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Old 11-28-2007, 08:01 PM #1
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Default Formula for finding current

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


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Old 11-28-2007, 08:11 PM #2
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Default Re: Formula for finding current

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
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Old 11-28-2007, 08:14 PM #3
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Default Re: Formula for finding current

awesome thanks
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Old 11-28-2007, 08:25 PM #4
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Default Re: Formula for finding current

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,
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Old 11-28-2007, 08:57 PM #5
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Default Re: Formula for finding current

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

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Old 11-28-2007, 09:00 PM #6
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Default Re: Formula for finding current

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.
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Old 11-29-2007, 02:11 AM #7
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Default Re: Formula for finding current

What is the sense voltage and also what would be the max current getting to a blu-ray diode with a 15 ohm resistor.

i don't get it
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Old 11-29-2007, 03:13 AM #8
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Default Re: Formula for finding current

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!
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Old 11-29-2007, 04:12 AM #9
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Default Re: Formula for finding current

Quote:
Originally Posted by jayrob
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!
:P{sigh} that secret knowledge has been revealed many times in these pages...
but once more:
  • the LM317 is a voltage regulator.
  • it maintains a voltage of 1.25V between its Output and Sense pins. That's its job.
  • therefore, in "Daedal's circuit', the sense resistor always has 1.25V across it.
  • once the resistor/pot is set/adjusted (by you), the resistance between those pins stays constant.
  • constant voltage and constant resistance means constant current.
  • your laser diode is in series with all that, so it gets the same current.
to figure what resistance to use:
  • ohm's law: Voltage = Current X Resistance (E=IxR)
  • algebra: E=IxR => R=E/I
  • therefore: Resistor Needed = 1.25V / Current Desired
hope this helps...

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Old 11-30-2007, 12:21 AM #10
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Default Re: Formula for finding current

DanQ, nice use of color code. [smiley=thumbsup.gif]
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