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Old 04-16-2008, 03:36 PM #1
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Default Question about DDL driver resistor

OK, I have made many standard DDL circuits and they hace all worked fine, even for the blu-rays. But I mostly just built them exactly as stated. Now I got a long open can from TheMonk and a Jayrob heatsink and want to crank up the current. So I thought by adding another 10 ohm resistor to the other 2 10 ohm resistors in parallel would raise the current significantly. But I can only get about 246 mA out of it. Now the only 10 ohm resistor I had left is 1/8th watt. Is the LM317 using more than 1/8th of a Watt to regulate? Does it actually pass the full current throug the adjusting resistor? Or just through the output?

Thanks


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Old 04-16-2008, 05:19 PM #2
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Default Re: Question about DDL driver resistor

The wattage of the resistors adds up. So 4x 10ohm resistors would give you 2.5 ohms, which will give have .5 amps of current. That, with 1.25V across the resistors, gives .625W, divided by 4 resistors is .156 watts apiece. That is more than 1/8 (.125) watt, so it may not be advisable. Plus, I haven't seen anyone run one of these diodes that high in current, it'll probably need to have active cooling to take that.

I just typed all that, and realized that maybe you meant that the 3rd one you put in already was only 1/8W rated, and that maybe what I said above had no bearing on what you're doing at all. So, let me write what I would mean if the 3rd one you put in was only 1/8W rated and that's what resulted in your low current reading. In that case, the 3.33 ohms from the 3 resistors should give .375A, resulting in .468W of power being put through the resistors total, or .156W of power through each resistor, same as with the other, and still more than the 1/8W rating of the resistor. So that may be your problem with what you're experiencing, that it's not allowing as much current through as it should be.

So yes, the full current is going through the sense resistor, so it does have to be rated to carry the current that you want to use. Hope that helps.
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Old 04-16-2008, 11:34 PM #3
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Default Re: Question about DDL driver resistor

Quote:
Originally Posted by desslok
OK, I have made many standard DDL circuits and they hace all worked fine, even for the blu-rays. *But I mostly just built them exactly as stated. *Now I got a long open can from TheMonk and a Jayrob heatsink and want to crank up the current. *So I thought by adding another 10 ohm resistor to the other 2 10 ohm resistors in parallel would raise the current significantly. *But I can only get about 246 mA out of it. *Now the only 10 ohm resistor I had left is 1/8th watt. *Is the LM317 using more than 1/8th of a Watt to regulate? *Does it actually pass the full current throug the adjusting resistor? *Or just through the output?

Thanks
Steve, didn't I send you a 3 ohm 1 watt resistor with the heatsink? This would be perfect for the open can build. The 3 ohm resistor will give you 420mA's. One other thing...at this current, you must have 2 X 3.6 volt batteries. 6 volts will not work for long.

LM317 DDL driver/watts for resistor:
Examples:
1.25 X .420(mA's) = .525 watt (needs a minimum of 1/2 watt resistor - 1 watt is better!)
1.25 X .200(mA's) = .25 watt (1/4 watt resistor or pot - 200mA max)

If I did not send you the resistor with the heatsink, let me know and I'll send you one...
Jay
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Old 04-18-2008, 01:47 PM #4
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Default Re: Question about DDL driver resistor

Quote:
Originally Posted by jayrob
[
Steve, didn't I send you a 3 ohm 1 watt resistor with the heatsink? This would be perfect for the open can build. The 3 ohm resistor will give you 420mA's. One other thing...at this current, you must have 2 X 3.6 volt batteries. 6 volts will not work for long.

LM317 DDL driver/watts for resistor:
Examples:
1.25 X .420(mA's) = .525 watt (needs a minimum of 1/2 watt resistor - 1 watt is better!)
1.25 X .200(mA's) = .25 watt (1/4 watt resistor or pot - 200mA max)

If I did not send you the resistor with the heatsink, let me know and I'll send you one...
Jay

Ok thanks you two for the replies. I think you did send me that resistor Jay, I think I put it in my parts box thinking it was the one I usually cut off the MXDLs. I'm sure I can find it though.
Thanks!
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