Action instruments provides unparalleled assistance in helping customers select and use our products. However, Action also provides excellent after sales support.

One of our customers is using the Q476, AC powered, frequency to DC isolators. In this installation, the 10V excitation source is being utilized, standard on the Q476, to drive a relay. The relay is providing an input frequency from 0-2.32Hz and the Q476 is set to provide a 0-1mA output to a chart recorder. The graph of their chart recorder, scaled for 0-1mA, is shown below. The horizontal axis shows a time frame of about eight hour.

As can be seen, whenever the output rises to about 500uA, the output starts to intermittently slam high.

The module was sent back Action to determine why the module was creating a spike on the output. A relay was connected to the input terminals and sure enough, the spike was observed when input frequencies were above 1Hz. After simulating the customers problem (and important step in the process), the question is what is causing the output to be driven high? What is the root cause? The cause was traced to the relay which was occasionally sending a false trip, or an additional closure of the relay, to the Q476.

Now that the problem has been identified, what is the best way to resolve the problem? Adding a debounce circuit to the input of the Q476 is the simplest and most effective solution. A 47W resistor in series with a 0.1mF film capacitor across the Q476's input terminals completely suppresses the unwanted signal.