Wilkerson Instrument Company Inc. – Blog - Solutions provider for signal conditioning and process control instrumentation

Prevent Overflow – Duplex Lift Station Back-Up Pump Controler

June 9, 2014 John Wilkerson DC Voltage and Current, DC/AC Isolators, DC/DC Isolators, Signal Conditioners No comments

The DR1920 Back-Up Pump Control is a compact DIN rail mounted unit used to monitor
a backup High-High Level Alarm in a tank or wet well and to start up to two pumps when the High-High Alarm switch closes.

It prevent overflows in situations where the primary level control system fails.

When the High-High Alarm switch in the tank or well closes, the DR1920 closes a relay that starts Pump 1 and starts an internal Timer 1. When Timer 1 reaches its set time, and the High-High Alarm switch is still closed, Pump 2 is started.

Pump 1 and Pump 2 will run until the High-High Alarm switch opens. When the
High-High Alarm switch contact opens, Timer 2 is started and both pumps continue to run until Timer 2 reaches its set time.

If the High-High Alarm switch opens during the Timer 1 interval, Timer 2 is started when the High-High Alarm switch opens. Pump 2 is not started and Pump 1 will run until Timer 2 reaches its set time. Timer 2 is set by the user to lower the level ito the desired low level.

The DR1920 also has provisions to alarm on a switch opening.

ALARM SWITCH MONITOR

The DR1920 monitors the open or closed status of a switch contact that is already
wired into a control system. This switch may have any of several voltages that are
used to control the pump system. In addition, if the primary control system fails, the normal voltages may not be present.

The DR1920 is wired across the alarm switch contacts and uses a unique circuit
that measures the impedance present between the switch contacts.

When the DR1920 senses a low impedance, it begins its control function, regardless of any absence or presence of voltage across the switch contacts. This feature allows the
DR1920 to be wired in parallel across the switch contacts without regard to the
existing control system, permitting simple installation in existing systems.

For situations where the existing wiring prevents this circuit from being used, an
isolated contact must be provided. A second input, which requires an isolated contact or
open collector transistor (optical isolator) is also provided.

A Reset input is provided to stop the pumps and reset the timers. It requires a contact
closure or open collector transistor which will conduct. Since the Back Up Pump Control is used
only when there is a problem with the primary control, the digital alarm inputs, as
well as the reset inputs, are provided to allow the periodic exercising of the system
to verify functionality.

Isolated DC Signal Conditioners Applications

October 23, 2013 John Wilkerson AC Voltage and Current, DC Voltage and Current, DC/AC Isolators, DC/DC Isolators, Signal Conditioners No comments

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.