Old 01-14-2009, 09:36 AM #1
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Default DDL Circuit Values

Hi

I wonder if I use a DDL circuit, what will the resistor values to use with the LM317 for long life of of the diodes

(a) Red - 360 mA?
Resistor value set at 3.5 ohm

(b) PHR803 - 100 mA?
Resistor set at 12.5 ohm

The voltage will not matter, right? Because the current is regulated.

Any advise greatly appreciated.


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Old 01-14-2009, 10:33 AM #2
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Default Re: DDL Circuit Values

More voltage in means more volts are wasted and burned up by the regulator (i.e it gets hot). You want to avoid having 12 volts in if it needs only 7.5v to reach regulation.

I'd highly recommend getting a reasonably cheap multi meter than can measure current to accurately set your current using a test load.
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Old 01-14-2009, 02:25 PM #3
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Default Re: DDL Circuit Values

I've made a LM317-based driver for a PHR, running at 125mA, it runs well off a 9-10V (3xCR2) supply. The driver does have a dropout of about 1.5V at 100mA, you must add also 1.25V lost in the resistor for current sense... we're at 5.5V (PHR) + 1.5V (317) + 1.25V (resistor) = 8.25V. reccomeneded powering it off a 9V compact rechargeable cell or anything like it, but able to source for most of the time MORE than 8.5V, or you'll get your diode starting to dim at medium battery charge.

On another side, I've noticed that putting a 50 ohm resistor parallel to the 100 ohms pot gets a better regulation, allowing to get the diode start lasing at the minimum pot value (about 30mA in my build) and a discretely linear power gain up to 60-65% output. Being the output current formula 1.25 / R, as R decreases, power doesn't increases linearly with the bare potentiometer, placing a resistor in parallel will give you some more room for near-linear operation, still getting half of the power control between 90 and 100% of your pot :-[

If I'll be able to test a decent setup with a mosfet and a comparator to keep the current stable I'll post a tutorial about it.
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Old 01-15-2009, 05:19 AM #4
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Default Re: DDL Circuit Values

Are my resistor values correct for PHR and red?

Correct me if I am wrong, as long as the voltage is enough, the current will stay constant with the fixed resistor values. I am thinking of not using any pot at all to save space. Just want a safe value for the current...

Is my method doable?
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