RMS meter Peak detector AC/DC converter
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- Опубліковано 24 сер 2024
- AC to DC converter analog circuit is analyzed in this example. Circuit output voltage is shown that is proportional to the peak voltage Vm of the input sinusoid. The circuit is designed using two op amps and two diodes. There are two cascade stages in this circuit. The first stage stage is a half wave rectifier with inverting amplifier gain. The second stage is a lowpass filter in a inverting amplifier topology. The cutoff frequency of this lowpass filter needs to be set sufficiently less than the frequency of the input sinusoid so that this low-pass operational amplifier filter practically completely removes all sinusoidal components in the Fourier series expansion of the half-wave rectified sinusoid. Such removal results in a purely DC or constant component appearing at the output of this circuit. The DC component can represent either the peak value (peak detector case) or RMS value (RMS meter case) with proper choice of the values of the four resistors in this circuit.
First circuit I have seen drawn in a normal way. Circuits are easier to understand when drawn with signal direction from left to right, with feed back from right to left.
Thanks for watching and sharing your thoughts.
Very interesting circuit. I am enjoying your channel quite a bit.
Thanks for watching. Glad that you are enjoying my channel and this video.🙂 For more Analog examples and also my Digital/DSP playlist pls see: AC/DC Adapter Circuit with Op Amp and Diode ua-cam.com/video/3rXW5bPlgd8/v-deo.html
Converting Capacitance to inductance ua-cam.com/video/6IYz4iaa05c/v-deo.html
Analog Logarithm Calculator with Op Amp and Diode ua-cam.com/video/w4DuaCJGRRY/v-deo.html
and my Analog circuit playlist ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
For Digital Signal Processing and DSP pls see my second playlist ua-cam.com/play/PLrwXF7N522y6cSKr0FmEPP_zQl011VvLr.html
I hope you also enjoy the Digital video collection. 🙋♂️
Thanks for watching. Please note that this circuit example reports the RMS or Peak of a sinusoidal input. In that context it is practically accurate and is able to report either RMS value or peak value of the sinusoidal input as explained in the video. Note that the circuit does not directly measure the peak of input sinusoid. Instead it rectifies the input sinusoid and then the second stage filters all Fourier harmonics and only allows the DC component to appear as the final output voltage of the circuit. I hope this additional explanation is helpful.
For more circuit examples see:
AC/DC Adapter Circuit with Op Amp and Diode ua-cam.com/video/3rXW5bPlgd8/v-deo.html
Converting Capacitance to inductance ua-cam.com/video/6IYz4iaa05c/v-deo.html
Analog Logarithm Calculator with Op Amp and Diode ua-cam.com/video/w4DuaCJGRRY/v-deo.html
and ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html which is the collection of Analog & Digital Circuits videos.
Interesting circuit. While the calculations are correct, Vx signal must be drawn upside down (inverted).
Thank you very much Dariush for watching and your detailed attention. In the visualization time domain plot of signal patterns I forgot to invert the half-wave rectified signal at mid point Vx in the circuit. As described in the video, whenever input voltage is positive, the Diode D2 turns on and the first op amp acts as an inverting amplifier with a negative gain that inverts the positive input to a negative amplified voltage at mid point Vx which (assuming freq < cutoff) is then further amplified by the negative gain of the next LPF stage to generate the positive RMS or Peak value as a DC voltage at the output of the circuit.
Why in other circuits for "Half-wave rectifier circuit" diodes are directed opposite. (this is as Full wave)
That's a good question. Because it depends on whether the circuit is a positive half Rectifier or negative half Rectifier. I hope this is helpful. 🙋♂️
For more circuit examples see: AC/DC Adapter with Op Amp and Diode ua-cam.com/video/3rXW5bPlgd8/v-deo.html , Converting Capacitance to inductance ua-cam.com/video/6IYz4iaa05c/v-deo.html , Analog Logarithm Calculator with Op Amp and Diode ua-cam.com/video/w4DuaCJGRRY/v-deo.html
Analog Multiplier Circuit ua-cam.com/video/VP53A2zpVMQ/v-deo.html
and ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html which is the playlist of my Analog & Digital Circuit videos.
The implementation of peak detection is inaccurate
The first stage half wave rectifier cannot detect peaks as implemented because it lacks a peak storage mechanism at the output of the rectifier.
The cascaded second stage lowpass filter cannot detect peaks at all because it filters them out for any signal that is not identically DC.
The assumption that the peak value is 1.4 times the RMS value is only true of a *pure sinusoid*.
This circuit can be modified to detect the positive peaks by adding three more rectification components as follows:
- attach another diode to the output of the first op amp, with the arrow on the additional diode pointing at the output node of the op amp.
This diode is an additional stage of rectification with an additional output node that functions independently of the input impedance at the cascaded second stage filter of the original circuit.
- at the other terminal of that additional diode, attach a parallel connection of a grounded charge storage capacitor with a bleeder resistor across it. Choose an appropriate RC time constant for the cutoff frequency. In this case, probably the time constant we want is the same as the time constant for the RMS detector portion of the original circuit, since we likely desire the response time of both the peak and RMS meters to track each other.
Now we've got a true peak detector, because we are directly detecting the peak voltage of the half-wave rectified signal with a diode and a low-pass filter.
The detected peak value is inverted, so if we are interested in detecting it with the sense non-inverting, we have to add an inverting op amp buffer stage. An additional buffer is probably a good idea anyway, because we don't want the complicated output impedance of the peak detector loaded by whatever is connected afterward, such as a mechanical meter movement and/or a light emitting diode.
Thanks @CherylJosie for watching this video and for sharing your thoughts. This circuit is presented in the context of a circuit example that reports the RMS or Peak of a sinusoidal input. In that context it is accurate and is able to report either RMS value or peak value as explained in the video. The circuit is not directly measuring peak, instead it rectifies the input sinusoid and then the second stage filters all Fourier harmonics and only allows the DC component to appear as the final output voltage of the circuit. I hope this is helpful. Thanks again.
You can see that the ouput is Vmaximum / pi. That is not the peak voltage, it's the medium DC voltage. Considering an sinusoidal wave, the relationship between medium DC voltage and peak voltage is constant. So you just need to apply gain to the medium DC voltage to have the peak output voltage. It can detect peak voltage of a SINUSOIDAL WAVE. Now if you wanna see peak voltage of anything at the input (not just a sine wave) then you'll need the peak storage mechanism. Wich is not the case here.
@CherylJosie
@marconis.giacomini1543 Thank you for your clarifying comment and your interest in this Engineering Channel.