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Class : Apprenti
Posts : 2
Registered on : 12/10/2008
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Hello, I want to try making a 3-LED clip with USB-power from AL107B LEDs, which seem to have nominal 100 mA direct current and 1.8 V voltage. It looks like I can connect them in 2 ways: 3x(LED + 32 Ohm resistor) or ((LED + 32 Ohm resistor) and (2 LEDs + 14 Ohm resistor)) parallel-connected. In first case, the normal summary current will be 300 mA, with all LEDs disrupted - almost 469 mA; in the other case, normal current will be 200 mA, with all LEDs disrupted - 513 mA.
I'm going to use a PTC or meltable fuse, but I can't understand, what is the maximum current that USB can give without theatening a motherboard.
Wikipedia tells that default current is 100 mA, but device can request up to 500 mA using the data channel (D+/D-), and the cite-note about requesting the high-power is broken; in you FAQ a 200 mA PTC fuse is strongly recommended, but I don't understand, what did this value appear from.
So, what current can I use? Can I request 500 mA in a simple way, if needed? What will be, if 100mA<I<200mA? 200mA<I<500mA? I>500mA (10-20%)?
Thanks and sorry for my poor English...
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Class : Apprenti
Posts : 6
Registered on : 13/08/2008
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Don't be afraid. I used USB power with serial attached 3 IR leds and a 40 Ohm resistor. Its working ok, even 24/7 when you forget to turn power off.
The brightness is good enough, so you can use your system not only in dark conditions.
Edited by gearman on 12/10/2008 at 21h48. |
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Class : Electronicien en chef
Posts : 651
Registered on : 10/07/2007
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Hello chaos.
100mA are the leds nominal current value.
You don't need to drive them at full power.
You can try running them at 50mA, so you won't overload your USB power.
Also if you can , always choose the less current consuming setup.
That's a serial/parallel setup, with 1 led in a serial circuit and 2 leds in the other.
To acheive 50mA, you'll get :
2 leds and a 27 ohms resistor
1 led and a 68 ohms resistor
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