"Greg Hines" wrote in news:OtjHe.492$ snipped-for-privacy@nnrp1.ozemail.com.au:
Hi all,
>
> Don't know if I have the correct Newsgroup for this type of question,
> and if not please let me know the best place to post it.
>
> I'm after an explanation of a circuit which in attached, or can be
> found at:-
>
>formatting link
>
> It's a circuit that controls the speed of a air pump to maintain a
> constant airflow. It uses PWM for this purpose. The PWM test point
> at the far RHS of the circuit goes to the gate of a MOSFET that
> switches the pump.
>
> DELTA P is the output of the flow transducer that's in the pump air
> flow after it has been amplified. R28 sets the airflow for the pump
>
> My questions concern U5C and U5D and the associated components.
> What's the purpose of these 2 OpAmps? Is it adding some sort of
> hystersis and if so the 10K feedback on U5C provides an awful lot of
> hystersis? Why would you use 2 200K resistors in series as R30 and
> R31?
>
> Any insight to the above questions would be appreciated.
>
> Greg
> -
> snipped-for-privacy@bigfoot.com.NOSPAM
> Remove NOSPAM when replying
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> begin 666 circuit.jpg
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> Attachment decoded: circuit.jpg
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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.