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Circuit distorting AC signal from CT

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Red_Micro:

--- Quote from: NiHaoMike on August 27, 2020, 01:46:54 am ---Move C21 to the other side of the analog switch, that way the ESD diode in the switch will not clamp the negative peaks.

--- End quote ---

I will try your suggestion. What do you think of my thought of referencing L2 to Vcc/2 as L1? Originally I reference to GND because the test_square signal to the test winding comes from a micro. The current from the micro has to return to the micro through ground and if I reference L2 to the VCC/2 point then the return current may affect the reference. Not sure actually.

srb1954:
Omit analog switch Z1/Z2 altogether.

When you aren't driving the test winding tri-state the micro output so it doesn't load down the CT winding. You will need some protection diodes on the micro output so it is protected against transients feeding back from the CT.  These diodes will need to have a reasonable current rating (>100mA) as they also have to handle any charging current through the coupling capacitor C21.

Jay_Diddy_B:
Red_Micro,


**************************************************
This analysis is wrong!!

The REF pin on the LM4041 is referenced to pin 2, the cathode.

Ignore
**************************************************

I think you need to revisit this part of the circuit:






With the component values shown, the LM4041 will try to regulate TP9 at

VTP9 = 1.225V (1 + R33/R35)   = 1.225 (1+ 30K/10K)= 4.9V

This can't happen because you only have a 3.3V supply.


To regulate mid-rail, 1.65V

Make

R33 = 10K
R35 = 30K

VTP9 = 1.225 (1 + 10K/30K) = 1.633V


(With the original values, the voltage on TP9 = 3.3 x (10K +30K)/ (10K + 30K + 3.3K) = 3.05V )

In addition this circuit is a shunt regulator, it has limited ability to source current. You need add a bypass capacitor from TP9 to ground.

Regards,
Jay_Diddy_B



Red_Micro:

--- Quote from: Jay_Diddy_B on August 27, 2020, 11:50:34 am ---Red_Micro,

I think you need to revisit this part of the circuit:

(Attachment Link)




With the component values shown, the LM4041 will try to regulate TP9 at

VTP9 = 1.225V (1 + R33/R35)   = 1.225 (1+ 30K/10K)= 4.9V

This can't happen because you only have a 3.3V supply.


To regulate mid-rail, 1.65V

Make

R33 = 10K
R35 = 30K

VTP9 = 1.225 (1 + 10K/30K) = 1.633V


(With the original values, the voltage on TP9 = 3.3 x (10K +30K)/ (10K + 30K + 3.3K) = 3.05V )

In addition this circuit is a shunt regulator, it has limited ability to source current. You need add a bypass capacitor from TP9 to ground.

Regards,
Jay_Diddy_B


--- End quote ---

I measure 1.65V at TP9 so the LM4041 part is OK. According to the datasheet: VO = VREF[(R2/R1) + 1] or VO = VREF[(R35/R33) + 1]

Jay_Diddy_B:
Red_Micro,

You are right. I missed that the reference in the LM4041 is with respect to the cathode.

Can you check the voltage on TP9 with a scope?

Jay_Diddy_B

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