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- Posted on
August 1, 2005, 11:17 pm
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I'll have a quick go at explaining. U3B and U5A form an oscillator: note
the feedback from the output of U5A to U3B's non-inverting input ie +ve
feedback. This forms a sawtooth oscillator. U3A is simply a buffer of half
the supply voltage to set the DC operating point. Probably a single supply
or battery supply. U5B is a comparator without hysteresis. Now, the Delta P
signal is first amplified by U5C, a DC stage. And no, the 10k feedback
resistor does NOT add hysteresis but simply puts a single pole in the
response to roll off high frequency response, probably pulsations in the
flow or pressure sensor.
The next stage is U5D forming an integrator. Of Course this has open loop
gain at DC which means that the PWM is capable of going to 100% and 0%. I
am assuming here though that the sawtooth amplitude is less than the supply
rails, you can verify with calcs or simulation. When the system is in
balance, the set point voltage of R28 will equal the value of Delta P
average voltage and there will be a constant duty cycle value. Q2 is
obviously an enable function.
The two 200k resistors in series in the integrator would have been chosen
by the designer presumably because that was an available value, and/or to
minimise the number of different values for a SMT design to minimise the
number of different reels used.
Re: Can someone explain how this circuit operates
** I think that Geoff has supplied very good explanation - especially as
the nature of the feedback sensor signal at "Delta P" is unknown. The
circuit is part of a feedback control loop so it is designed to interface
with the specific electro / mechanical items involved.
The "Delta P" signal may well be a rectangular pulse train derived from an
opto-interrupter reacting to tiny fan blades breaking a beam - or it may
be something entirely different.
The OP should have supplied that info.