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Old 14th December 2011, 11:08 AM   #1
AndrewT is online now AndrewT  Scotland
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Default high current CCS using lm317

Hi,
my searching abilities are proving to be severely incapable.

How do you wire up an lm317 with a booster transistor, so that it operates as a CCS at currents that exceed the 317's 1.5A current rating?
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Old 14th December 2011, 06:04 PM   #2
Nisbeth is offline Nisbeth  Denmark
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Forgive me for stating the obvious, but wouldn't it be easier to just use an LM350/LM338/LT108x with a higher output current rating? This would only work up to the 7.5A of the LT1083 of course.

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Old 14th December 2011, 06:52 PM   #3
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Why did not adding current boost (pnp) transistor and current limiting transistor (npn) + current limit resistor work?

Check data sheet page 20 for 6V Charger and go add the boost transistor...
Change resistor values as needed...
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Old 14th December 2011, 07:04 PM   #4
AndrewT is online now AndrewT  Scotland
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A 317 CCS is a two component assembly.
Is starting with voltage regulator and modifying it to add on a current limiter the only way to get a >1.5A CCS?
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Old 14th December 2011, 07:11 PM   #5
SY is offline SY  United States
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Let's say you have a power NPN. Add a resistor in series with the emitter, and take the CCS output from the "far" end of the resistor. Now, connect the collector to the power source, the "in" of the 317 to the power source. Connect the 317's "out" to the transistor base, then the 317's "adj" terminal to the "far" end of the resistor.

The resistor is calculated as i(set)/0.55. That value is approximate, you may have to diddle the resistor value a bit.
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Old 14th December 2011, 07:17 PM   #6
AndrewT is online now AndrewT  Scotland
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I have drawn that description out.
I will plug board it this PM.
Thanks.
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Old 14th December 2011, 09:26 PM   #7
AndrewT is online now AndrewT  Scotland
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It works, with just 3 components.
Thanks again.
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Old 14th December 2011, 09:41 PM   #8
SY is offline SY  United States
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No prob. I love simple circuits.
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Old 16th December 2011, 12:21 AM   #9
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I just happened to be working on a 0V to 20V regulator to replace one side of my Microanta bench supply that quit working as it uses a UA723 and I don't have one to replace it.

It seems to be working good as a simulation in this configuration as high power CCS.

As a regulator R4 is a .5 ohm for the existing metering and R1 is the load and R5 would be shorted or nonexistent.

Pretty cool as this would/could be the Basis to a bufferd version of Elvee's LM317 Class A amplifier.

Also I used the 1N4002's for the simulation and Diodes of the proper current rating should be substituted for your application.
Without the diodes the output voltage won't go to 0V as a regulator.
I haven't spent much time on this as I just now whipped up the CCS configuration.

Enjoy
jer
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Last edited by geraldfryjr; 16th December 2011 at 12:28 AM.
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