Calculating resistor value?

[Lumencraft (Matt)]

Hello everyone,

I need to know how to calculate resistor value. Example, I want to use a resistor to reduce the current of a driver board from 1400ma to 1200ma. Can a resistor be used for this? How do I know what resistor to use? Where is a good place to get resistors?

Thanks

 

[Mr Happy]

I need to know how to calculate resistor value.

Certainly. Resistor values obey Ohm's law:

R = V / I

where the resistance value R in ohms equals the voltage difference across the resistor V in volts divided by the current flowing through the resistor I in amps.

Example, I want to use a resistor to reduce the current of a driver board from 1400ma to 1200ma. Can a resistor be used for this?

Perhaps, but most probably not. A constant current driver board is programmed to output a constant current. You cannot change that current without adjusting the driver (such as by replacing the current sense resistor on the board with a different one).

How do I know what resistor to use?

By reading the specifications and/or applying the correct formulas for the application where the resistor will be used.

Where is a good place to get resistors?

From electronics supply houses or sometimes B&M stores like Radio Shack.

 

[Illum]

Where is a good place to get resistors?

www.allelectronics.com

radioshacks gone downhill for awhile now, not much selections and a little expensive.

 

[Lumencraft (Matt)]

Certainly. Resistor values obey Ohm's law:

R = V / I

where the resistance value R in ohms equals the voltage difference across the resistor V in volts divided by the current flowing through the resistor I in amps.

So 7 volts in across, the resistor I have 6.5 volts. difference of .5v
.5 volts divided by the 4 amps I have coming out of the resistor means I need a resistor with a value of .125 oms ? getting warm?

Perhaps, but most probably not. A constant current driver board is programmed to output a constant current. You cannot change that current without adjusting the driver (such as by replacing the current sense resistor on the board with a different one).

So going battery to driver to resistor to led would probly not work?

 

[Lumencraft (Matt)]

Lets revisit this old thread and make sure I am on track here.

If I want to run an XML2 off of a power supply rated at 4V 2A since my cree is 3.1V @ 2000 ma then I need to drop .9V across the resistor right? So my resistor value needs to be .9V/2A= .45 ohm resistor? Yes? No?

Also to know how many watts the resistor needs to be capable of I do what? .9V x 2A? 1.8watt resistor needed?

Thanks in advance everyone.

 

[hiuintahs]

Yes you are correct in your calculations. However it is a good practice to oversize the wattage on the resistor and use at least a 3W resistor or better a 5W. 1.8W will get pretty warm.

Power in = power out + power of the resistor

Power in = 4V x 2A = 8W
Power consumed by resistor is 0.9V x 2A = 1.8W
Power out to LED is 3.1V x 2A = 6.2W.

Efficiency is 6.2 / 8 = 77.5%

0.45 ohms is a non standard value. Will probably have to settle for a 0.47 ohm. I have a 0.47 ohm laying around if wanting to experiment. If you have to have 0.45 ohms, then a 12 ohm, 1/4 watt in parallel with the 0.47 ohm will give you 0.452 ohms.

http://www.mouser.com/Passive-Components/Resistors/Through-Hole-Resistors/_/N-7h7z5Zscv7?P=1z0wszxZ1z0wui7Z1yzoxqbZ1z0vij2Z1z0z819Z1z0wljoZ1z0x6fr&Keyword=0.45+ohm+resistor&FS=True&Ns=Power%20Rating|1

http://www.digikey.com/product-search/en?FV=fff40001%2Cfff80482%2C40533%2C406c4%2C4076c%2C4076f%2C40770%2C409f7%2C40a3d%2C40e51%2C40e94%2C80008%2C80009%2C80069%2Cc0001%2Cc0002%2C1c0002%2C1c0003%2C400001&k=resistor&mnonly=0&newproducts=0&ColumnSort=-2&page=1&stock=1&pbfree=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=25

 

[Lumencraft (Matt)]

Awesome. Thank you.:twothumbs