?5v regulator instead of lm317

Thread Starter

Gadersd

Joined Dec 8, 2012
98
I am going to make a laser driver. Most of the schematics I see use a lm317 regulator. Would it be fine if I used a 5v regulator that outputs 1.5 amps instead?
 

crutschow

Joined Mar 14, 2008
34,201
Depends upon whether they are using the LM317 as a voltage regulator or a current regulator. Lasers typically require a current regulator.
 

Thread Starter

Gadersd

Joined Dec 8, 2012
98
It is used as a current regulator. Couldn't I just use resistors to keep the current at the right value?
 

Thread Starter

Gadersd

Joined Dec 8, 2012
98
Could you explain to me how that works? How would connecting the ground pin to the vout pin regulate the current?
 

MrChips

Joined Oct 2, 2009
30,618
The voltage regulator tries to maintain a constant voltage across R1.
Using Ohm's Law you can calculate the current through R1. This is your constant current.
 

MrChips

Joined Oct 2, 2009
30,618
The GND pin is actually a reference pin. The constant voltage regulator tries to maintain a constant voltage between the output pin and this reference pin.

You can offset this reference pin to a voltage different from 0V.
 

wayneh

Joined Sep 9, 2010
17,495
Yes, but notice that you need a supply voltage high enough to reach 7 at the output plus the drop across the regulator.
 

Thread Starter

Gadersd

Joined Dec 8, 2012
98
It looks like I have to use a lm317. I just read somewhere that a laser diode's voltage drop will increase as it gets hotter, so I guess I am going to have to use a lm317 to keep the current constant.
 

crutschow

Joined Mar 14, 2008
34,201
The disadvantage of using the 5V regulator as a current source is that it drops 5V across the sense resistor R1 whereas the LM317 only drops 1.2V. The resistor must thus dissipate 5/1.2 = 4.16 times more power for a given current output.
 
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