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11282007, 08:01 PM  #1  
 
Class 2 Laser 
What is the formula to find the current that would be allowed to a LD using a resistor of different values? for example what would be the current to one of senkats LD's if i used a 5.1 ohm resistor. i have a fiew choices of what resistors to use and i want to make the right choice. BTW: this is using deadals driver these are my options: 5.1 Ohms 3.6 Ohms 5.6 Ohms Two 10 ohm resistors in parallel (5 Ohms) 6.2 Ohms 6.8 Ohms i can get all of these for a couple of dollars i just need to know how much current will geet to my diode. if any one can help or give me the formula that would be great Thank you, Mitch  
11282007, 08:11 PM  #2  
 
Class 2 Laser Join Date: Nov 2007
Posts: 317
Rep Power: 11 
Well, current = voltage/resistance, I=V/R So if you put one volt across one ohm you'll have one amp. If you know your voltage, and the voltage drop of the diode, you can calculae the current that you should be delivering to the junction. Is this the info you were looking for? Dave  
11282007, 08:14 PM  #3  
 
Class 2 Laser 
awesome thanks
 
11282007, 08:25 PM  #4  
 
Class 2 Laser 
well know i know i am doing somthing wrong. with deadals circut the ld should get around 2.6v and with a 5.1 ohm resistor the driver should output a max of 250ma but when i use this formula i get about 500ma what am i doing wrong. sorry i am so ignorant, well i guess every body has to lear some where Thanks, Mitch  
11282007, 08:57 PM  #5  
 
Class 2 Laser Join Date: Nov 2007
Posts: 317
Rep Power: 11 
Are you accounting for the voltage drop of the diode junction? I may not be understanding here myself  and I do not know the Vdop of the diodes  but if you've got a 0.7v drop on the diode, and feeding it with 2.6v, I'd use the adjusted voltage of 2.6  0.7 = 1.9 volts to figure my current with. But now I'm confused, and without knowing for certain the Vdrop of the diode... you'd probably get better advice from SenKat, Daedal, BlueFusion or one of the other gurus here. If you've gotten no better advice by the time I've returned (heading out right now) I'll dig up my thinking cap and see what I can figure by actually looking at Daedal's schematic. Dave  
11282007, 09:00 PM  #6  
 
Class 3B Laser Join Date: Jun 2007 Location: Dallas Tx
Posts: 3,199
Rep Power: 180 
A better way with the LM317 is to take the sense voltage and divide it by the current to figure the value of the resistor you want to use. So R = 1.25/.250ma's for example. In this case you would want to use a 5 ohm resistor. The voltage drop of the diode doesn't matter, this is the way to calculate the resistor needed.
 
11292007, 02:11 AM  #7  
 
Class 2 Laser 
What is the sense voltage and also what would be the max current getting to a bluray diode with a 15 ohm resistor. i don't get it  
11292007, 03:13 AM  #8  
 
Class 4 Laser Join Date: Sep 2007 Location: Northern CA, U.S. Astrophotography! (Some of my Photos)
Posts: 9,851
Rep Power: 3922 
Hi rubberband, Gazoo showed me that formula a while back and I have found it to be excellent for calculating what resistor to use for a desired current. If you are using DDL's driver with the LM317, just always use the formula of 1.25/desired current and you will get the ohms of resistance needed! Example: 1.25/250 is 5 ohms 1.25/291 is 4.3 1.25/83 is 15.06 ohms So, a 15 ohm resistor will give you around 83mA of current... Jay P.S. I couldn't tell you why the formula works ( I don't know about these things), but I do know that it works!
__________________ Personal lasers: (just a few  too many to list) RPL 425, Sypder III Krypton, RPL 35 (473), GLP473, GLP593.5, HeNe 632.8, Many DIY 445, red, bluray, green, 635, and 450 builds... Looking for a DIY kit? Take a look at some of my more popular kits linked in color... * FS: 405G1 Glass Lens Mod for Bluray! 31% increase in power vs AixiZ acrylic! * FS: New! 1000 Lumen XML2 Maglite Modification With Voltage Monitor!  Parts or Complete Light! Must see! * FS: Polished Stainless Steel Green Kit! Easy assembly in minutes! * FS: Large Maglite Monster Kits! Side Button Forward Clicky! Custom Head Option! Gnarly... Click here for a list of my build kits! Projects and tutorials too!  
11292007, 04:12 AM  #9  
 
Class 2M Laser Join Date: Sep 2007 Location: The Left Coast
Posts: 835
Rep Power: 17  Quote:
but once more:
DanQ  
11302007, 12:21 AM  #10  
 
Class 2M Laser Join Date: Aug 2007
Posts: 790
Rep Power: 23 
DanQ, nice use of color code. [smiley=thumbsup.gif]
 
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