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Step 2Resistor Sizing
Last step I called out an 18 Ohm resistor - why 18 Ohm?
The product page gives the following MAXIMUM specs for the LED. You could use the NOMINAL specs, but hey, I live on the edge. The effect of using the maximum values will be to increase brightness at the expense of life. AS the life is quoted at 100,000 hours, even a 50% decrease would still be 5.7 years continuous use - I'll take that deal.
Vmax = 4.5 V
Imax = 30 mA (=30/1000 = 0.03A)
The capacitor can supply 5.5V, but since we will be feeding it from a USB port the voltage should only reach 5.0V.
The purpose of a resistor in series with an LED is to limit current, preventing thermal runaway. Thermal runaway occurs when an LED draws too much current, therefore heating up a little, which in turn causes a little bit more current to be drawn, which causes an increase in temperature in a viscous cycle.
So to determine the size of the resistor, we use Ohm's Law, where:
I = current
R = resistance
V = voltage
V= I x R
This can be arranged for each of the terms, here we want to know the resistor required for the maximum voltage and current.
R = V / I
The 'left over' voltage after the LED has taken it's 4.5V need to be paired with a 30mA current (this is how I think of it anyways - other people teach/learn it different ways).
(5.0 - 4.5)
R = ------------
0.030
= 16.66666666666...6
(Instructables won't let me format that properly - hopefully you get the idea, (5.0-4.5) is the nominator, 0.030 the denominator)
Resistor come in standard values... rounded UP (I'm turning soft!) the next standard values is... you guessed it... 18 Ohm.
Of course you could have just used a calculator like the one here
The colour code for an 18 Ohm resistor is brown-grey-black
The product page gives the following MAXIMUM specs for the LED. You could use the NOMINAL specs, but hey, I live on the edge. The effect of using the maximum values will be to increase brightness at the expense of life. AS the life is quoted at 100,000 hours, even a 50% decrease would still be 5.7 years continuous use - I'll take that deal.
Vmax = 4.5 V
Imax = 30 mA (=30/1000 = 0.03A)
The capacitor can supply 5.5V, but since we will be feeding it from a USB port the voltage should only reach 5.0V.
The purpose of a resistor in series with an LED is to limit current, preventing thermal runaway. Thermal runaway occurs when an LED draws too much current, therefore heating up a little, which in turn causes a little bit more current to be drawn, which causes an increase in temperature in a viscous cycle.
So to determine the size of the resistor, we use Ohm's Law, where:
I = current
R = resistance
V = voltage
V= I x R
This can be arranged for each of the terms, here we want to know the resistor required for the maximum voltage and current.
R = V / I
The 'left over' voltage after the LED has taken it's 4.5V need to be paired with a 30mA current (this is how I think of it anyways - other people teach/learn it different ways).
(5.0 - 4.5)
R = ------------
0.030
= 16.66666666666...6
(Instructables won't let me format that properly - hopefully you get the idea, (5.0-4.5) is the nominator, 0.030 the denominator)
Resistor come in standard values... rounded UP (I'm turning soft!) the next standard values is... you guessed it... 18 Ohm.
Of course you could have just used a calculator like the one here
The colour code for an 18 Ohm resistor is brown-grey-black
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15
Jul 4, 2007. 10:57 AM
Yerboogieman
says:
Yerboogieman
says:
i've had thermal runaway a few thousand times, thanks for the tip
Reply
1
Jan 2, 2008. 12:17 PMsimharry
says:
the led s smell when they have thermal runnaway
Reply
1
Apr 1, 2007. 3:34 PMaiden120000
says:
is 18ohm the same as 18R?
Reply
4
Apr 1, 2007. 7:17 PMtrialex (author)
says:
Sure is. You can use the colour bands or a multimeter to check.
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