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4x questions about SMD LEDs.? Answered

Hi ... I would like to ask :

1. how many SMD LEDs I can power from USB (in general)
2. would and USB port handle 90 of these (uk.farnell.com/avago-technologies/hsms-c191/led-smd-he-red/dp/1058373) LEDs ???

3. Would you rather buy 2pcs of these LED STRIPS (cgi.ebay.com/45-SMD-White-LED-Strip-Lights-Lamp-Waterproof-Flexible_W0QQitemZ370324943281QQcmdZViewItemQQptZLH_DefaultDomain_0) to get the 90 pcs ???
4. Could I power these 90 SMD LEDs from the strip by USB (after modifying it...'cose its for 12 volts).

Thank you in advance, for your answers.




Best Answer 8 years ago

Those LEDs that you posted have a max forward voltage drop of 2.6V and a max current draw of 20ma.
USB puts out 5V and 100ma.

So, to keep it simple, you would be able to power 5 LEDs (5 X 20ma = 100ma) by hooking them up in parallel.  If you put a larger value current limiting resistor to get the current draw down to 10ma, you would be able to put 10LEDs in series, but they would have similar overall brightness as the 5 LEDs at max power.

There are some fancy tricks you might be able to incorporate like charlieplexing, which will greatly increase the number of LEDs, but they won't be full brightness and it requires some controller circuitry.

If you bought those LED strips which are powered at 12 V, you would not be able to have them run at 5 V since they are probably wired 3 or 4 LEDs in series for each parallel branch. Since 2.6 X 3 > 5V, you would have to essential rebuild the circuit to power it with 5 V, so its not worth it.


Aside from multiplexing, you can connect 10 leds , in five parallel groups of 2 seris connected leds.

This is because the forward voltage is 2.6 v, roughly half that of the voltage source. Dropping the forward voltage to 2.5v per led will do no harm.

So how much voltage are you planning on dropping across the current limiting resistor?

none. a pair of 2.6Vf leds in series attached to a 5V source requires no limiting resistor. You could put a 100ohm in series to avoid short circuit issues, but if the circuit is built correctly, none is required.