[Diy_efi] I need to rectify a low ac voltage.

[Jörgen Karlsson]

Thanks Brian!

My original post seems to have been delayed somewhere and I had already
found a good way to rectify the signal when I received your post. The
information about the instrumentation chips was however very interesting.

Jörgen

> -----Original Message-----
> From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org]On
> Behalf Of Brian Dessent
> Sent: Sunday, July 21, 2002 9:50 PM
> To: diy_efi at diy-efi.org
> Subject: Re: [Diy_efi] I need to rectify a low ac voltage.
>
>
> Jörgen Karlsson wrote:
>
> > Can anyone please help me to rectify and filter the signal, I
> can't come up
> > with a good way to do this.
>
> If you want an ideal rectifier (i.e. no forward diode drop) you can put
> the diode in the feedback path of an op amp.  I'm certain you can do
> half wave, and pretty sure you can do full wave (worst case -- make two
> half wave rectifiers, one with an inverted input, and sum the outputs.)
> Look at op amp cookbook websites or books for the details.  A quick
> google search found this:
>
> http://www.deas.harvard.edu/courses/es154/lectures/lecture_2/diode
_circuits/diode_appl.html

You can probably combine filtering in the feedback loop and do it all in
one step.  If you provide details I can help more.

Also, depending on your application, you might try a ready-made
instrumentation chip.  For example I know there are chips out there that
take arbitrary AC input and output a (quite filtered) DC voltage
corresponding to the RMS value of the input.  These are intended for use
in meters and such.  I think a lot of them use bolometric sensing (i.e.
differential heating) but some might also use analog multiplication or
digital sampling.  For example, Analog Devices' AD536A / 636 / 637 / 737
/ 736.  I think the cheapest of these is under $5.

Brian

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