okay since I'm way below max mA.

Here to my questions:

1) I was planning to test the Vf of each LED (I have 10) by hooking them up to a AMC7135-350mA driver powered by a 4.5V power supply. This should let me choose the ones with similar Vf (at least two and two) - does this make sense or do I need to take other things into account e.g. burn in of the LEDs?

2) Do you think I will be able to see any difference between 350mA and 500mA? (maybe I should go for 700mA each I just don't know if it is safe with a plastic body)

Kind regards Magnus

1) That'll work. You might run 'em all for a while on a heatsink, but the burn-ins tend to run in parallel (High Vf is still high Vf)

2) Brightnesses:
XR-E datasheet:
350 mA is 100% luminous flux
500 mA is 125%
700 mA is 160%

These aren't widely distinguishable settings - it's a common problem with AMC7135 lights; the modes don't space very far apart. When you change modes (350 to 500 or 500 to 700), your eyes would see a pulse of brighter light, then they'd adjust and it'd look the same (Despite killing your batteries faster).

 

[mastou]

Thanks then I will probably just go for the initial Vf testing. I thought there was bigger difference in brightness between 350 and 500mA, maybe I should go for 350 and 700mA or drive all 4 XR-E in parallel with either one AMC7135-700mA (thus 125mA per LED) for mode 1 and two AMC7135-1050mA or 1400mA? (thus 500mA or 700mA? per LED).

 

[mastou]

I have now used the mentioned setup to measure the Vf of the 10 XR-E LEDs I have and got the following:

1) 3.2
2) 3.2
3) 3.18
4) 3.13
5) 3.19
6) 3.17
7) 3.16
8) 3.15
9) 3.13
10) 3.17

Since I just needed to identify the ones that have similar vf it is ok for me, but it seemed a bit low so I also tried to test the mA output from the AMC7135 driver (only one AMC7135 chip I removed two from the 1050mA driver) powered by a cheap power supply set to 4.5V and only got ~100mA?

Can this be due to poor connection, wrong size wires, or is it more likely to be the power supply that doesn't provide the expected 4.5V ?

Just out of curiosity is there an easy way to test if the removed amc7135 chips still work ?

 

[MikeAusC]

. . . . Just out of curiosity is there an easy way to test if the removed amc7135 chips still work ?

A 7135 will deliver 350mA provided that the input is 0.1 volt higher than the LED voltage - and it's kept cool - and the third pin connects to earth.

They couldn't have made it simpler

 

[MikeAusC]

2) Do you think I will be able to see any difference between 350mA and 500mA? (maybe I should go for 700mA each I just don't know if it is safe with a plastic body)

Because of the way the eye responds to brightness, most torches have minimum step sizes of 2:1 or 3:1 - anything less seems barely noticeable.