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ddl circuit resistor values

nkgamer1990

nkgamer1990

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hey there,
so im making a 445 burner in a project box and wanted to control the output but had a few questions.
first, with a ddl circuit would i be able to control the output variably with minimal risk to the diode(by variably i mean change the output while the circuit is powering the ld?
second, if so what resistor value would i need for the full 1 amp output of the lm317.
third, if a ddl circuit cant safely do what i need can someone supply a diagram of a circuit that can?
help would be MUCH appreciated.
-Nicholas
 





nkgamer1990

nkgamer1990

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by overpowering do you mean to much input voltage causing the resistor to heat up until it fails?
if so can this be avoided by using a 1.3ohm resistor at 5watts?
-Nicholas
 
kiyoukan

kiyoukan

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no. i mean power being pulled threw the resistor.
AKA if you used a 1w resistor and was driving the driver at 2w you would burn out the resistor setting the driver into a free run.
If you are unsure of what you are doing you might just want to buy a driver.
 
nkgamer1990

nkgamer1990

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thanks for the advice, im much more of a do it yourselfer and try to live by that old teach a man to fish philosophy.
anywho would this circuit even work for actively controlling the output? and wouldnt i need a 5 watt resistor because 4.5-5 volts for the diode multiplied by 1amp gives you 5watts?
-Nicholas
 
Last edited:
kiyoukan

kiyoukan

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you could but to driver a blue laser at that power you will need more than 4.5-5v.
The laser needs 5vs and the lm317 drivers take about 2.8v so you need a powersupply of about 8 or 9v and you can use 12v psu no problem.
And about the resistors no. i dont know where you figure v * x = wattage of resistor but for the circuit in question the max resistor you will ever need is a 2w.
As the driver cant go above 1.5w.
This is a must read for you!
Laser driver - It can be done
Read all of it including testing. it will show you what you need.
Good luck and remember to discharge the caps before you connect a laser to it or else it will no longer work.
 
nkgamer1990

nkgamer1990

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well versed with rog.
but i guess i wasnt being clear, i know the regulator has a dropout voltage and that plus the voltage of the ld equals input voltage. not clear where you get 1.5watts from.
-Nicholas
 
kiyoukan

kiyoukan

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lm317s can output 1.5a max.
also it says the lm317s only take 1.5v but they take 2.8v in this setup.
so you will need 5 for the laser and 2.8 for the driver that means you need 8v psu at 1a
 
nkgamer1990

nkgamer1990

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i understand all that, i dont understand where you get 1.5 watts going through the resistor like you stated earlier.
-Nicholas
 
kiyoukan

kiyoukan

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i was saying you dont ever need a 5w resistor as the max it can go is 1.5w
 
nkgamer1990

nkgamer1990

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i get that, 5watt resistor is excessive however im just trying to figure out how you get 1.5watts when an lm317 can put out 1.5amps at 5volts. doesnt ohms law say amps x volts=watts?
-Nicholas
 
Blord

Blord

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If you are going to variable the resistor maybe a rheostat is better solution. Variable Resistors

These are variable resistors which can handle much more power.
 
T

Things

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For my 445nm box build, I use a 5W resistor to set the highest current setting, and I have a 15W (overkill) rheostat to adjust the power from just below lasing, to the max current set by the resistor, You put the rheostat in series with the resistor on the LM317.

Make sure you heatsink the LM317 very well, running them near their max power rating generates a LOT of heat.
 
nkgamer1990

nkgamer1990

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can someone supply a schematic by any chance?
also could i swap the lm317 for an lm338 or would that require a totally different circuit?
-Nicholas
 
Ramsey_innovations

Ramsey_innovations

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After looking at the data sheet for the lm338 it looks like it should work fine however I'm sure of the V.drop. There is a schematic towards the bottom of the data sheet under typical applications labeled "precision current limiter" follow that schematic for the most basic setup. Best of luck!

P.S. if you decide to add any capacitors to the circuit make sure they are fully discharged prior to connecting the L.D.
 
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