Need help in designing a circuit - Process Instrumentation


Help me design a circuit to meet the following requirement.

I am driving a transformer coupled load through a Phase angle controlled thyristor power pack (single phase). The mains is 240 volts (R phase and neutral) to the Power pack. The output of the thyristor is controlled (0 - 220 v) via a potentiometer of

10k ohm. The output of the thyristor is put to the primary of the step down transformer.

The current flowing from the output of the thyristor and the primary of the transformer with the help of CT (50 / 5 ratio).

I now need a circuit to trigger a relay with reference to a set point (to be adjustable from

0.5 to 5.0 amp via a potentiometer / trimpot). Display of the current would be a parameter of luxury.

Can some one please guide with their response on


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Rectify the output of the CT, filter it, and scale it (with an opamp stage) to the range of a DC panel meter. To trip the relay, just tap the output of the opamp stage with a trimpot, send that into a comparator or schmitt trigger, which drives the relay.


Dave M
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The current waveshape is the nasty, hacked up, remnants of a sinewave so must be measured as a 'True RMS' value. Simplest and best just to use a AD736 'true RMS converter' chip. Set the current transformer load resistor to give a scaled value (upto

+200mV DC) out of the chip when maximum transformer current is flowing. Use the chip output to directly drive a 200mV FSD digital voltmeter diplay (or a cheapo DVM set to 200mV range). i.e. get say 200mV from AD736 chip and display it as "20.0" Amps. Use chip DC output, the setpoint pot' and a LM393 comparitor to drive a relay (relay may need the help of a transistor).
Reply to
john jardine

A more physics based solution would use the relay trip level because the actuation threshold will be a function of RMS squared coil current:-) A simplified block diagram of the circuit would then be: View in a fixed-width font such as Courier.

. . . V+ . | . |\\| . .------|+\\ . | | >---+--. . --- .--|-/ | | . /// | |/| | | . | | | k|| R1 . | V- === k|| (1+ -- ) x Iac . | | k|| R . | | k|| set . | | | . Iac-> | | | . -------+-------+--[R1]---+--' . ( | | . CT ( [Rc] [Rset] . 10:1 ( | | . ( | | . -------+-----------------' . --- . /// . . . .

Reply to
Fred Bloggs

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