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15th December 2007, 08:30 AM  #1 
diyAudio Member
Join Date: Oct 2006

LM317 CCS  Any limitations?
Just my purely curiousity and would worth make my understanding of LM317 CCS more clear. Hope somebody could point me out.
My question will be, if LM317 forms a CCS of 1.5A, then i put 1 meg resistor to its downstream to the ground, will I get 1.5A through this 1Meg resistor? I don't think it will work but having limited knowledge in electronics make me seek for the explanation. Many thanks, AK 
15th December 2007, 08:44 AM  #2 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Hi,
the 317 adjusts it's throughput to ensure ~1.25V between it's output and adjust pins. Placing a resistor between these pins, as shown in the datasheet, allows the regulator to behave as a CCS. Insert a 10r resistor and the 1.25V will give a 125mA CCS. Now measure or calculate your worst case maximum input voltage to the CCS and use this to determine the maximum voltage drop across the CCS. then check how big the heatsink needs to be. If you need 1.5A CCS then the resistor will need to be ~830milliohms. The 317 won't last very long.
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regards Andrew T. 
15th December 2007, 09:12 AM  #3 
diyAudio Member
Join Date: Nov 2006

Power = Current (squared) x Resistance
so 1.5 Amps of current through 1000000 Ohms of resistance would produce 2,250,000 Watts of power. Voltage = Power / Current so 2,250,000 Watts of power divided by 1.5 Amps of current requires 1,500,000 Volts. The 317 is good, but that may be pushing things a little! 
15th December 2007, 09:12 AM  #4  
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Join Date: Jun 2002
Location: Melbourne, Australia

Quote:
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Bestever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269 

15th December 2007, 09:16 AM  #5  
diyAudio Member
Join Date: Jun 2002
Location: Melbourne, Australia

Quote:
__________________
Bestever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269 

15th December 2007, 06:57 PM  #6 
diyAudio Member
Join Date: Oct 2006

Hi AndrewT,
Thanks as usual. I mean after I form a 1.5A of CCS by placing your 0.83R between it's output and adjust pins, I should get 1.5A out of that. Then I place 1Meg resistor from this point to ground. I'm sure the current wouldn't be that much but cant really explain what will really happen. Hi Gordy, I think you are right. Let's say if I have only 10volts before the 317, I forms 1.5A CCS as per AndrewT's, then I place 1Meg in downstream, what's actually happens? Will the current only be at 7.5(roughly)/1,000,000 Ampere? Then will there be a problem in CCS loop as they set out to be 1.5A at first? Hi Circlotron, I got your point. 317 is a floating type and my example here is quite a bit exagerated though. 
16th December 2007, 10:03 AM  #7  
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Quote:
a large value resistor fed from a high voltage operates as a current source. Small variations in the load make almost no difference to the CCS current. To avoid the need for these very high voltages and large resistors, we instead use solid state CCS circuits, that mimic this behaviour from more usable voltages. If you fit a 317 CCS and use it to feed a high value resistor then the resistor will dominate the CCS behaviour. Only when the current in the resistor has increased sufficiently to activate the 317 CCS action will the 317 become the controlling element. To pass 1.5A through 1M will require 1.5MV. for all supply voltages below 1,499,999V the resistor controls the current. when the voltage increases to 1,500,000 the 317 starts to take over control. at 1,500,003 the 317 has 3V across it and is starting to work well as the CCS controlling element. But, it is now dissipating about 2.625W [(31.25)*1.5]. it will need a heatsink. Increase the supply voltage another few volts to 1,500,007 and dissipation has gone to >8.6W. You are close to blowing up the317 for a 0.0003% increase in voltage. Forget it.
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regards Andrew T. 

16th December 2007, 10:58 AM  #8 
diyAudio Member
Join Date: Sep 2002
Location: Lakewood, Ohio

In AudioXpress magazine Walt Jung wrote a 2 part article on current regulators.
http://www.audioxpress.com/magsdirx/...enda/index.htm 4/07 Sources 101: Audio Current Regulator Tests for High Performance. Part 1: Basics of Operation Walt Jung Jung2778.pdf 5/07 Sources 101: Audio Current Regulator Tests for High Performance. Part 2: Precise High Current/Voltage Operation Walt Jung Jung2779.pdf
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Kevin 
16th December 2007, 04:12 PM  #9 
diyAudio Member
Join Date: Oct 2006

Hi AndrewT,
This is what I 'm looking for. The basic behind this thing. Thanks for the explaination. I really appreciated your help. My example here is too extreme. Just to clear up my head how to use 317 as a CCS properly. Hi Kevin, Thanks for the links. Definately will have a look in to it. Cheers everyone. AK 
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