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Circuit distorting AC signal from CT

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Red_Micro:

--- Quote from: Jay_Diddy_B on August 27, 2020, 12:19:33 pm ---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

--- End quote ---

Voltage at TP9 is a clean DC signal 1.65V. I will try to also upload the graph. But it looks pretty good.

Jay_Diddy_B:
Hi,

What is the part number of the opamp?

Jay_Diddy_B

Red_Micro:

--- Quote from: Jay_Diddy_B on August 27, 2020, 12:28:12 pm ---Hi,

What is the part number of the opamp?

Jay_Diddy_B


--- End quote ---

It's OPA317. Good news. I disconnected L2 and now the circuit works well for L1. It seems like the switch after L2 is distorting the signal and that is reflected into L1. Also, I had to lower the bias resistor R31 for D12 because the reference was affected by the load current. By lowering R31 I allow more current. Now I just need to fix the test coil circuitry. I need to find a more reliable way of injecting a current from the micro. I will try moving C12 to the other side of the switch as suggested, above but still unsure if L2 needs to be referenced to GND or D12.  If I'm injecting current from the micro, then it would need to be referenced to ground so it finds the return path.

Red_Micro:

--- Quote from: srb1954 on August 27, 2020, 06:42:30 am ---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.

--- End quote ---

I will try that also. The thing is that when I'm not driving the test winding and I let it open, a reasonable voltage develops across L2. Even L1 has a burden.

srb1954:
If the two secondary windings have identical number of turns the voltage developed across the drive winding should be the same as that across sense winding in normal operation. Therefore the effect of a load resistor on the L1 winding will also be reflected into the L2 winding. You should never operate a CT without a low-value load resistor on the secondary winding to avoid the generation of high output voltages from the CT.

However, under certain transient conditions you can get spike voltages on the windings caused by the effects of transformer leakage inductance, which is due to imperfect magnetic coupling between all the windings of the CT. In this case the TVS on winding L1 might not full protect circuitry connected winding L2. You should perhaps also put a TVS on the L2 winding to clamp any such spikes.

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