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At various times and in various places I've seen several requests for how to set up LEDs on a sub. Here's the skinny on how they work and how to calculate the required resistor value.
LEDs have a very low resistance and must be hooked up to a battery through a current limiting resistor. When in an on state, they will drop a fairly constant voltage independant of the current they pass. ('Vf' on a data sheet) For most LEDs (green/yellow/red) this is about 1.2V. (I believe white and blue are up around 3V.) Most LEDs will give good illumination at around 20mA. ('If' on a data sheet) These two figues are approximate but are a good starting point for setting things up. With this understanding we can now calculate the required value for the current limiting resistor with this formula:
R=(Vbatt - Vf) / I
And here's an example:
Subdriver A wants to hook up a single LED to a 12V battery.
Vbatt = 12V
Vf = 1.2V
I = 0.020A [20mA = 0.020A]
R= (12V-1.2V) / 0.020A
= 540ohms
Subdriver B wants to hook up two LEDs to a 6V battery.
R= (6V-[2x1.2V]) / 0.020A
= 180ohms
Some other useful tidbits:
"I calculated my resistor value, but the LED won't light up. What could be wrong?"
-Unlike a bulb, LEDs are "one way" devices. Hooking them up backwards generally won't kill them, but they won't light up either. Round LEDs usually have a "flat" spot next to one of the leads- this is typically the lead that ought to be hooked up to the negative lead on your battery. (The shorter lead also often indicates the negative lead.)
"Can I hook up LEDs in parallel?"
-Yes, but it's not the best way to do it because they tend to glow with a very uneven brightness when you do. It's better to run them in series.
"How many LEDs can I string in series?"
-That depends on your battery voltage. Your battery voltage must be greater than the total Vf for all your LEDs.
"Can I hook up 10 LEDs to a 12V battery and skip the resistor?"
-Don't go there!
"My LEDs are too dim, can I make them brighter?"
-Yes, to a point. Some LEDs will handle 100mA of current. Some may burn out at 30mA. You'll know for sure what it can take if you can locate a data sheet, but it may be easier to just re-run the calculations for a higher current and try it out again.
No more excuses guys- time to light up those subs!
LEDs have a very low resistance and must be hooked up to a battery through a current limiting resistor. When in an on state, they will drop a fairly constant voltage independant of the current they pass. ('Vf' on a data sheet) For most LEDs (green/yellow/red) this is about 1.2V. (I believe white and blue are up around 3V.) Most LEDs will give good illumination at around 20mA. ('If' on a data sheet) These two figues are approximate but are a good starting point for setting things up. With this understanding we can now calculate the required value for the current limiting resistor with this formula:
R=(Vbatt - Vf) / I
And here's an example:
Subdriver A wants to hook up a single LED to a 12V battery.
Vbatt = 12V
Vf = 1.2V
I = 0.020A [20mA = 0.020A]
R= (12V-1.2V) / 0.020A
= 540ohms
Subdriver B wants to hook up two LEDs to a 6V battery.
R= (6V-[2x1.2V]) / 0.020A
= 180ohms
Some other useful tidbits:
"I calculated my resistor value, but the LED won't light up. What could be wrong?"
-Unlike a bulb, LEDs are "one way" devices. Hooking them up backwards generally won't kill them, but they won't light up either. Round LEDs usually have a "flat" spot next to one of the leads- this is typically the lead that ought to be hooked up to the negative lead on your battery. (The shorter lead also often indicates the negative lead.)
"Can I hook up LEDs in parallel?"
-Yes, but it's not the best way to do it because they tend to glow with a very uneven brightness when you do. It's better to run them in series.
"How many LEDs can I string in series?"
-That depends on your battery voltage. Your battery voltage must be greater than the total Vf for all your LEDs.
"Can I hook up 10 LEDs to a 12V battery and skip the resistor?"
-Don't go there!
"My LEDs are too dim, can I make them brighter?"
-Yes, to a point. Some LEDs will handle 100mA of current. Some may burn out at 30mA. You'll know for sure what it can take if you can locate a data sheet, but it may be easier to just re-run the calculations for a higher current and try it out again.
No more excuses guys- time to light up those subs!
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