# Help me with designing a circuit to convert AC to DC signals

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#### garg29

ac to dc

hi friends
i want to design a simple circuit to convert AC signal source into a DC output signal of 5 volts cause i want to measure its amplitude with an ADC . the ac input signal can be square ,triangular, sine etc...i mean generalized one.

thanks
with best regards
amit

#### v_c

Re: ac to dc

What you need is a full bridge rectifier followed by a capacitor to give you the dc.
Take a look here for the structure Bridge Rectifier

You may not need the transformer if your ac signal not too high in amplitude. Once you rectify the waveform (put all the negative regions to be positive), the waveforms looks like bunch of M's (like the M in McDonald's signs). Assuming the ac waveform has an amplitude of $V_m$, the average value of this rectified waveform will be $2 V_m/\pi$, assuming the diode drops are negligible. So when you filter this rectified voltage, the dc voltage should be around this value. So knowing the dc value you can infer what the amplitude of the ac waveform. If there is a transformer used, you also have to take into account the turns ratio. If you used a 10:1 transformer, then your rectified waveform would be 1/10 of the magnitude of the original waveform, so when you calculate the amplitude of the original waveform just multiply by 10.

Best regards,
v_c

#### garg29

Re: ac to dc

please provide me the circuit daigram cant understand without it.

#### IanP

Re: ac to dc

Another option of measuring AC voltage is to feed it to a voltage divider followed by a peak detector and "scale" the whole circuit in such a way that the output is in the 0-5V range..
See examples of circuits here:

Regards,
IanP

#### garg29

ac to dc

Thanks for replying everyone. Ianp the links and idea is really very useful, but i just want to ask will this idea provide me a "True RMS" reading?

Thanks once again,
With best regards,
Amit

#### IanP

Re: ac to dc

In theory, if you know the "shape" of the signal, you can apply formulas which link peak values with RMS values ..
In practice, a sine wave is never "pure" sine wave, so for exact measurements of RMS values it is better, but more expensive, to use RMS-DC converters (example: https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1086,D4563 ) ..

Regards,
IanP

#### garg29

ac to dc

right. that's what i was thinking of. is there any other method which i can use, if waveform shape is unknown to me. i mean without "linear" IC's, if there could be some technique which does this by using simpler ICs, opamps etc.

Added after 4 hours 13 minutes:

and just for a thought, after seeing the diagram ofin the 2nd link, should there be a buffer stage between resistor divider network and LF353?

#### Kral

Re: ac to dc

garg29,
An RMS Converter IC such as the Analog devices AD737 will convert any arbitrary waveform into a DC voltage that is proportional to the True RMS value of the input. A precision rectifier, followed by a low pass filter will convert any arbitrary waveform into a DC voltage that is proportional to the average value of the input.
Regards,
Kral

### garg29

Points: 2

#### garg29

ac to dc

Thanks a lot kral for the solution. I'll try to do that.

#### lats

##### Full Member level 4
Re: ac to dc

hi kral
can u please provide the circuit daigram of low pass filter which u

suggested for the above application. i will be thankful to u.

#### garg29

ac to dc

hi ianp
the circuit of peak detector which u said is now working fine at low voltages but for high voltage like 380volts rms ac ,the circuit shows 426volts rms and also there is fluctuation. i think it is because of harmonics/noise .how can i suppress them and make my circuit work acuurate for high voltages...please help me out

#### sivani_vs

##### Junior Member level 3
Re: ac to dc

U can use a full wave rectifier. Its better to use more number of stages in the filter, sothat the repple factor will be less.

#### IanP

Re: ac to dc

Because you are dealing with the mains voltage (50Hz) you may try a filter with as low corner frequency as 100Hz ..
Initially you can try simple RC filter with R=1kΩ and C=1.6µF but it will "decrease" the voltage at 50Hz to ≈75% ..
Finally you can build 2-nd order active filter with 0dB gain and, to make life easier, you can use this tool from AD: **broken link removed**

Regards,
IanP

#### Kral

Re: ac to dc

lats,
If you use the precision rectifier shown in figure 4 at
~
h**p://sound.westhost.com/appnotes/an001.htm,
~
then simply connect a capacitor in parallel with the feedback resistor R5. Select the value such that R5 X C is muchh longer thatn the period of the AC waveform. For example for 50 Hz, (period = .02 Seconds), a capacitor value of 100uF would work. This gives a time constant of 100X the period. You can reduce the capacitance value by scaling the resistor values up. By using 200K resistors, you could reduce thae capacitance to 10uF. The value depends on how much ripple that you can tolerate in the output.
Regards,
Kral.

#### garg29

Re: ac to dc

hi kral i tried the circuit on a simulator ,the output m getting is a full wave. can i feed this to pic adc directly? the picture is attached along with the output wave form.

#### Kral

Re: ac to dc

garg29,
I'm not failiar with the pic ADC In general, an ADC requires the input to be sampled, so that the input to the ADC proper remains constant during the conversion. The output of the precision rectifier should be filtered before it is applied to the ADC. Otherwise, you would get different answers depending on what part of the waveform was sampled prior to the conversion.
Regards,
Kral

#### arul_81284

##### Newbie level 5
ac to dc

I need your help.. can anybody give me the design of the circuit for thyristor rectifier

#### rajasekaranma

##### Newbie level 4
Re: ac to dc

normally we are use 5v regulation voltage by 7805
is there any ic instead of 7805?

#### zenisle

##### Full Member level 2
ac to dc

AC-DC is mainly in power managment domain.

#### yousafzai

##### Full Member level 5
Re: ac to dc

AD536 is an excellent AC-DC converter which is easy to use.

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