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# Help me build my fixed driver!

#### Flugufrelsarinn

##### New member
Okay, so I have two questions.

On the LM317, how do I know which prong is VIN , VOUT, ADJ?

And to figure out how much resistance I need, would I use the formula...
R= 1.2/Iout? So if I want 150 Ma I would use a 8 ohm resistor?

D

#### Deleted member 8382

##### Guest
It's r= 1.25/Iout

You can find everything on the datasheet, google it

#### pyrotech

##### New member
Hallucynogenyc is right on with the formula. I come up with an 8.3 ohm resistor. You may not find that, but an 8 ohm will give you about 156Ma. Bear in mind almost any resistor can be 5% or 10% off and still be considered good, that will affect your Ma.
The pinouts confused me too. the schematics are for ease of drawing and do not necessarily represent accurate pinouts of the LM317. Pin 1 is Adj, there should be a dot on the plastic case to denote Pin 1. Pin 2 is Vout and Pin 3 is Vin.

#### Flugufrelsarinn

##### New member
Okay, thanks

Just to be safe with the PHR, I'll go with a 10 ohm resistor and 1k resistor. I was being dumb when I though 150 mA would last on the PHR. I'll ask here if I have any more problems!

#### Devilz Rej3ct

##### New member
If your running the 10 ohm and 1K resistor in parallel then correct me if I am wrong but you end up with 9.9 ohms.

#### pyrotech

##### New member
We kinda need to see your schematic.

#### Flugufrelsarinn

##### New member
Here's the best I could do

EDIT. Oops, forgot a switch, there will be a switch before the LM317

Last edited:

#### Bluefan

##### Well-known member
I =U/R -> I = 1.25/10 = 125mA
The PHR uses about 4 volt, so there's 4mA through the 1K resistor -> 121 mA left, but check the resistor you're using, 1% is qute common, but 5% may be around too. Measuring low resistances is best done with a 4 wire method, if available.

#### pyrotech

##### New member
Bluefan has electronics experience that I lack. My only concern is that if you want long life out of a PHR you may want to target >100mA. I've burned them out at 120Ma in less than a second. Not all PHRs are created equal. If you start low you can always replace a resistor and boost things up a bit.

#### Bluefan

##### Well-known member
from the diode compilation thread for the PHR:
Safe operating current: 90mA
Enthusiast operating current: 110mA
Maximum operating current: 120mA

But I'd question the lifetime of the diode when running close to the maximum, I'd go for 90mA and be happy with a longer life.

#### Flugufrelsarinn

##### New member
I'll probably play middle ground with it and run ~100 mA with a 12 ohm resistor.