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Old 01-09-2012, 07:31 AM   #1
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Default Resistors for LM317

The output of my DDL is going to be approx 500mA, I've bought 0.25w resistors (10ohm) I'm going to stack 4 of them in parallel... does that pose a problem?
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Old 01-09-2012, 08:47 AM   #2
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Default Re: Resistors for LM317

10 ohm x4 in parralel is going to be 2,5 ohms, which is exactly 0.5 amps in theory...

Nope, no problem posed. Just a question, what diode do you intent to power with half an amp?
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Old 01-09-2012, 04:00 PM   #3
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Default Re: Resistors for LM317

Weird error when editing post

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Old 01-09-2012, 04:01 PM   #4
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Default Re: Resistors for LM317

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Originally Posted by Eudaimonium View Post
10 ohm x4 in parralel is going to be 2,5 ohms, which is exactly 0.5 amps in theory...

Nope, no problem posed. Just a question, what diode do you intent to power with half an amp?
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Has a suggested safe drive current of 400-500mA so I'm modifying this DDL Laser driver - It can be done with 4x10ohm resistors in parallel to create 2.5ohms (at 500mA)

Was just concerned since the resistors I've bought are 0.25a (Standard), but if you say that's cool, I'm happy

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Old 01-09-2012, 07:52 PM   #5
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Default Re: Resistors for LM317

Resistors in parallel share the current divided between them but have the same voltage drop across them, so what you get is each resistor in parallel shares the wattage requirement. Four 250mW 10 Ohm resistors in parallel will yield the same as one 1W 2.5 Ohm resistor.

The current level seems fine but 500mA x 3V Vf = 1.5W load, your resistors might get hot with long duty cycles. Either upgrade to 1/2 watt resistors or heatshrink the leads and thermal epoxy the resistor bundle to a heatsink.
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Old 01-09-2012, 11:25 PM   #6
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Default Re: Resistors for LM317

I've ended up with enough components to build about 20 DDL drives lol... so I'll leave it as it is for now, it'll do the trick for my first build, I'm just trying to get a feel for things before I go on to bigger and better builds, I hadn't taken the power ratings of the resistors into account, it doesn't really mention that kind of thing in DDL build guides, I guess it's just assumed. So... Off I go to eBay and buy the cheapest components I can find lol..
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Old 01-09-2012, 11:32 PM   #7
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Default Re: Resistors for LM317

Before you check eBay, try Mouser.com If you're buying bunches of parts you can really save a bundle there. Just be sure you pick USPS for shipping because everything else is $$$.

There's some nice 5 and 10 watt resistors there in assorted values from half an ohm to 10 ohms, which when used in the right combination will work for any current setting you could need and will handle up to a 2000mW 405nm build (if we ever get it up that far, haha!).
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Old 01-09-2012, 11:53 PM   #8
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Default Re: Resistors for LM317

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Originally Posted by Sigurthr View Post
The current level seems fine but 500mA x 3V Vf = 1.5W load, your resistors might get hot with long duty cycles. Either upgrade to 1/2 watt resistors or heatshrink the leads and thermal epoxy the resistor bundle to a heatsink.
The voltage drop in the LM317 is 1.25V. The power is 500mA x 1.25V = .625W
The four 1/4W resistor will do fine at 500mA setting. They can handle this wattage.
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Old 01-09-2012, 11:54 PM   #9
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Default Re: Resistors for LM317

Yea, suppose I should have scouted around rather than sticking to eBay, I'm always unsure of websites shipping as I live in the 51st state (AKA The UK), Our good friends in China seem to want to sell me all sorts of components cheaply and with free postage, which is nice... even so, I've been buying components for this build for a couple of months with the change left in my bank account after all the bills are paid, I've been keeping a log of what I've bought, where from and how much, I decided to add it all up today and came up with 30 which is depressing since I can probably buy a nice powerful green laser for less than that. On the up side, I will have enough components for various DDLs so all I'll have to do next time is tweak the design and purchase a new LD and housing.

So far the most expensive thing was the eye protection (They are more expensive than the damn LD lol)

Once I'm done with this build, I'm going to try and build the most powerful laser my budget will allow lol.
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