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Tobi

MOVs To Protect Outputs

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I am installing several 1746-OW16 relay output modules in an AB SLC 5/05 PLC. The relay contacts are being used to turn on a 24VDC isolation realy. I have read that add a MOV accross the coil of the isolation realys will extend the life of the realy contacts on the output modules. Can anyone suggest what type of MOV to use in this situation. Thanks

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I would suggest using diodes across the isolation relay coils, 1N4001 diodes would be a good place to start.

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He has a relay ouput ,diodes across the isolation relays are used with transistor outputs.

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The MOV protects against over-voltages. It protects the coil insulation (when connected at the coil side) and the relay contacts and body (when connected at the contact side). In 24 VDC work the coil is usually more valnurable to over-voltages than the contact side. An anti-parallel diode across each DC-driven coil will conduct inductive current at turn-off and as vettedrivr suggested is the least you can do. An RC-snubber (a series connected resistor and capacitor circuit) across the contacts reduces the rate of voltage change (rise or fall) and so reduces/eliminates arcing when the contacts bounce at turn-on/off. It also allows for some leakage current to flow, but with 24 VDC circuits this usually is no problem. Ideally you should use all three methods. MOVs are selected according to clamping voltage and energy capacity. When selecting them check the data sheet DC clamping voltage (some manufacturers mark their MOVs for AC clamping voltage while others mark them for DC). The MOV diameter/size specifies the energy capacity and it is better to err on the larger side. The IN400x line is a typical choice for relay coils when the current is under 1 A. The RC-snubber values are calculated on the allowed maximum rate of voltage change, the driven coil current and the voltage (24 VDC in your case) and are not that critical for relay work. There are many ways (simple or more complex) to calculate them and a web search should provide a suitable method. You can either roll your own circuits or buy them ready. Most relay manufacturers offer sockets/bases/add-ons for their relays such as indicating LEDs, anti-parallel diodes, MOVs as well as RC-snubbers for various load currents.

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So the intrface ralays for 24DC are with current in the range 5-20ma (coil current) .the new 1000A contaactors are with electronic control PCB with optocopler isolated inputs -current<10ma, the choise of 1N4001 diode is recommendet when we have a PLC with transistors outputs. In my aplicationen i have the faellow power suply 220V/220volt transormer to the 24Vdc electronic powersuply (Omron,IFM,or Sitop) the 24Vdc is for the PLC and all of interface,relays ,contactors i think this is a classic configuration

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I'm with vettedrivr on this one. A diode across the 24vdc relay coil will do fine. MOVs are normally used on 120VAC inductive loads. Diodes are normally used on 24VDC inductive loads regardless of whether the output is a transistor or a relay contact. Edited by mellis

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ok,transistor for example NPN with relay between colector and +VDC in this case the diode is compulsory! Why?because we have a inductive load and thats induts a inductive overvoltage peak -500v for 2ms and the transitor will be damage!sure! With the diode to the relay, in teh OFF time (1-2ms) we have a over voltage with opposite polarity,and in this moment the diode is conductive this is a secret (Faradey and Lentz laws) if we have reley to reley circuit where is a sense with the diode.the diode is a transistor guardian sorry for my horryfied english Edited by alex31

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Some relay manufacturers provide relays with built in flyback diodes. It will simplify your installation.

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see this technote http://rockwellautomation.custhelp.com/cgi...HN1cHByZXNzZXI* http://rockwellautomation.custhelp.com/cgi...amp;p_topview=1

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