Silviron
Flashlight Enthusiast
There are people on this forum that are WAY better at math and electronics than I am, but so many people have asked how to calculate resistors for LEDs that I am posting this as a seperate topic so it might be easier to find.
(If any of the experts find errors, feel free to smack me upside the head and I'll correct them.)
FORMULA FOR DETERMINING RESISTOR VALUE FOR VOLTAGE DROP WITH LEDS:
Examples:
To run 1 Nichia on three 1.5 V batteries it would be:
4.5 - 3.6 / .02 = 45 ohms That would be the value to run the Nichia at 3.6V at 20 mA as the manufacturer recommends.
If you want to run 2 Nichias in parallel, at the recommended current it would be a total of 40 mA, so: 4.5-3.6 /.04 = 22.5 ohms
Since easily acquired resistors (like say from Radio Shack) are rarely available in the values you calculate, the easiest (and safest) thing to do is to use the next higher rating. 47 Ohms is a common value, and very close to the 45 you need for one Nichia on 4.5V.
If you want to run three SERIES {3.6V X 3 = 10.8V} connected Nichias on 12 V you would need a 60 ohm resistor: {12 -10.8/.02 = 60)
<HR>
If you can't find a resistor in the neighborhood of what you need, you can "construct" one using easily found resistors in parallel or series using the following formulae:
<IMG SRC=http://www.az123.com/LED/resseries.jpg>
For instance if you need a 20 ohm resistor and can only find some 10 ohm values, just put two in series with each other.
<IMG SRC=http://www.az123.com/LED/resparall.jpg>
If you need a 70 ohm resistor and only have 220 ohm resistors available, three of them in parallel will give you 73.333 ohms -close enough.
For working with "regular" LEDS, including Nichias a 1/4 watt resistor is usually enough (although if you are using 20 or more LEDS, I'd move up to at least 1/2 watt sizes for single or series resistors). If using resistors in parallel their wattage is increased (2 parallel 1/4 watt resistors will handle at least 1/2 watt.) so you don't need to use 1/2 watters in this case.
If you want to run a single Nichia at 20 mA on 6 Volts you need a 120 ohm resistor:
(6.0 - 3.6 / .02 = 120)
If you want to overdrive 2 SERIES Nichias to 30 mA on a 9V battery you would need a 30 ohm resistor (9V - 7.2V /.06) = 30 ohms. (this calculation isn't exact, {since as you vary the current, the voltage also increases if it is available} but it is close enough, I think). Since 30 ohm resistors are hard to find, you can use the more common 33 ohm resistor or use three 100 ohm resistors in parallel.
If you are using LS modules (Luxeon Star), you will vary the numbers accordingly: they are supposed to run on 3.4 Volts @ 350 mA rather than the 3.6V @ 20 mA for Nichias.
(I'd also recommend using at least 1/2 watt resistors with an LS module - others may disagree, but I say better safe than sorry.)
So, to run an LS on 4.5V you would want a 3.15 ohm resistor- (4.5 - 3.4 / .35 = 3.1428..) Since resistors in this range are hard to find, use three 10 ohm resistors in parallel which will give you 3.333 Ohms - close enough.
{ 1 / (1/10 + 1/10 + 1/10) = 3.333}
To run an LS on 3 NiCd or NiMH rechargable batteries which are nominally 1.2V each you would need a .57 ohm resistor- { 3.6 - 3.4 /.35 = .571 }
As usual these are hard to find- I'd recommend a stepping up to a 1 ohm resistor anyway for LED safety's sake. Most Radio Shacks don't carry 1 ohm resistors in anything less than 10 watt units and those are too big physically for most conversions. So, you can use six 10 ohm resistors in parallel. {1 / (1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10) = 1.0166).
<HR>
Because most easily found resistors have what they call a 5% (sometimes 10%) tolerance you might get an actual value of anywhere from 90 ohms to 110 ohms out of a 100 ohm resistor. So it is always a good idea to go to a higher value in selecting a resistor- better to have a slightly dimmer than optimum light rather than burning out the LED. This is somewhat less critical on lights that will be used only for a couple of minutes at a time with fresh batteries, but for lights that will burn for longer periods at a time, it is very important.
Also, most fresh batteries will have a higher than nominal voltage, and you should take this into account. A fresh Alkaline may have slightly over 1.6 volts, a fresh Lithium AA can actually have 1.8 V and a freshly charged NiCd or NiMH can have 1.45V initially. Although this over-voltage usually disappears quickly with heavier current draw things like incandescent bulbs, digital cameras etc., When dealing with sensitive solid state components (including LEDs) at a low current draw, this initial over voltage CAN be a disaster. Again, this is less likely on lights that are just used for a moment at a time, but you should be aware of it.
Just yesterday, I accidentally plugged a colored LED that was supposed to run on 2.1 to 2.4 V into a freshly charged NiCd 3.6 V battery pack (that actually had 4.04 V on it)---- It only took about 2 seconds to completely destroy the LED. It didn't explode as is possible, but it did turn brown, burn my fingers, and stink to high heavens before it burnt out.
Some multiple LED arrays may require series/parallel arrangements to run most efficiently on the available voltage. The best way to do this is to calculate the resistor value for each series chain and put that value of resistor on each series chain, rather than just putting a single resistor on the whole array.
