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Old Dec 16, 2010, 11:12 AM
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You can set up the lm317 as a current regulator, 250ma would require a 4.8 ohm resistor. This way you only lose about .3 watts in the resistor. Be careful though since a resitor is added, several LM317's cannot be electrically connected through the heatsink (separate heatsinks or isolation required).
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Last edited by spog; Dec 16, 2010 at 11:21 AM.
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Old Dec 16, 2010, 11:23 AM
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Hi!

Quote:
250ma would require a 4.8 ohm resistor.
Really? But this way, only 1.2V would drop at the resistor.
If I used 3.3V regulators, and I want 220mA, then the voltage drop of a 15ohm resistor would be exactly 3.3V.
Or am I misunderstanding the concept?

Is the electrical connection through the heatsink a band thing? As I read, the case is ground. But even if I isolated them, they still would be connected via ground, as all are connected to the same cable from the power supply.
Would it not even be possible to save ground cables, and let the heatsink carry the current?
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Old Dec 16, 2010, 12:29 PM
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Julez,

The LM317 is for practical purposes a 1.2 volt fixed regulator. It will work exactly like the 3.3V regulator except the regulator will drop 10.8V instead of 8.7V.

The case of the regulator is the output. In Spog's circuit output = ground. But put the resistor in as you suggest and regulator output is some voltage above ground. The ground has to be on the other side of the resistor from the regulator output.

The problem Spog was poining out is that regulators are not perfectly identical in output voliage so connecting all the outputs together, even if that is not ground, forces output voltage to be exactly the same. Some regulators will think it is too high snd some think it too low. They will try to adjust for this and the result is they won't share the load evenly, perhaps very unevenly.
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Old Dec 16, 2010, 12:50 PM
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Spog,

I very much like your design for its simplicity. I have a bunch of LM317's around. I don't have a big heat sink and fan. How dumb would it be to use a smaller heat sink and toss the whole thing in water for cooling? Should be fine for low voltage, any idea how high could the voltage go? Ideally I'd like to go as high as a 10S Lipo.
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Old Dec 16, 2010, 12:53 PM
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Whoops, it just occurred to me that max input to an LM317 is like 37V? Wouldn't work for 10S
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Old Dec 21, 2010, 12:56 PM
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Quote:
How dumb would it be to use a smaller heat sink and toss the whole thing in water for cooling?
Great idea for increasing heatsink capacity for cheap. This should be ok, and you might even be able to run very high voltages with distilled water or transformer oil. I may find some time to play with this idea. Just BE CAREFUL, electrocution is not fun.
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Last edited by spog; Dec 21, 2010 at 01:05 PM.
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Old Dec 23, 2010, 06:08 AM
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There are plastic sprays available to protect PCBs from moisture. I heard that clear spray paint works also well. So give it a paint job, and it should work well submerged.
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Old May 09, 2012, 02:31 AM
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Why would this not work? I built it with 6 ST LM317T 's and the only load I measured was 35 ma from the fan running....
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Old May 09, 2012, 05:15 AM
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Quote:
Why would this not work?
It may be that the negative is not connected? The picture of my load depends that the LM317's are electrically connected to the heatsink. If the LM317's are isolated or are on separate heatsinks, the negative must be connected to the jumpered ADJ-OUT terminals of each LM317.
Attached is a schematic and wiring diagram. A .1uF capacitor should be added near the LM317's.
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Old May 10, 2012, 12:06 PM
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It is 1.5 amps per 317 regulator, you can also use a LM150/350 for 3 amp output.
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Old May 10, 2012, 12:19 PM
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Originally Posted by 7500rpm View Post
It is 1.5 amps per 317 regulator, you can also use a LM150/350 for 3 amp output.
No.
The LM317's are shorted, so it will operate at current limit. The current limit for the LM317T is approximately 2.5 amps at 12VDC. This decreases with increased voltage since the current limit is designed to limit output power to about 25-30 watts.
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Old May 10, 2012, 01:37 PM
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http://theparanoidtroll.com/electron...rceload-lm317/

Go down the page, more efficient way to do things than running parts on the edge of distruction...FYI
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Old May 10, 2012, 02:09 PM
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Quote:
Originally Posted by 7500rpm View Post
http://theparanoidtroll.com/electron...rceload-lm317/

Go down the page, more efficient way to do things than running parts on the edge of distruction...FYI
The resistor is not required, the device will not destruct. I have been using these as cheap heaters for years, none has ever failed.
The whole idea of this is to be cheap, safe and easy. The resistor defeats the design being inexpensive.
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Last edited by spog; May 10, 2012 at 02:27 PM.
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Old May 13, 2012, 07:57 AM
ltc
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I think it would be more prudent to install current output resistors, along with resistors to program the output current with respect to the internal VREF.

Here is an example

http://reprap.org/wiki/File:LM317-12V_Nx1A.jpg

Interesting use of am LM317 nonetheless.
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Old May 13, 2012, 01:55 PM
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Hi all,

Out of interest, here's my own adjustable electronic dummy load complete with LCD and pot to adjust the load current. My own requirement is for up to 5A loads.......but would be very easy to adjust the schematic for much higher.

Ian.



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