What Values of CR and/or Varistor Can I Use to Protect My FP-RLY Module From Inductive Loads?Hardware: FieldPoint>>Discrete I/O Modules>>FP-RLY-420
Problem: I need to use a contact-protection circuit to protect my FieldPoint Relay modules from inductive loads. How do I know what values of CR and/or Varistor I should use? Solution: CR Circuits: The selection of the resistor and capacitor value should be sized to solve the noise problem, but not so large as to cause other concerns such as size or cost. A good rule of thumb is to start with a resistor value equal to the rating of the circuit voltage. Then, choose a capacitor sized to the current level of the inductance. For current values of 1 Ampere, a 0.1mFd capacitor is a good starting point. For larger current values, the capacitance should be increased. Therefore, a 120 VAC, 0.4 Ampere inductive load would use an CR circuit with a resistor value of 120 ohms and a capacitance value of 0.1 mFd. If this capacitance value does not solve the noise problem, then first step should be to increase the capacitance value followed by decreasing the resistance value. For example: A 120 ohm/0.1 mFd combination and a 47 ohm/0.2 mFd combination have approximately the same electrical time (T=RC) constant for energy absorption, but the latter combination may work better in a particular application then the former because of the smaller resistance value. Generally, there is no one perfect combination for a given application, but rather a series of combinations that could work. Varistor Circuit: You should use a varistor when adding an absolute voltage limit across the relay contacts. Varistors are available for a wide range of voltage and clamp energy ratings. Once the circuit reaches the voltage rating of the varistor, the varistor's resistance declines rapidly. In the case of a 220 VAC inductive load, use a Varistor with a voltage rating slightly above 220V Related Links: KnowledgeBase 2MDAQBOH: I Am Using a FP-RLY Module and I Get Intermittent Errors Attachments:
Report Date: 07/23/2002 Last Updated: 12/11/2003 Document ID: 2NMCRAQ2 |
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