HOPE THIS HELPS (and that I didn't make too many errors.)
(If any of the experts find errors, feel free to smack me upside the head and I'll correct them.)
FORMULA FOR DETERMINING RESISTOR VALUE FOR VOLTAGE DROP WITH LEDS:
Examples:
To run 1 Nichia on three 1.5 V batteries it would be:
4.5 - 3.6 / .02 = 45 ohms That would be the value to run the Nichia at 3.6V at 20 mA as the manufacturer recommends.
If you want to run 2 Nichias in parallel, at the recommended current it would be a total of 40 mA, so: 4.5-3.6 /.04 = 22.5 ohms
Since easily acquired resistors (like say from Radio Shack) are rarely available in the values you calculate, the easiest (and safest) thing to do is to use the next higher rating. 47 Ohms is a common value, and very close to the 45 you need for one Nichia on 4.5V.
If you want to run three SERIES {3.6V X 3 = 10.8V} connected Nichias on 12 V you would need a 60 ohm resistor: {12 -10.8/.02 = 60)
<HR>
If you can't find a resistor in the neighborhood of what you need, you can "construct" one using easily found resistors in parallel or series using the following formulae:
<IMG SRC=http://www.az123.com/LED/resseries.jpg>
For instance if you need a 20 ohm resistor and can only find some 10 ohm values, just put two in series with each other.
<IMG SRC=http://www.az123.com/LED/resparall.jpg>
If you need a 70 ohm resistor and only have 220 ohm resistors available, three of them in parallel will give you 73.333 ohms -close enough.
For working with "regular" LEDS, including Nichias a 1/4 watt resistor is usually enough (although if you are using 20 or more LEDS, I'd move up to at least 1/2 watt sizes for single or series resistors). If using resistors in parallel their wattage is increased (2 parallel 1/4 watt resistors will handle at least 1/2 watt.) so you don't need to use 1/2 watters in this case.
If you want to run a single Nichia at 20 mA on 6 Volts you need a 120 ohm resistor:
(6.0 - 3.6 / .02 = 120)
If you want to overdrive 2 SERIES Nichias to 30 mA on a 9V battery you would need a 30 ohm resistor (9V - 7.2V /.06) = 30 ohms. (this calculation isn't exact, {since as you vary the current, the voltage also increases if it is available} but it is close enough, I think). Since 30 ohm resistors are hard to find, you can use the more common 33 ohm resistor or use three 100 ohm resistors in parallel.
If you are using LS modules (Luxeon Star), you will vary the numbers accordingly: they are supposed to run on 3.4 Volts @ 350 mA rather than the 3.6V @ 20 mA for Nichias.
(I'd also recommend using at least 1/2 watt resistors with an LS module - others may disagree, but I say better safe than sorry.)
So, to run an LS on 4.5V you would want a 3.15 ohm resistor- (4.5 - 3.4 / .35 = 3.1428..) Since resistors in this range are hard to find, use three 10 ohm resistors in parallel which will give you 3.333 Ohms - close enough.
{ 1 / (1/10 + 1/10 + 1/10) = 3.333}
To run an LS on 3 NiCd or NiMH rechargable batteries which are nominally 1.2V each you would need a .57 ohm resistor- { 3.6 - 3.4 /.35 = .571 }
As usual these are hard to find- I'd recommend a stepping up to a 1 ohm resistor anyway for LED safety's sake. Most Radio Shacks don't carry 1 ohm resistors in anything less than 10 watt units and those are too big physically for most conversions. So, you can use six 10 ohm resistors in parallel. {1 / (1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10) = 1.0166).
<HR>
Because most easily found resistors have what they call a 5% (sometimes 10%) tolerance you might get an actual value of anywhere from 90 ohms to 110 ohms out of a 100 ohm resistor. So it is always a good idea to go to a higher value in selecting a resistor- better to have a slightly dimmer than optimum light rather than burning out the LED. This is somewhat less critical on lights that will be used only for a couple of minutes at a time with fresh batteries, but for lights that will burn for longer periods at a time, it is very important.
Also, most fresh batteries will have a higher than nominal voltage, and you should take this into account. A fresh Alkaline may have slightly over 1.6 volts, a fresh Lithium AA can actually have 1.8 V and a freshly charged NiCd or NiMH can have 1.45V initially. Although this over-voltage usually disappears quickly with heavier current draw things like incandescent bulbs, digital cameras etc., When dealing with sensitive solid state components (including LEDs) at a low current draw, this initial over voltage CAN be a disaster. Again, this is less likely on lights that are just used for a moment at a time, but you should be aware of it.
Just yesterday, I accidentally plugged a colored LED that was supposed to run on 2.1 to 2.4 V into a freshly charged NiCd 3.6 V battery pack (that actually had 4.04 V on it)---- It only took about 2 seconds to completely destroy the LED. It didn't explode as is possible, but it did turn brown, burn my fingers, and stink to high heavens before it burnt out.
Some multiple LED arrays may require series/parallel arrangements to run most efficiently on the available voltage. The best way to do this is to calculate the resistor value for each series chain and put that value of resistor on each series chain, rather than just putting a single resistor on the whole array.
HOPE THIS HELPS (and that I didn't make too many errors.)
