Archive for Loop Powered Display

How Loop Powered Isolators Work

How Loop Powered Isolators Work

Loop powered isolators are necessary when an extra isolated 4/20mA output is required in areas where no power is available.

A loop powered isolator’s power is provided by its 4/20mA input current.  This makes it possible for an existing 4/20mA loop to be used to provide another isolated 4/20mA output through a loop powered isolator.

dm4391 loop powered isolator

DM4391-2 Dual channel DC isolators With and without front cover

A loop powered isolator is basically a center tapped transformer which has the input 4/20mA current flowing through its center tap on the primary. An oscillator circuit alternately switches the current through each half of the primary.  This alternating current creates a current in the secondary of the transformer. The voltage out of the secondary is rectified and filtered.

The turns ratio on the transformer is designed with a slight step down so the current out of the secondary is slightly greater than the input current.

This is a simple form of a loop power isolator, but it has shortcomings in that the transformer is not perfectly linear over the current range. The current is also sensitive to the load resistance on the secondary.

A more complex version of a loop powered isolator has basically the same transformer circuit, but it has a circuit which monitors the current into the unit. The circuit passes this data through another isolation circuit to the secondary side of the unit.  This circuit controls the current out of the isolator so it exactly matches the input current.  This control circuit makes the loop powered isolator as accurate and linear as any powered transmitter.The DM4391 utilizes this feed forward circuit to provide an accurate and linear 4/20mA output.

The DM4391 can be connected anywhere in an existing 4/20mA loop. It is available as a single channel or dual channel unit.

For specifications and information about the DM4391 see our Catalog Page (pdf) or Installation / Operation Manual (pdf)

Differential Pressure Measurement Across Liquid Filters in a Beverage Processing Plant



Measure the pressure drop across each filter in a bank of filters to determine when to switch to a fresh filter(s) so the clogged one(s) can be cleaned.

Provide electrical isolation between each input signal as well as between input to output to prevent noise pickup and noise from ground loops.

Wilkerson  Instrument Co. Products Used


Mighty Module Wilkerson Instrument

Mighty Module


Mighty Module Wilkerson Instrument

Mighty Module


LP400 Loop Power Display

MM4408 MM4300 LP4OO
Specification PDF Specification PDF Specification PDF
I/O Manual PDF I/O Manual PDF I/O Manual PDF
How To Order How To Order How To Order



Many beverage processing plants utilize filters at various stages of the process to remove unwanted materials from the liquid. As these filters load up doing their job, the flow passing through the filter is reduced. If the filters are not replaced or cleaned, the flow can be impeded enough to disrupt the total process.

This particular application surfaced when the customer was faced with replacing an old pneumatic system. The customer wanted to install an electronic system using strictly “off-the-shelf” commercially available equipment. He was faced with very limited control panel space and limited budget. Therefore, the system must use the fewest components possible and the smallest displays available.

The Fluid Flow Diagram shows the basic arrangement of the pressure transducers and the filters. Only one prefilter and one two-stage filter are in service at any given time.


Fluid Flow Diagram - Bottling Plant

Fluid Flow Diagram - Bottling Plant


This allows the other filter(s) to be cleaned, sterilized and made ready for service.

USDA approved pressure transducers that are cleaned and sterilized in place were selected. By placing transducers 1, 2 and 5 so their use is common to either prefilter and either filter bank in use, the customer needed only five pressure transducers to do the job.

The Signal Flow Diagram shows how the customer’s judicious use of a switch, two MM4300 DC to DC transmitters, and three MM4408 difference transmitters achieves the needed input to input and input to output signal isolation with only five modules.

Signal Flow Diagram

Signal Flow Diagram


The product chosen to measure the differential pressure (pressure drop) across the filter was the MM4408 and the MM4300.

The MM4408 inputs are not isolated from each other, but the output is isolated from both inputs.

The MM4300 signal conditioner has isolation from the input to output.  It is used to provide one input to the MM4408. This provides isolation for both inputs.

Three LP400 loop powered displays in series with the three 4/20mA outputs provides an indication of the pressure on the output of the filters.  The Display can be calibrated in engineering units to fit the need.

Wilkerson Instrument Company Inc Is a solutions provider for signal conditioning and process control instrumentation  –   Copyright 2012



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