> Resistor Question
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syncop8r |
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Class : Apprenti
Posts : 3
Registered on : 27/02/2009
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Hi all. I have purchased some SFH485P IR LEDs and am now trying to work out which resistors to use.
Current, Forward If:100mA
Voltage, Vf Max:3V
I wish to use 2xAA batteries to power them.
However entering these values into the wizard doesn't seem to work. I have seen another thread where someone has recommended inputting 50mA and 1.5V (why?). Should I use these values?
Using them gives me 30ohm 5%.
I have found resistors at my local electrical store but they are 30 ohm 1%. Would they be ok?
Any help much appreciated.
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benmeijer |
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Class : Habitué
Posts : 208
Registered on : 19/07/2008
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For the SFH485P IR LEDs 1.5 V is enough to reach the voltage when it gives (invisible) light. I measured them actually its 1.43 V).
calculation:
Resistor value (ohms) = [battery voltage (V) – LED’s forward voltage rating (Vf)] / LED’s current rating (If)
Resistor value (ohms) = [3 V - 1.5Vf] / 0.050 If = 1.5/0.05 = 30 ohm
30 ohm 5% means it could be 28.5 - 31.5 ohm
30 ohm 1% means it could be 29.7 - 30.3 ohm
for my own I used 33 ohm. Rule: lower the value the leds are brighter / rise the values and the leds are dimmed...... But don't go to far!
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Incisal |
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Class : Apprenti
Posts : 8
Registered on : 27/02/2009
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Ah so the % value is the tollerance/quality of the resistor! That explains alot to me now... that was so confusing before.
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asparagin |
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Class : Apprenti
Posts : 5
Registered on : 02/03/2009
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hi, i left my physics what electronics is concerned in highschool.
for 3 led i would use than 3 x 30 ohm resistances @ 1.5 V (2xAA) batteries?
thanks.
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