PortenoSuenos

1756 Fast Dry Contact Output

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Hi All, I have an existing 1756-OW16I output module that we are switching an output (very low load) on/off every second. It works but about once a year I have to move it to another output due to failure. Im just switching a 12vdc signal to a relay coil. There has to be a better solution to this type of output. I just need a dry contact but switchable at a cycle rate of 1 second. Any suggestions..??? Thanks Edited by PortenoSuenos

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The relay is going to fail at the rate you are switching! Use a DC output module.

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... driving a separate, easily replaceable, relay. If you are switching a low level signal (I hope) get 'signal-level' contacts. These may be bifurcated and gold or silver plated. You may still have to replace it often but it will be less of a hassle.

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Is it possible to use something like these. http://www.farnell.com/datasheets/1500104.pdf Steve

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Adding interposing relays will NOT help resolve this issue in any way. Only a transistor output will work at these switching rates.

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Im using this to trigger (reset) a watchdog timer relay, so it has to be every second. The watchdog timer is made for this fast triggering. The problem isn't want Im driving...its the output module in my PLC. The watchdog timer sends out a 12vdc signal and this is what you use to send back to the reset signal. Im using a dry contact on a OW16I module but I feel there is a better way. Edited by PortenoSuenos

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Given the stated REQUIREMENT for a dry contact the external relay was suggested to ease the replacement given the inevitable failure.

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and you have already received solution - do not use electro-mechanical relays in applications requiring frequent or fast switching, use solid state (transistor output when DC application). OW16 is a relay output card. don't use it for frequent switching because it will fail. use output card with transistor outputs such as OB16 or OB32. of course their output is 24V and not Galvanically isolated. this can be resolved by adding interposing device such as relay or optocoupler or SSR but again - do NOT use electromechanical relay for frequent or fast switching, use solid state (transistor output), for example one like in reply #4 by nehpets. this will give you something that will practically never need any replacement. note, solid state devices will have voltage drop that is greater than mechanical contacts of a relay (for example 0.1 - 0.3V) which is insignificant for most applications (including I/O operations which is what you need). but some devices with solid state outputs also include current limiting circuit in their output stage. in such case voltage drop may be as high as 3V which can be quite significant. solution is to read the datasheets and test things before implementation. when permanent solution is not possible or not economical, you can also use sacrificial approach where interposing replay is part of consumables. according to datasheet of the product in reply#4, voltage drop is less than 200mV even at full current. in other words - I would go for it. just make sure to watch polarity.

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I didn't realise that it was a relay output card (thats what you get for typing before you read), however after reading the spec. for this card I am gobsmacked that the rated mechanical rating for these relays is 100,000 operatations. How can Allen Bradley justify the extortionally high prices the charge for these cards. Steve

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So IF Im understanding correctly.... Use a OB16 module firing a SSR to trigger/reset the WDT relay.

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