Archwn said:
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,
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 [(3-1.25)*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.
