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ac to dc conversion using bridge rectifier

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enrrique

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I'm designing a circuit to continuously monitor the voltage level of the ac power line. For that i use a bridge circuit to rectify. Due to ripples, I'm forced to use a capacitor at the output. but i want to instantly monitor the dc level of the bridge output. Since I'm using a capacitor, instantaneous monitoring is not possible. If i don't use capacitor, then ripples are really huge. Can anybody suggest a method to solve this problem???

Thanks in advance
 
Are you trying to make an AC voltage monitoring circuit that is powered by DC power, and use it to monitor the same AC that's powering the power transformer of the DC powered monitor circuit?
 
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What will the voltage monitor connect to?
 
I had a similar problem in a control loop I once worked on. The voltage was the average of the pulses from a magnetic pickoff, and I also needed a rapid response time without ripple. My solution was to use a short time-constant filter, which had a fast enough response time for my requirements, but thus had too much ripple. I then just sampled the waveform at the same time in each period with a Sample & Hold. That eliminated the ripple while still giving fast response to any changes in the voltage.

To do that in your case would require generating a sample enable signal precisely synchronized to the same point on the AC waveform every cycle. A zero-crossing detector with a fixed delay added to the output may work best for that since it's insensitive to the voltage variations you are trying to measure.
 
I'm designing a circuit to continuously monitor the voltage level of the ac power line. For that i use a bridge circuit to rectify. Due to ripples, I'm forced to use a capacitor at the output. but i want to instantly monitor the dc level of the bridge output. Since I'm using a capacitor, instantaneous monitoring is not possible. If i don't use capacitor, then ripples are really huge. Can anybody suggest a method to solve this problem???

Thanks in advance

Hello there,


The only way to get this for sub cycle monitoring is to use a synchronous reference voltage. That's basically a reference voltage that is phase locked to the line you are monitoring. This way you can tell within a very short time if your input has changed and how much it changed.
Most applications however can work with a simple low pass filter, which is made from a cap and resistor. Other applications can work with a solution like Carl's suggestion, where you sync to the line and sample at the same point every time.
Some applications out there use multiple sample periods, where they sample at several places during the cycle and that works as fast as you need it to also because you can choose as many points as you need. This is similar to a synchronous voltage reference except it's in digital form. To implement this you need to store the relative voltages of several places in the sine wave cycle and compare to the measured values (ie a sine lookup table more or less). You may however want to ignore some places in the cycle especially close to the zero crossing to eliminate false readings.
 
Are you trying to make an AC voltage monitoring circuit that is powered by DC power, and use it to monitor the same AC that's powering the power transformer of the DC powered monitor circuit?

Thats exactly what im intending to do. And I want it to be really fast because its the power supply to a device that im trying to monitor
 
You don't say how you are monitoring it, but if it is fast you should be able to use a peak detector to output a pulse at each peak. This could trigger your sample. Assuming its a micro you might want to take several to reduce the effects of noise. Do you need 1 cycle response?
 
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