Measuring the loop current using test diode is one of the important troubleshooting methods. A Test diode can be placed anywhere in the current loop forward biased.
The rectifying diode can be placed in series at anywhere in the loop.
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During normal condition the diode conducts easily, that only shows a drop of 0.7 V which is common for forward biased silicon diode.
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To measure the loop current, we connect an ammeter parallel to a diode.
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The ammeter resistance is much lower than the diode resistance.
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As current tends to flow through lower resistance path, the loop current flow through the ammeter.
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Without enough current to bais the diode, entire current flows through the ammeter. the diode effectively turns off and conducts 0 mA, leaving the entire loop current to pass through the ammeter.
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During this current flow the ammeter reads the amount of current through the loop.
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When the ammeter is disconnected the diode resistance produce a 0.7V again and starts to conduct again.
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Such a diode may be installed at the nearest junction box, between terminals on a terminal strip, or even incorporated into the transmitter itself. Some process transmitters have an extra pair of terminals labelled “Test” for this exact purpose.
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Ammeter is connected to the terminals labelled as “TEST” below and to the right of the main screw terminals where the loop wiring attaches.
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Transmitters equipped with analog meter movements for direct visual indication of the 4-20 mA signal usually connect the analog milliammeter in parallel with just such a diode.