Old 03-07-2011, 02:05 PM #17
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Default Re: Lm317 Help

RHD,
Well,,KICK ME IN THE HEAD!! I finally got it! I was all wrapped up in total current of the output loop being exponentially higher than current in the reference and internals of the 317 (quite simple really) and blah, blah, blah.... I couldn't see that I was calculating the power dissippated by the LD, NOT the resistor... I feel like such a dummy (probablly rightfully so...). I bow to your experteese and command of Voodoo Magic! Keep on shining!
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Old 03-11-2011, 02:25 PM #18
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Default Re: Lm317 Help

Ok, now I have a question!

When we build our typical test loads, is there a rationale for the 1 ohm resistor, other than merely creating a measurement point where mV = mA ???

And either way, why do people prefer measuring mV at the resistor, when you could just use current mode and put your meter in series ?!?
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Old 03-11-2011, 04:13 PM #19
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Default Re: Lm317 Help

Only reason I can see is people are lazy.
Dividing something by 1 is infinitely more immediate and easy than dividing by anything else.

Having SOME resistor always in series is important though to provide a solid continous electrical circuit,
because apparently some circuits like the flexdrive, go POOF when run without a load (complete circuit).
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Old 03-11-2011, 05:34 PM #20
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Default Re: Lm317 Help

Quote:
Originally Posted by anselm View Post
Only reason I can see is people are lazy.....
ROTFL




@rhd: seriously, the 1 ohm resistor is the more easy way for measure the current in the test load ..... diodes FV and internal resistence changes with current, so you cannot measure at them with precision ..... also, 1 ohm value is choosen cause this way, each mA flowing through the tesistor cause an 1mV voltage difference at its sides (old good Ohm law, V=R*I), so you can simply hook the DMM in parallel to the resistor on mV range, and read directly the millivolts as milliamperes.
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Old 03-11-2011, 09:27 PM #21
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Default Re: Lm317 Help

I'm sorry - I still don't understand why it wouldn't be easier to just turn the dial on your multimeter to CURRENT, and read the current directly while in series??
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Old 03-11-2011, 09:45 PM #22
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Default Re: Lm317 Help

You can do that just fine, I have done it. At least with a LM317 circuit.
But I read different things about switching drivers.
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Old 03-11-2011, 10:26 PM #23
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Default Re: Lm317 Help

Wow this thread has been busy, I have upgraded my host to a mag light with 8 AA batteries inside.

I also built my self a driver with some random parts from radio shack.
Op-Amp, transistor ect.
It is still linear so its inefficient, but now I get a nice even spread when adjusting the brightness.
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