Archive for DC Voltage and Current

Isolated DC Signal Conditioners Applications

Value of Isolated 4ma to 20ma Signal Conditioners

DC signal conditioners with 4mA to 20mA outputs provide the ability to send signals over long wires to more than one instrument.  The current from the signal conditioner is a constant current proportional to the signal into the signal conditioner.  The constant current allows long leads.  The level of the measured signal is proportional to the voltage drop across the resistor in the input of the instrument receiving the signal.  Voltage drops across the long wires have no effect on the constant current.  The constant current allows several instruments to have their inputs wired in series and the current through each input is identical.

Isolated Signal Conditioners provide a further important value.  The isolation circuit has no direct electrical connection between the input and output circuits.  The input signal is passed through a transformer as an AC signal or through an optical isolator as a light beam.

These two types of circuits make the signal conditioner able to have a high voltage common mode signal while still processing a very low level signal from a sensor or other instrument.

The isolation from input to output also keeps extraneous currents from odd sources from flowing on the common signal lead through the signal conditioner.  This extraneous current is usually called a “ground loop”.  The isolation breaks this ground loop.

Prevent Ground Loops

A typical ground loop is created when a thermocouple has its welded junction grounded to earth through its mounting hardware.  The thermocouple signal is very low in amplitude.  If the thermocouple wire has a completed circuit from the thermocouple mount to the earth at the signal conditioner and a current flows in the thermocouple wire, it will impose a voltage on the thermocouple signal.  The typical ground loop noise is caused by AC current flowing in the wire.  With an isolated signal conditioner, there is no path for current to flow to earth at the conditioner.  The isolator breaks the ground loop.  The thermocouple signal is amplified and passed on through the isolation circuit to the conditioner output with no noise from the ground loop.

Process Small Signals On High Common Mode Signals

A common mode signal is a signal which is connected to both inputs of a signal conditioner and the conditioner has an output equal to a zero input.  An example would be a thermocouple welded to the positive terminal of a 138V battery so the temperature of the battery terminal can be ascertained.  The conditioner processes the thermocouple voltage but is not affected by the 138VDC common mode voltage from the battery.

Another example would be a battery with a resistor in series with the + terminal.  A signal conditioner can measure the voltage drop across the resistor so the current through the resistor can be determined.  The measurement is unaffected by the battery 138V common mode voltage.

A great value of isolated signal conditioners is that common mode ranges of 1000V to 2000V are not difficult to find.

Split One Signal into Many Separated Individually Isolated Signals

A common requirement of 4mA to 20mA outputs from DC input isolated signal conditioners is a need to measure 1 sensor and send the signal to a number of locations in the 4mA to 20mA form.

One method of creating several isolated signals is to connect the inputs of several 4mA to 20mA input/output signal conditioners in series.  Each conditioner will provide an isolated output.

Multiple output conditioners are available.  If dual output conditioners are used as above, one gets 2 outputs per each input.

Another method is to use one 4ma to 20mA input conditioner.  Using voltage input conditioners with 4mA to 20mA outputs, connect the voltage inputs in parallel across the input of the current input conditioner. 250 ohm loads are common inputs for current inputs.  This creates 1 to 5V drop across the 250 ohm resistor.

Isolator Creates a Separately Isolated Signal From An Existing Signal With No 2nd Power Supply Required

In areas where power is lacking and another isolated signal is required, a loop powered isolator can do the job.  A loop powered isolator uses the 4mA to 20mA current from a signal conditioner to power another circuit to create another isolated output.


MM/FR Series DR Series DM Series SR Series TW8 Series

Mighty Module

DR Series DIN-Mod
SR Series TW8 Series

Frequency Input Two Wire Transmitters

Frequency Input Two Wire Transmitters

Frequency input 2W transmitters are widely used with flow meters to measure flow rate and total flow for a period of time.  They are used in many other applications to measure the rate of movement of many other devices.

Typical Applications:

Flow meters

  • Turbine meters
  • Paddle wheel meters
  • Positive displacement meters

Conveyor belt speed

Fan motor speed

AC generator speed for correct frequency

Speedometer for boats, trains, and other vehicles

Wind velocity


Applications are only limited by the creativity of the human mind.  Creativity is improved by knowing how an intended useable device works.



Frequency input TX’s come in analog circuits and digital circuits.  Analog circuits came first and are still in great use.

An analog frequency circuit works on a simple principle.  A pulse of a fixed width and amplitude is created in a circuit which is triggered by the zero crossing of the input signal.  The width is narrower than the period of 1 cycle of the input frequency.

