LED and resistor
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The LED should have a mA rating. If it's 20mA, that is the maximum current it's rated for, so you want to put a resistor on it that will keep it under that level at your maximum voltage (4.2V for Li-Ion). 4.2 / .020 = 210, so find a 200-220 ohm resistor and put it in series with the LED (doesn't matter if it's ahead or behind it). Rework for whatever the mA rating of the LED is.
Normally you'd also worry about the wattage rating of the resistor, but 1/4 watt is going to handle anything you're likely to be putting in an ecig. "Forward Voltage" rating isn't something you need to worry about unless you're wiring multiple LED's in series or using much higher voltages than we are.
--Dave
Normally you'd also worry about the wattage rating of the resistor, but 1/4 watt is going to handle anything you're likely to be putting in an ecig. "Forward Voltage" rating isn't something you need to worry about unless you're wiring multiple LED's in series or using much higher voltages than we are.
--Dave
Thanks Dave. I picked up some 100, 150 and 220 1/8 watt resistors on the way home.
The 100 and 150 resistors @ 4.2v will be pull more than 1/8th of a watt (176mW for the 100 ohm), so try to either stick with the 220s or go up to a 1/4 watt resistor. The 220 is pulling 76mW, so you're good to go with a 125mW (1/8W) resistor there.
As a rule of thumb, if you're getting close, you want to step up to the next one - the 150ohm resistor is technically 117mW, but thats close enough to the limit that you'll want to step it up to a 1/4W.
As a rule of thumb, if you're getting close, you want to step up to the next one - the 150ohm resistor is technically 117mW, but thats close enough to the limit that you'll want to step it up to a 1/4W.
Well, the LED's actually create some resistance as well (my actual measured current on a 25mA LED with a 220 ohm resistor was 10mA). With a 100, it was 27mA at 4.45V (driven by a wall wart), which worked out to 120mW. For a homebrew mod that's probably already overdriving a switch by a factor of 5+, being right at the margin of your resistor's rating isn't really going to do much. On the other hand, unless you're *really* hurting for space, there's no reason not to use the larger resistor, either.
--Dave
--Dave
Actually, Ohm's Law states:
Resistance = Voltage/Current
Amp(s) is what the current is expressed in
Actually, the forward voltage of the LED is very important. It respresents basically a fixed voltage drop across the LED, ahd the resistor has to handle the rest. I have used LED's that have a forward drop ranging from .7V all the way up to 3.9V depending on the composition and style.
The correct way to do this is to first determine the forward V drop of the LED. Subtract that from the source voltage, and then use Ohms law to determine the resistor size for the desired LED current.
Example:
My LED has a 2.5 volt forward voltage drop.
My source is 3.7V
I want 10mA (.01A) of LED current.
R = (3.7 - 2.5) / .01
R = 120 Ohms
P = (3.7 - 2.5) x .01
P = .012 Watts
As you can see, as the source voltage increase the forward voltage drop becomes less and less of a factor, but in low voltage applications (like these), it is important.
The correct way to do this is to first determine the forward V drop of the LED. Subtract that from the source voltage, and then use Ohms law to determine the resistor size for the desired LED current.
Example:
My LED has a 2.5 volt forward voltage drop.
My source is 3.7V
I want 10mA (.01A) of LED current.
R = (3.7 - 2.5) / .01
R = 120 Ohms
P = (3.7 - 2.5) x .01
P = .012 Watts
As you can see, as the source voltage increase the forward voltage drop becomes less and less of a factor, but in low voltage applications (like these), it is important.
Actually, that works out to 73mW being dissipated by the resistor.
This is why techs make lousy circuit designs: We use rules of thumb to tell if something "looks right", then check it against the specs, instead of really knowing the math behind it.
--Dave
--Dave
I know its a dumb question but why do you need the resistor. I dont know anything about that kind of stuff, but i put an led in my mod but no resistor.
Not all LEDs need one for the standard voltage we generally use. This is easy mode:
LED calculator for single LEDs
You guys make my brain hurt.
My suggestion is to try a 330 ohm resistor. If the LED doesn't light up, step down to a 220. Lather, rince, repeat until the LED lights up.
My suggestion is to try a 330 ohm resistor. If the LED doesn't light up, step down to a 220. Lather, rince, repeat until the LED lights up.
Not that simple, the LED will probably light on the 330, it will just be dimmer (you can actually light a small LED with the ohm or continuity functions on your multimeter). It will get brighter as you drive more current through it, but once you pass the rated current level you have an increased risk of burning it out. You could hook it straight to the source voltage with no resistor, and it will probably work fine, for a while.
So either run the math, or hook up the multimeter and check the current with various resistors until you find the right resistor value.
--Dave
So either run the math, or hook up the multimeter and check the current with various resistors until you find the right resistor value.
--Dave
There are many commercial LED's out there that have resistors built into them. For example, by design and chemistry, I don't know of a "true" 5V LED. But I can buy what looks like a standard TO LED package that is rated for 5V operation. That's because it has a resistor built into the die.
And you really don't want to run a LED above it's rated forward voltage without a dropping resistor. Above its rated voltage an LED loos a lot like a short. It will draw tons of current, be very bright, and fail very quickly. Ideally a LED will be driven via a "current driver" and not simply off of a base voltage.
And you really don't want to run a LED above it's rated forward voltage without a dropping resistor. Above its rated voltage an LED loos a lot like a short. It will draw tons of current, be very bright, and fail very quickly. Ideally a LED will be driven via a "current driver" and not simply off of a base voltage.
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