Author Topic: questions on dave's psu design  (Read 1929 times)

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Offline amateur_25Topic starter

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questions on dave's psu design
« on: April 07, 2012, 01:10:01 pm »
The U3A opamp controlling the voltage is an noninverting amplifier. However am unsure what the opamp at the bottom is. I understand it there for the current limiting but it doesn't have any resistors so it can't be an noninverting/inverting amplifier. Is it a comparator?

Offline amspire

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Re: questions on dave's psu design
« Reply #1 on: April 07, 2012, 01:35:59 pm »
It is an opamp - basically a LM324 opamp IC in a 2 opamp package instead of a 4 opamp package.

It is running in linear mode - it is not running as a comparator even though it is functionally described as a comparator. Dave is just using the word "Comparator" in the sense that it is comparing the actual current to the set current.  It only really gets out of saturation when the current out reaches the current limit.

Its feedback path includes at the current sense input, then there is the opamp U3B, then through Q2 that can reduce  the voltage on the LT3080 regulator that lowers the output current -  which of course lowers the current sense voltage completing the loop. An opamp does not necessarily need feedback directly across the opamp - it just need some feedback path from the output back to the input. U3B has such a path.



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