Floating Signal Source Wiring Guide
A floating, non-referenced signal source is a signal that is not connected in any fashion to the building ground system but rather has an isolated ground reference point. Examples of floating signal sources are outputs of transformers, thermistors, and battery-powered devices. To measure these devices, the ground reference point must be set to the analog ground reference of the SeaI/O-570 as shown in the below schematic. Without this connection, the signals will float and an accurate measurement is not possible. The SeaI/O-570 is configured to measure floating, non-referenced signals by default.

Ground-Referenced Signal Sources Wiring Guide
A ground-referenced signal is a signal that is connected in some fashion to the building system ground and therefore is already connected to a common ground point with respect to the SeaI/O-570. Examples would be non-isolated outputs of instruments and devices that are connected to the same building power system as the SeaI/O-570. The difference in ground potential between two devices connected to the same building power system is typically 1mV to 100mV and can be much higher with improper power system grounding. This ground potential difference can show up as a measurement error without using the proper measurement connections. The connection scheme below is designed to eliminate this ground potential difference from the measurement. This setting can be configured and verified via Sealevel SeaMAX software.

Thermistor Inputs
On the SeaI/O-570 PCB, Switch 5 (SW5) allows each of the eight A/D channels to be individually configured for a thermistor by moving the corresponding channel selector (1-8) to the “On” position. This will apply a bias voltage of +5V through a 10K Ohm resistor. Connect the thermistor between the input and ground reference on the SeaI/O-570’s terminal block connector. This will create a voltage divider and as the resistance of the thermistor changes so will the voltage read by the application. Please reference the data sheet of the chosen thermistor to calculate the measured temperature.

Application Example:

Highly inductive loads, those that use magnetic fields such as DC motors, produce a surge of voltage (referred to as "blowback voltage") when a relay is opened, breaking power to the electrical circuit. This surge of blowback voltage is created by the collapsing of the armature coil's magnetic field. In cases with large or higher wattage motors, this can cause an overvoltage condition on the motor and could cause the relay contacts to arc and possibly fuse. To protect against this overvoltage condition, care must be taken to dissipate the energy stored in the DC motor's coil when power is removed and there is no longer a path for current to flow.

In this example, we will demonstrate why an open-collector output with a flyback diode is better suited for controlling applications that are considered highly inductive.

Application Explanation

Reed Relay Implementation

Figure 1 shows a simple circuit for controlling power to a 24V typical DC motor using a Reed relay on a Sealevel SeaI/O-440U distributed I/O module. For this example, we will use an oscilloscope to measure the voltage change when power is removed (turned off) using this circuit.

Figure 1

Figure 2 represents a graphic from an oscilloscope measurement of the voltage spike that is induced at the Reed relay terminals when the relay is opened, removing power from the DC motor. This graph shows a delta voltage (delta Y on the screen) of 127V. During testing there were approximately 200V (delta) spikes routinely observed when the Reed relay is turned from on to off, indicating that the Reed relay did not properly dissipate the highly inductive load energy from switching the DC motor's power.

Figure 2

Open Collector Output

Figure 3 shows a circuit for a controlling a 24V DC motor with Sealevel's SeaI/O-540U open-collector output module with integrated flyback diode controlling power:

Figure 3

Figure 4 shows a graphic using an oscilloscope measurement of the voltage change that is induced when power is removed from the DC motor when using an open-collector output from the SeaI/O-540 module. During the open-collector output testing, the largest observed voltage change was 30V DC at the output terminal.

Figure 4

Conclusion

When power is removed from a highly inductive DC motor, the motor's blowback voltage must be properly dissipated to avoid damage to the electrical control circuit and/or the DC motor. This example clearly verifies that our open-collector output with a flyback diode circuit works very well at controlling highly inductive load energy associated with DC motors.

Sealevel's SeaI/O-530 and SeaI/O-540 modules use open-collector outputs with integrated flyback protection diodes making them a good choice for this application. The flyback diode provides a path for current to flow and dissipate in these conditions.

If Reed relays are used for moderate inductive load applications, they may require employing circuit protection measures. Reed relays are perfectly suited for a broad range of control applications but are not typically called upon as a solution for highly inductive loads.

Equipment Specifications:

DC motor: Pittman/CPR: 500
Power supply: Bench top 24VDC
Oscilloscope: Rigol DS1052E

SeaIO-540U: (32) Open-Collector Outputs (each containing a flyback protection diode)
SeaIO-440U: (32) Form A - Reed Relay Outputs

Hardware: A PC with Sealevel DIO-16.PCI Adapter (Part# 8002) or USB SeaLINK (Part# 8208), connected to Sealevel TB02 terminal block with CA112 cable. The TB02 and CA112 cable are available in a kit as Part# KT101.

Software Control: Reed Relay output provides contact closure to ground (common) causing the CD player to enter the selected mode of operation (Play, Pause, Stop, Forward, or Reverse).

Hardware:A PC with Sealevel PIO Adapter (Part# 8005, 8008, 8009, 8010) or USB SeaLINK (Part# 8203, 8205), connected to Sealevel TA01, Relay Rack I/O Simulator terminal block with CA167 cable. The CA167 cable from the PC to terminal block is available as an option.

Software Control: Run VBTest or VCTest to cause the LEDs to turn ON and display the switch inputs. Set the ports to input or output through the control panel. Use the TA01 to develop your application program and simulate a solid state relay rack before going live on-site.

VBTest

Hardware: A PC or Laptop with Sealevel SeaLINK USB PIO-48 Adapter (Part# 8203) connected to an industry standard relay rack on DIN rail mounting track controlling a light pole or other industrial device. The USB cable (Part# CA179) is included with the 8203. Ribbon cables (Part# CA167) can be used to connect the 8203 to up to two standard PB24 style relay racks.

Software Control: Our included VCTest and VBtest sample applications can be used to control relay closure and input monitoring.

VBTest

VCTest

Hardware: A PC with Sealevel PLC-16.PCI (Part# 8011) board installed, connected to Sealevel TB08 terminal block. The cable from the PC to terminal block is supplied.

Visual Basic Application to turn on fan:

Hardware: A PC with Sealevel PIO-48.PCI (Part# 8005) or USB SeaPORT PIO-48 products, connected to Sealevel TB07, terminal block with CA167 cable. The cable from the PC to terminal block is available as an option.

Software Control:Run VBTEST or VCTEST to monitor switch inputs. Click on Outputs (B0) to turn LED #1 ON/OFF. Click on outputs (B1) and (B2) to turn LED #2 from Red to Green.

VBTest

VCTest