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### Topic: simple IR leds in series resistor question (Read 2753 times)previous topic - next topic

#### zehneinsvierzehn

#15
##### May 30, 2011, 12:07 pm
ah okay i misread. 60 mA is maximum forward current. so would 47R be enough?
you told me that their voltage can differ due to age or temperature. how much can it differ? if the "typical forward voltage" is 1,25V and the "maximum forward voltage" is 1,6V, what would you think? is it possible the LEDs draw even less than 1,1V?

still the usb question: can i get little more than 100mA from my usb port without setting anything?

#### sarahwilson

#16
##### May 30, 2011, 02:13 pm
Thanks all of you. You people are brilliant. It seems you have very good knowledge in this field. It helped me a lot.

#### Grumpy_Mike

#17
##### May 30, 2011, 11:47 pmLast Edit: May 31, 2011, 09:12 pm by Grumpy_Mike Reason: 1
Quote
is it possible the LEDs draw even less than 1,1V?

Wrong use of words. Current is drawn, voltage is dropped. Typical is a word that covers a number of sins but I would expect about 10% variation with temperature and 40% with life time.
The point is that a design that relies on undefined values is not a good or stable design, in other words you are not in control of what happens you are a hostage to fortune.

Quote
can i get little more than 100mA from my usb port without setting anything?

Yes 100mA is the default maximum current rating for a USB device. In the device's parameters it can say that it can request up to 500mA but in the absence of anything it's 100mA.

#### zehneinsvierzehn

#18
##### May 31, 2011, 02:05 pm
thank you! really

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