Using loop Calibrators

Special-purpose electronic test instruments called loop calibrators are manufactured for the express
purpose of 4-20 mA current loop circuit troubleshooting. These versatile instruments are generally
capable of not only measuring current, but also sourcing current to unpowered devices in a loop,
and also simulating the operation of loop-powered 4-20 mA transmitters.

Here, the loop wiring is broken at the negative terminal of the loop-powered transmitter, and
the calibrator connected in series to measure current. If this loop had a test diode installed, the
calibrator could be connected in parallel with the diode to achieve the same function. Note the
polarity of the calibrator’s test leads in relation to the circuit being tested: the calibrator is acting
as an unpowered device (a load rather than a source), with the more positive loop terminal connected
to the calibrator’s red test lead and the more negative terminal connected to the black test lead.
The same loop calibrator may be used to source (or drive) a 4-20 mA signal into an indicating
instrument to test the function of that instrument independently. Here, we see the calibrator used as a current source to send a 16.00 mA signal to the PV (process variable) input of the controller:

No transmitter need be included in this illustration, because the calibrator takes its place. Note
how the calibrator is used here as an active source of current rather than a passive load as it was in
the last example. The calibrator’s red test lead connects to the controller’s positive input terminal,
while the black test lead connects to the negative terminal.