The filter in the diagram has a cutoff frequency of about 1kHz because the 1.5k resistor plays a dominant role; however could you post also the voltage measured at the input of the filter (i.e. directly on the shunt) in the same conditions you get the oscilloscope picture you posted ?
The effective low-pass R is 1.5k || 15 k = 1.36k rather than 1.5k + 15k. But it still doesn't explain the observed waveform.
Although a first order filter won't remove all switching transient, the signal should look at least more smoothly. So bad measurement setup, as suggested by godfreyl is the only plausible explanation. For a comparison, you can tap the filter output signal with a coaxial cable soldered directly to the filter capacitor.
It seems you have a grounding problem. The signal you captured is not passing through the filter. You can try to:
1. Shorten the ground wire from circuit ground to probe as much as possible
2. Make a differential measurement: connect a probe to CH1, another to CH2. The ground wires of the two probes connected toghther a to any other point. Then connect the center tip of CH1 to Vout and tip of CH2 to ground of the circuit. Set the scope to visualize CH1-CH2. If the circuit ground is isolated from the main earth it should work.
Looking closely at your scope traces...
It looks as though you are pulsing the motor at about 60 percent duty cycle. That appears to be the On time anyway.
The coil rings with greater amplitude after switch-off (as compared to switch-on). That is not unusual.
I believe your goal is to filter out the AC part of the ringing? That is the chief noise on your signal.
During 'Off' time, we do not necessarily expect the reading to fall to zero. Inductive action wants to continue pulling current around the power loop (or any available loop). This may be happening in your case.
So the question arises: Is current going through the shunt during Off time? Furthermore could the ringing be on a resonant frequency involving your filter capacitor?
Although you installed a 15k resistor, there may be another current/voltage path that the motor coil is acting through.
Just seeing what results from some brainstorming.
Coils have a way of making odd things happen when you switch current through them. Did you try using a resistive load instead of the motor? That will tell you a lot as to whether your filter arrangement is working.
The effective low-pass R is 1.5k || 15 k = 1.36k rather than 1.5k + 15k. But it still doesn't explain the observed waveform.
I will post measurements of drain to source MOSFET voltage compared to the voltages droped by the shunt too. If I recall correctly, that weird ringing at the turn-off of the MOSFET is not present in the MOSFET itself.
I could use a resistive load but that would produce no ringing so, how would I know if I am filtering any AC component?.
I think I can see a subtle ramp down in the voltage when the MOSFET is off, possibly indicating current is still flowing and slowly decreasing?
PWM at about 20 KHz, a mosfer controller UCC37325P and a MOSFET IRF3707 protected by a fast diode DSEP 29-06A. (in a low-side drive configuration).
hey FvM,
I am wondering why its in ll rather than series?
K
Yes, this is why I asked if you had tried a resistive load.
While the mosfet is off, your scope trace would fall to zero. Or should fall to zero. A resistive load would let you test your setup without the somewhat unpredictable nature of a switched coil.
Because when looking back to your OP:
I wasn't sure if this meant you installed a diode across the motor. (My simulation does.)
If no diode is across the motor, and you have a volt reading during Off-time...
Then it means current is going through your motor during Off-time...
And it suggests current is going through your mosfet when it should be off. Of course we can't be certain as to your motor's characteristics.
But suppose your low-pass filter is working? And suppose your motor is generating ringing with an amplitude of tens or hundreds of volts? That could still show up on your scope. (My simulation has it producing over 1000 volts, but I'm not claiming that your motor behaves exactly the same.)
Again I can't be sure that is really what's happening, but it is a way to explain the ringing waveform that was supposed to be gone.
It seems, FvM is not available - thus, I will answer:
* Try to calculate the transfer function and you will be convinced
* The corner frequency (3db) is the inverse of the time constant tau. And tau is the product of C and the resistors which determine discharging of the C. Here, both R's act in parallel.
It seems, FvM is not available - thus, I will answer:
* Try to calculate the transfer function and you will be convinced
* The corner frequency (3db) is the inverse of the time constant tau. And tau is the product of C and the resistors which determine discharging of the C. Here, both R's act in parallel.
Current drain at best efficiency: 19,3A (7,2V)
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