Question

You are an electrician working in a plant that uses programmable controllers to perform much of the logic for the motor controls. The plant produces highly flammable chemicals, so the programmable controllers use a 24-volt DC output that is intrinsically safe in a hazardous area. You are having trouble with the output modules of the programmable controllers that are connected to DC control relays. These outputs are going bad at an unusually high rate, and you suspect that spike voltages produced by the relay coils are responsible. To test your theory, you connect an oscilloscope to an output that is operating a DC relay coil and watch the display when the programmable controller turns the relay off. You discover that there is a high voltage spike produced by the relay. What device would you use to correct this problem, and how would you install it?

Answer 1

To solve the problem of relay coil spikes damaging programmable controller output modules, install a flyback diode in parallel with the relay coil, ensuring proper polarity to short-circuit and safely dissipate the spike.

Step-by-step explanation:

To correct the problem of high voltage spikes produced by the relay coils in a programmable controller, you would use a diode installed parallel to the relay coil. This diode is often referred to as a flyback diode, snubber diode, freewheeling diode, or suppression diode. Its purpose is to suppress or eliminate the high voltage spikes that occur when the magnetic field in the relay coil collapses, which happens when the controller turns the relay off.

The correct installation of the flyback diode is in parallel with the relay coil, but it must be installed in the correct polarity. The cathode (usually marked with a stripe) should be connected to the positive side of the coil, and the anode should be connected to the negative side. This way, when the coil is de-energized, the diode provides a path for the current induced by the collapsing magnetic field, thus protecting the output modules from high voltage spikes. The diode essentially short-circuits the spike, allowing the current to circulate through the coil and diode instead of damaging the controller's output modules.