As the input frequency changes, the rate of these pulses changes with the frequency.  The circuit measures the average DC voltage created by these pulses.  At the  highest frequency  measured, the voltage is at its highest level.  As the frequency goes down, the average voltage gets less in proportion to the decrease in frequency.   This circuit is the frequency to DC converter.

Average_VDC Frequency Two Wire Transmitter

Average VDC Frequency

The output of the frequency to DC converter drives the TX output circuit to give 4mA at zero frequency and 20mA at the full scale frequency.

The frequency to DC converter circuit output has to be filtered to make the output a smooth DC voltage.

The filter used can only work to a limited lower frequency below the full scale frequency.  As the frequency goes down, ripple on the DC voltage gets higher and higher.

Due to the filter limitations, the TX can only be used from full scale down to where the ripple becomes excessive.

The filter also limits the response speed of the TX to a change in input frequency.

The filter can be modified to improve response speed and ripple by having the frequency to DC converter circuit create a pulse for each zero crossing of the input signal.  This provides 2 pulses per 1 cycle of the input.  The filter can be faster because it is filtering a frequency 2 times higher.

The double pulse circuit requires a reasonable duty cycle on the input signal, because it must respond to 2 zero crossings.

The single pulse circuit can measure the frequency of a very narrow pulse, since it only responds to 1 zero crossing per cycle.



First – Some Math and a Counter

The period of 1 cycle of a frequency signal is:  (1000 Hz (1KHz) used as example)

Period = 1/freq      Period = 1/1000 = .001S = 1 millisec = 1mS

If you open a gate into a counter for 1mS and let a 1,000,000 Hz signal go through the gate for the 1mS, the counter will display a count of 1000.  This is our TX input signal (1000 Hz).

A digital Frequency 2W TX processes input signals by the counter method above.  The input signal is divided by 2 to make a pulse the width of 1 cycle of the signal.  This pulse opens the gate to the counter in the microprocessor.  A clock generator (1,000,000 Hz in our example) pulses are counted for the time the pulse holds the gate open.

The count made is scaled mathematically so the resulting number can be input into a digital to analog converter.  The analog signal from the A/D converter drives the output circuit to create a signal between 4mA to 20mA.

The A/D converter holds this value until the next sample is made.  It is then updated to the new measured value.

The microprocessor allows a display to be part of the 2W TX.  It also allows the frequency range to be changed with a keypad.

The lowest frequency which can be easily measured is limited by the size of the counter in the microprocessor and the size of the registers used for the math required.



SR2700 Frequency Input Field Rangeable  Two-Wire Transmitter

SR2700 Frequency Input Field Rangeable Two-Wire Transmitter


Most Frequency  input 2W TX are purchased with a fixed frequency operating range.  If the installation changes the device being measured, a different TX must be acquired.


The SR2700 can accept several inputs.

The most common signal source is a magnetic pickup which creates an AC sine wave when an iron object is moved passed the magnetic end of the sensor.  The frequency of the sine wave created is proportional to the velocity of the iron passing the sensor.  The TX creates an output current proportional to the frequency created.  The common current used is a 4mA to 20mA output. Over current protection is provided.The TX input also has a pullup resistor which can create an input to the TX by pulling the resistor to common circuit and then releasing the resistor.

Breaking a light beam or a high speed switch closure can be used.  Transistors can be used to drive the TX input with suitable signals driving the transistor.  Creativity can add more.

A typical signal is a sine wave.  Square waves can be processed.  The time between narrow pulses, which are proportional to the frequency, can be processed.  The input which can process the fore mentioned signals will also process an AC signal riding on a DC voltage.

The SR2700 can be  changed to a different frequency input in the field.  The frequency range and the filters required are in a small module which plugs into the top of the enclosure.

The small plug-in module on the SR2700 contains all the components to make the width of the square wave pulse (Full Scale) and the filters for a 20 to 1 or a 10 to 1 turn down.  10 modules cover the Full Scale range from 12.5Hz to 12,800Hz.  One TX can cover any Full Scale in this frequency range with the small plug-in Module.

The  product has the standard 4 to 20mA output.

It is available as a Private Label product.




DIS 975 Frequency Input Process Indicator with Optional Alarm(s) and Isolated Transmitter, 4.5 Digits

DIS 975 Frequency Input Process Indicator with Optional Alarm(s) and Isolated Transmitter, 4.5 Digits

MM1700 Frequency Input, Single Alarm, Fixed Range, DPDT Relay

MM1700 Frequency Input, Single Alarm, Fixed Range, DPDT Relay


© Joe E. Wilkerson 2012


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