Low-Hassle Options for Control Connections

Oct. 9, 1997
Labor-saving terminal blocks can minimize installation time and cut the cost of controls as well.

Joe Pickell
Phoenix Contact
Harrisburg, Pa.

Intense competition is forcing system integrators to get smarter about fielding control equipment that minimize materials and labor costs. It is also increasingly important that the start-up period be relatively painless. For these and similar reasons, makers of control equipment have developed connection modules that simplify the wiring of cabinets and are economical as well.

An example is in applications using pilot lights. Such indicators frequently serve as the main machine interface for automation or process alarms. It is critical that operators be able to periodically test the integrity of the lamp. The typical solution is to utilize a push-to-test type pilot light rather than a standard pilot light. The drawback, though, is that push-to-test models cost more.

Multichannel diode arrays, however, can be used to test multiple pilot lights using one push button. This approach costs much less than deploying one pushbutton for each light. The arrays provide diode OR gate connections to facilitate lamp testing and sit in a single module which mounts to a DIN rail. This eliminates the need for duplicate terminations.

Many control-system assemblers encounter another problem involving polarity of field connections. Field sensors and controller I/O cards are designated as either sinking or sourcing devices. Often, as the end of the job draws near, assemblers discover they have a mismatched set of sensor and controller terminals, where both are either sinking or sourcing devices.

Logic inverters placed between the sensor and controller solve the problem. Terminal blocks with inverters already built in are available, so there are no extra connections required. They convert the sinking field device to a sourcing device or vice versa, allowing direct connection to the controller.

A number of packaging and conveyer applications require high-speed sensing. This is typically not a problem for field sensors which sport sensing speeds ranging from 0.3 to 1.0 msec. The difficulty is that standard controller input cards have typical speeds of 7 to 8 msec. The normal solution is to purchase specialized high-speed cards. But these cards come at a tremendous premium and typically handle only one channel each.

Another possible approach for some applications would be to install a pulse-stretching module. It takes a short-duration pulse and stretches it sufficiently to register on a standard controller input card. Besides eliminating the need for a specialized high-speed input card, it lets one multichannel card handle tasks normally requiring multiple high-speed cards, again with attendant cost savings.

One area that system integrators sometimes struggle with is providing a manual speed-override option for variable-speed drives. Most makers of these drives also offer as an accessory a through-panel-mounted speed-override potentiometer. Many integrators specify that such overrides be locked inside a cabinet so unauthorized users can’t make speed adjustments.

There is no standard way of mounting the override control. A typical solution is for the panel assembler to build a makeshift angular bracket which bolts into the control panel. This is somewhat time consuming, as is the typical connection strategy of soldering leads to the potentiometer.

However, a more elegant approach uses a DIN rail-mounted, fully enclosed potentiometer module. In addition to being easy to mount, the device is also easy to wire. Termination is via screw clamp terminals.

When installing electromechanical relays, frequently users face two problems. One arises in applications where the controller uses a triac output device which, in turn, must switch an electromechanical relay. The problem comes from the triac off-state leakage current. Many triac output cards have up to 6 mA of leakage. Unfortunately, this is enough to keep the coil energized on many of today’s high-density relays. Normal solutions involve hard-wiring resistors across relay coils directly at the coil terminations. This can create a shock hazard as well as a heat-dissipation problem.

Specially developed relay packages address the behavior with an integral bleeding resistor across the coil. This bleeding resistor drains the triac leakage current during the off state so the electromechanical relay can reliably drop out.

The second problem involves using high-density PC-based control to switch electromechanical relays. Often, outputs from PC-based controllers are TTL-level signals. They lack sufficient power to drive most electromechanical relays.

In the presence of highcurrent loads, one option is to use the TTL level signal to control a solid-state relay. The solid-state relay then drives an electromechanical relay. But the resulting redundant relaying system takes up a lot of panel space, may complicate installation, and degrades system reliability.

Termination makers are addressing the problem with relay packages containing a transistor built into the coil circuit. The TTL-level control signal is used to switch the transistor. The transistor, in turn, switches a 24-Vdc power source to drive the relay coil. The entire circuit fits in one high-density DIN railmounted package.

© 2010 Penton Media, Inc.

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