Old 10-14-2008, 06:03 AM #1
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Default Fixed DDL, help...

i built an adjustable driver but for some reaosn both teh PHR diodes i used seem quite underpowered. so i think iv made a mistake in either the driver or the DMM readings.
I think the easiest thing to do for now is to build a fixed, but how can i build one wich will have a definate current of 90mA?

If your nice enough could you make or find a schematic, or explain to me how to mod the adjustable DDL.

thanks dudes...


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Old 10-16-2008, 06:02 AM #2
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Default Re: Fixed DDL, help...

mans... :-?
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Old 10-16-2008, 04:13 PM #3
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Default Re: Fixed DDL, help...

DDL driver is a constant current driver. The only component that's responsible for this is the LM317. And to set it up in current control configuration, is to connect a resistor to the output, and connect the adjust terminal to that other end of the resistor. Then connect the adjust terminal to the load of the circuit.

The LM317 works by ensuring a constant voltage, 1.25V, exists between the output and the adjust terminal. hence the current it outputs would be 1.25/resistance of that resistor only. It does not matter what the resistance of the load or other parts of the circuit will be. That's why it's called a constant current driver. (Get the datasheet for the LM317. They also explain several ways of using the LM317 along with how it works)

So if you'd want one with 90mA, make sure your power source is at least 6V (this is due to the dropout voltage of the LM317, i.e. the LM317 "eats" up some voltage itself), and then the resistance requirec would be 1.25V / 90mA = 13.9 ohms (rounded). Since they do not sell such precise resistors, it might be wiser to get a trim potentiometer and adjust till you get that resistance.
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Old 10-17-2008, 03:25 AM #4
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Default Re: Fixed DDL, help...

Hi

Use the online current calculator to get the ohms value:

http://www.reuk.co.uk/LM317-Current-Calculator.htm
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Old 10-17-2008, 06:32 AM #5
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Default Re: Fixed DDL, help...

well i made a driver with a trim pot... but im prety sure my DMM says 90mA. i have more trust in a fixed driver then setting my own...
but thanks for that. 14ohms...
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