Timeline for Is this op amp circuit a promising idea to measure 300VAC?
Current License: CC BY-SA 4.0
17 events
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| Apr 20, 2022 at 22:34 | comment | added | SKGadi |
I updated solution circuit on my post by placing a 1k ohm resistor and a 5.1 V Zener diode to protect the Op-amp. Also I changed the Op-amp to GS8331 for rail to rail compatibility. Is it good with the new changes?
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| Apr 20, 2022 at 21:12 | comment | added | Kuba hasn't forgotten Monica | Averaged over 10 seconds you can probably eke out 12 bits of amplitude in this application without trying too hard, and 16 bits of you use state of the art methods – when sampling is phase locked to line frequency. The phase lock can be closed in software, and you only need a digitally controlled oscillator chip to act as a sample clock for the ADC. Some MCUs offer that on-chip. | |
| Apr 20, 2022 at 21:09 | comment | added | Kuba hasn't forgotten Monica | You don’t care about the offset much: the software takes a running average over 1 line cycle. The exact offset is not relevant as long as the signal remains unclipped. You can always lower the gain a bit. BTW, you’ll get a resolution way better than 8 bits in this measurement done right. You have to fit a sine wave to the data, and you’ll easily get 10 bits of amplitude resolution even with very simple code. If anything, some noise added to the signal would reduce nonlinearities of the ADC. Line inputs are usually plenty good at that already :) | |
| Apr 20, 2022 at 21:05 | comment | added | Kuba hasn't forgotten Monica | LM358 and LM324 have input that goes past ground by 0.1-0.2V, and output that swings as close to zero as you want it with a pull-down. These circuits are can be simulated with a click. Please look at the waveforms, it literally takes a minute or less. You can easily virtually probe them in the simulator. That’s why I took time to ensure the simulations worked :) Your final solution will be destroyed by common spikes, you pretty much shorted the winding to op-amp input. Only low trace impedances will protect the op-amp, so design those for 1GHz bandwidth ESD spikes. | |
| Apr 20, 2022 at 20:59 | comment | added | Kuba hasn't forgotten Monica | @SKGadi LM358 in this application is swinging from about 6 to 16V, far from rail to rail. The output resistor network rescales it to 0-3.3V. The impedance to transformer secondary is virtual: a higher impedance is transformed into a lower one. You see that by looking at the current waveform coming out of the transformer. It’s close to zero ohms for LM358, and about 80 ohms for the discrete circuit. Ideal load for those transformers is 0 ohms, because they already have 150 ohms built-in resistance of the winding itself 50 ohms just adds to it. | |
| Apr 20, 2022 at 14:14 | comment | added | SKGadi |
Impedance to secondary of transformer is greater than 50 ohms for both Discrete Current Source I-V Converter and Cheap Op-Amp I-V Converter
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| Apr 20, 2022 at 14:06 | vote | accept | SKGadi | ||
| Apr 19, 2022 at 23:44 | comment | added | SKGadi | I updated my question with Edit 2 and Edit 3. Please give your thoughts. | |
| Apr 19, 2022 at 19:18 | comment | added | SKGadi |
I made an update based on your Circuit 3. I am using an instrument amplifier. Please look at it to see if it is a good solution.
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| Apr 19, 2022 at 19:17 | comment | added | SKGadi |
The circuit shown in the Cheap Op-Amp I-V Converter is the best and simplest, however, I see one problem. Again, it is because of my application specific problem. The 24VDC comes from a battery and its voltage may change with time. Since 24 is taken as reference, my output may change. Is it possible to use 3.3V source as reference to the positive pin? Also, are you considering that the op-amp LM358 is not rail-to-rail, so it may not reach zero?
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| Apr 19, 2022 at 19:00 | history | edited | Kuba hasn't forgotten Monica | CC BY-SA 4.0 |
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| Apr 19, 2022 at 18:55 | history | edited | Kuba hasn't forgotten Monica | CC BY-SA 4.0 |
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| Apr 19, 2022 at 18:43 | history | edited | Kuba hasn't forgotten Monica | CC BY-SA 4.0 |
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| Apr 19, 2022 at 18:02 | history | edited | Kuba hasn't forgotten Monica | CC BY-SA 4.0 |
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| Apr 19, 2022 at 17:37 | history | edited | Kuba hasn't forgotten Monica | CC BY-SA 4.0 |
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| Apr 19, 2022 at 15:44 | history | edited | Kuba hasn't forgotten Monica | CC BY-SA 4.0 |
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| Apr 19, 2022 at 15:32 | history | answered | Kuba hasn't forgotten Monica | CC BY-SA 4.0 |