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RE: LM317 as a CCS , how to?

Posted by Alex Kitic (A) on October 26, 2014 at 01:28:54

In Reply to: RE: LM317 as a CCS , how to? posted by DAK on October 25, 2014 at 13:16:07:

If one wants to set a constant current for a tube, and the CCS is going to be under the cathode, in my view there is going to be little difference between a "simple" (the circuitry inside is actually not so simple at all) IC like LM317, and some "elaborate" (actually simpler than the IC in terms of number of elements, most probably) discrete components solution - is going to be marginal, if any. Likewise, some claim being able to hear the difference between LM317 and TL783 in that position, but to me it is marginal if not placebo.

Besides the simplicity of implementation, parts count, and reliability, what matters is the voltage drop. The LM317 requires 1.25V for current setting and probably at least 2.5V across the regulator, thus a minimum cathode voltage is going to be 3.75V for correct operation.

While some "fancy" CCS made with discrete components might be applicable as current source above the anode, where the voltage drop is not an issue, as current sink with tubes that have rather low cathode bias voltages, in the range 5V to 10V, most of those solutions are not applicable.

Simplicity is also an important part of the equation: 1 IC with a series of protective features plus 1 resistor makes for easy installation and probably higher reliability, not to mention the cost...
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