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Old 02-13-2011, 04:51 PM #49
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Default Re: Is this a logical way to setup 2x LM317 in a driver?

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Originally Posted by 123splat View Post
Hopefully to clear up the issue for some: The idea of the 'Pass Transistor' hooked up in Common base mode (i think) is based upon adaptation of old non-IC PS design to a period of time when IC voltage requlators (used as current sources) could only pass much LESS THAN 1 Ampere. the Idea is that you use a, say, 150mA capable regulator to drive a High Current Power Transistor (2N3055 is an excellent example for this application) to get the current source up to Multiple Ampere output levels. For further design considerations, try looking up the data sheet for one of those older IC regulators (like a 4195 or uA723) on someplace like alldatasheet.com
As you might be able to tell, my EE skills are somewhat antiquated.... May not be the best solutions, but they got us to the Moon. Thanks for your putting up with the old fool...
I remember such circuits, but only as constant voltage sources. I'm not really sure to what extent they can be used as constant current sources though. Given the added complexity i'd just forget about the whole approach, and get a power transistor and drive it directly using an opamp based current source. Better performance, lower voltage drop, and not -that- much harder to build either.

As for the momentary switch: The circuit drawn above will work, but it will also drop some voltage itself. A better solution would be to use a FET in the negative line, and activate that by yanking the gate to positive when the button is pressed. Make sure to add a resistor from the gate to ground to discharge the fet gate after you let go of the button.

With an ompamp based driver there wouldn't be any problem in adding a momentary switch - just putting one in the resistor that feeds the voltage reference is usually sufficient.


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