Using test diodes to measure loop current

Another way to measure a 4-20 mA signal without interrupting it involves the use of a rectifying
diode, originally installed in the loop circuit when it was commissioned. A “test” diode may be
placed anywhere in series within the loop in such a way that it will be forward-biased. During
normal operation, the diode will drop approximately 0.7 volts, as is typical for any silicon rectifying
diode when forward biased. The following schematic diagram shows such a diode installed in a
2-wire transmitter loop circuit:

If someone connects a milliammeter in parallel with this diode, however, the very low input
resistance of the ammeters “shorts past” the diode and prevents any substantial voltage drop from
forming across it. Without the necessary forward voltage drop, the diode effectively turns off and
conducts 0 mA, leaving the entire loop current to pass through the ammeter:

When the milliammeter is disconnected, the requisite 0.7 volt drop appears to turn on the diode,
and all loop current flows through the diode again. At no time is the loop current ever interrupted,
which means a technician may take current measurements this way and never have to worry about
generating false process variable indications, setting off alarms, or upsetting the process.
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 labeled “Test” for this exact purpose. A diode is already installed in the transmitter, and
these “test” terminals serve as points to connect the milliammeter across.