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Subject: Re: design of 4-20mA circuit

Date: 08/11/00 at 12:03 PM
Posted by: Dean Huster
E-mail: dhuster@pb.k12.mo.us
Message Posted:

In Reply to: design of 4-20mA circuit posted by Bruce cincotta on 07/25/00 at 2:45 PM:

Hi, Bruce,

I assume you want to monitor the value of the ac voltage. I would use a small transformer to get a sample of the voltage and then rectify and filter it, keeping the filter components (the R and C) high enough to kill the ripple, but low enough that the circuit is responsive enough to changes in line voltage to follow to your satisfaction. Then run this dc into a voltage-to-current op amp circuit, easier to do with one op amp to derive the current and another to deliver the offset and range to get the 4 - 20 ma. You'll adjust the circuits to get 4 ma with 0vac in and 20ma with the maximum voltage you want to monitor, say 130vac.

If feel badly that I don't have a scanner, or I'd draw something out for you!

Dean


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