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Old 05-24-2009, 07:29 PM #1
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Default lm317 stability in question..

I have a question regarding the VOLTAGE stability of the LM317. I want to power 2 LM317 LD drivers in one enclosure. I want to be able to provide a steady 9 volts to each of their inputs. So I just figured I should use a third
LM317 in my complete circuit to be used as a 9 volt regulator, to feed the LD driver ics. I'm wondering if the VOLTAGE regulator's VOLTAGE will drop below 9 volts or not. Despite the LM317's voltage drop, would it be simply better to use an ML7812A 12 volt Voltage regulator even though the two LM317s call for 6-9 volts?


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Old 05-24-2009, 08:04 PM #2
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in other words... if I use an LM317 as a 9 volt regulator, should I expect it's output voltage to drop any when I connect two LM137s to its output?
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Old 05-24-2009, 08:30 PM #3
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Quote:
Originally Posted by Meatball View Post
in other words... if I use an LM317 as a 9 volt regulator, should I expect it's output voltage to drop any when I connect two LM137s to its output?
Why don't you just hook up a 9V battery , or a set of 18650 lithium batteries in series . Theres no point in over complicating things !
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Old 05-24-2009, 08:52 PM #4
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this is for a labby.. not only do I prefer mains power, but I haven't room for batteries inside my enclosure. maybe I should set an LM137 for 10.5 volts.... just in case...
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Old 05-24-2009, 08:59 PM #5
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If you're building a power supply from mains, you can either use a transformer, which after rectifying/smoothing is about 9V (or whatever voltage you want). It may be difficult to accurately get that voltage, though... Commonly, a regulated PSU will use a voltage regulator after the smoothing caps to get a more stable DC voltage (lower ripple associated with the rectifying/smoothing). The voltage should remain stable so long as the voltage supplied to the regulator is above it's necessary drop at it's intended supply current and that you don't overload/overheat the regulator IC.

So, yes, an LM317 can be used to regulate a voltage that is fed into some current regulating LM317s for the laser diode driver. Just make sure that your regulated VOLTAGE is high enough to account for the dropout of the LM317s to provide regulated CURRENT.

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Old 05-24-2009, 09:09 PM #6
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Ah! Thats the kind of answer I was looking for! So to compensate for the dropout of the LM317s, would it be better to set the voltage regulator a couple volts or so higher than the desired 9 volts, so that the dropout brings the voltage right back
down to say... ~9V?
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Old 05-24-2009, 09:22 PM #7
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I'm not entirely familiar with the dropout of the LM317 for voltage regulation, but I think it is somewhere around 2V or so. Shooting for at least a rectified 12V to the voltage regulating IC would probably be enough to get a stable 9V. If you go over, that's fine, but it would probably be a good idea to at least add a small heatsink to the IC, especially the higher the input voltage (greater input/output difference = more heat).

Then, add another 1.25V to the requirements of voltage regulation to get the required input voltage for the current regulating LM317s.
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Old 05-24-2009, 09:29 PM #8
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i think its 1.5V - 2V (the driver drop)
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Old 05-24-2009, 09:39 PM #9
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cool cool. I think I might have it under control then.. I'll use a 12v 1A wall adapter (puts out about 15-16v unloaded) to feed the voltage regulating IC set for 12 v. This will in turn feed about [(12 - (~2) - 1.25)v = ~9.75v] to each of the
current regulating LM317s. I like it! Thank you for the help!
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Old 05-24-2009, 09:52 PM #10
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Couldn't a LM7809-regulator work for you?
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Old 05-24-2009, 10:13 PM #11
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I did exactly what you are suggesting as I had to drop a supply from 12v to 7.5v.
It was better than wiring the other drivers direct and dropping 9v for the red and 7.5v for the BR both of which would have got very hot. I still used heatsinks and a fan just in case

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Old 05-24-2009, 11:21 PM #12
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I would, but all I have on hand is an LM7805 and LM7812. My local Radioshack doesn't carry the LM7809. But like what rog8811 showed, it would be nice to keep the voltage difference at the LD Drivers as low as practically possible. Plus a regulator would help protect my drivers from small spikes and main power imperfections if I set it up correctly with some capacitors across the inputs and outputs. I too, have heatsinks in place, and (even with my small enclosure) a small 12V fan. (thanks Radioshack). I'll post picts soon! I eventually plan to make a tutorial of my own... thank you all for your input!
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Old 05-24-2009, 11:29 PM #13
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Quote:
Originally Posted by FireMyLaser View Post
Couldn't a LM7809-regulator work for you?
It would work, but if he has the LM317, might as well use it... the 7809 would just save a couple resistors
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