I was thinking that a line frequency transformer would make the circuit unnecessarily bulky and so I tried to avoid it.Hi,
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What about a transformer, a voltage reference and a comparator? Why an op amp rather than a comparator, any reason, it saves a voltage reference could be one?
I read the information somewhere that an Opamp with a network of two resistors as in the diagram introduces hysteresis. I have realized that I interchanged the inverting and the noninverting terminals of the opamps. That was a mistake.Presently the OP circuit doesn't provide hysteresis (positive feedback missing). Both circuits seem to have a fixed threshold ("setpoint").
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Yes, I'll do that. I'm also thinking of connecting a capacitor across the bottom resistor of the rectified mains voltage divider to help reduce the ripple that the opamps see at the signal input.Things to consider:
1. the current to the opto-coupler LED needs to be limited - add a series resistor.
Oh! Thanks Brian for pointing this out... The ground symbol that references the opamps powersupply on the diagram is a mistake. I sketched the circuit without the opamp power pins.2. be very careful with 'ground' as the input part of the circuit is not isolated. As shown in the first schematic, you would either have a 'live' output or an exploded IC.
My initial plan was to power the opamp with a voltage that is output at the "Vref" pin of the power management IC that I am controlling the power supply with. It's not isolated anyways.3. think about where the supply for the op-amp, or better still a comparator comes from and whether is is isolated.
The Vref pin output voltage is actually a stable 5V reference. I think that it will be adequately stable if I use a fraction of that voltage as my opamp setpoint.You could make the whole input side 'live' and derive the comparator supply from the rectified AC, it would be a low cost option but you would have to add a stabilized reference voltage (a Zener diode maybe) because deriving the reference and measured voltage from the same source would not be reliable.
This sounds like a good idea but I can really picture how to implement the idea. Please explain it in more details.…
I was thinking - maybe accuracy-wise, not an alternative - about if the op amp could be replaced with a suitable voltage (series) Zener diode after the resistive divider, that way a pre-isolation side supply might be avoided and it could be used to create the digital on/off? Probably not accurate enough. Not sure if I'm asing a question or making a suggestion...
Haven't considered in detail actual implementation of this voltage reference in your circuit, but I believe the TL431 might be used as a comparator instead of the op amp here, too, for the sake of making another suggestion just in case it's of interest.
TL431 (or may be LM385-Adj. for lower quiescent current), primary supply derived from rectified mains and a high transfer factor opto coupler would be also my solution. But we didn't yet hear accuracy requirements.
Specifications are that the mains voltage fluctuates from 190Vac to 250Vac at the target location of use. I need to select a setpoint of the mains voltage that I desire operability.What is the main's voltage...
I am monitoring RMS voltage indirectly from the rectified mains voltage. I'd factor in diode drops and I'll provide a lot of filter capacitance to reduce ripple content to a tolerable level. Talking about noise, I have been wondering whether I should tap the rectified mains voltage for the power supply (SMPS) or I should have a separate bridge rectifier for this circuit to try and keep away from the noise within the power supply primary.…in my eyes it makes a big difference to monitor
* mains voltage or mains current
* mains RMS voltage
* mains rectified average voltage
* mains voltage waveform
* mains frequency
* mains overtones
* or something else.
It needs to be defined.
Mains voltage may be almost pure sine, but there is no must. In reality it will be distorted, there will be noise, there will be transient spikes. Thus every method will show different result, and different threshold levels (varying with distortion)
And what additionally needs to be defined: timing behaviour and hysteresis.
The given schematics is something inbetween RMS and rectified average .... somehow undefined.
Neither the one nor the other.
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Klaus
No.I am monitoring RMS voltage indirectly from the rectified mains voltage.
How about this approximation ad regards bridge rectification where the rectified output is about the same value as the peak:Hi,
No.
This simply is impossible.
You just monitor an undefined voltage signal and multiply it with a factor to get a value that looks like a RMS value.
But as soon as the waveform changes the factor should be adjusted - which most probably you don't do.
If you want to call it RMS you need to: square - average - squareroot.
The waveform changes... although in our region we have a very stable and clean mains supply, the sine is distorted. It's not easy to recognize this just by looking at the scope. Here the sine has a flatened top, caused by the rectifiers. And it's enough to make it impossible to get an +/-2% accurate RMS value with a non RMS measurement method.
Some values (I know it's not compareable with mains sine).
Take a square wave with variable duty cycle:
At 0% both RMS and average shows the same: 0%
At 100% both RMS and average show the same: 100%
But with 50%: the RMS value shows 70.7% while the average method shows 50%.
And both values are correct!
Klaus
Regarding Lm385 see figure 23 and 24 of the data sheet for comparator application. Same but inverted compared to TL431 which shouldn’t matter for an isolated AC application.
Hi,
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This short article, Shunt regulator monitors battery voltage, says: "A TL431 shunt regulator is a perfect choice for many applications. You can use it as a comparator with hysteresis by taking advantage of its inner voltage reference along with few additional components. You can use this comparator with hysteresis, like a Schmitt trigger, as a simple battery monitor (Figure 1)." As a battery monitor, it looks more parts than the op amp you have.
I've run a simulation and I could achieve some hysteresis.
This means peak value measurement.How about this approximation ad regards bridge rectification where the rectified output is about the same value as the peak:
Vdc=1.414*(Vac - 2*Vd) approx.?
It seems you mix accuracy and precision.The accuracy here is +-2% of the setpoint voltage and not for the entire continuum of instantaneous values.
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