R C Low pass filter not filtering at all

I worked it out but found the gain to be Load_R/(Load_R+R1)! So for Big loads the filter isnt very effective? Is that a correct observation?

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Nobody knows why the low-pass function has been combined with a voltage divider, although the shunt voltage is already small. This further complicates to acquire the filtered voltage without interferences.

Apparently the problem has been handled by the revised measurement circuit in post #9. It could be further improved by placing another capacitor parallel to the 100k resistor.

FvM said:

Nobody knows why the low-pass function has been combined with a voltage divider, although the shunt voltage is already small. This further complicates to acquire the filtered voltage without interferences.

Apparently the problem has been handled by the revised measurement circuit in post #9. It could be further improved by placing another capacitor parallel to the 100k resistor.

Click to expand...

The original configuration consisting in the RC filter plus an opamp set with a gain of 101 was replaced by a single resistor just to simplify. But perhaps I just made my problem harder to determine. I think I will get back to the measurement obtained at the begining, opamp included and connected to ADC port and continue testing there

BradtheRad said:

Yes, I see I had a good idea of your setup.

There's an image you haven't posted yet. Namely what does the scope show with no filtering at all?



Is that over 19 amps at 7.2 V? Sounds like a lot of inductive inertia. I wouldn't be surprised if the ringing is a couple of volts at least. Enough to have some amplitude remaining even if it is greatly attenuated through your working LPF.

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I was wondering... since the DC motor is an inductor, and it tends to maintain the current going through it, Am i "catching" the current of the motor going through it and through the diode at mosfet off time? (I fear I am not). To be able to include it in the measurement, then, do the shunt must be "inside" the diode - motor loop? (I mean, between B- and Mosfet Drain, and the diode conection from shunt leg connected to the Mosfet and motor B+)?

gonespa said:

I was wondering... since the DC motor is an inductor, and it tends to maintain the current going through it, Am i "catching" the current of the motor going through it and through the diode at mosfet off time? (I fear I am not). To be able to include it in the measurement, then, do the shunt must be "inside" the diode - motor loop? (I mean, between B- and Mosfet Drain, and the diode conection from shunt leg connected to the Mosfet and motor B+)?

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Yes, this is what I referred to earlier. Your schematic shows there should be no current through the shunt when the mosfet is off. However from your scope trace, it appears the mosfet continues to conduct even during Off time. I believe the motor coil continues to pull sufficient current during Off-time, to bias the mosfet gate on by pulling its source terminal low.

Substituting a resistive load will tell you a lot at this point. Such as whether your mosfet is still undamaged.

Questions that may determine the best arrangement:
* how the coil (coils) behave
* how many coil are in the motor
* how much current flows during off time vs on time
* how quickly current changes during off time vs on time