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IamJon

120vac or 24vdc relay output?

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It's been too long... How do I determine which to use, 120V or 24V relay outputs, if it even matters? MLX 1200 with 120VAC power Relay outputs to an autodialer. Does the PLC power or autodialer inputs determine which voltage to use? Thanks. EDIT: I'm looking at the 1762 OW8 and OX6I Edited by IamJon

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24 V is safer. You can always add an interposing ( slave) relay if your autodialer requires 120VAC.

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It depends. If the device(s) you are deriving require locally derived power to the point where you need individual isolation, this definitely makes relay outputs the most viable. Similarly, if output power is a concern, generally 120 VAC and/or relay outputs often spell the difference between needing an interposing relay (which creates many more additional points of failure with additional connection terminals, and either a solid state or electromechanical relay), and not. Then for cost and reliability reasons, you may have to choose one. As an example, it is much more cost effective and reliable to use 120 VAC coils and either triac outputs (if you use common control power) or else a relay output card (if you have separate control power) with motor starters. These days of course drives have gotten so cheap that often you can have your cake and eat it too on the low end. Otherwise the choice is between 120 VAC triac outputs and 24 VDC transistor outputs. Now even here if available (this is not something you find with Micrologix), if electronically fused or diagnostic IO is available, the cost of that over fused outputs generally makes the diagnostic IO the better choice even if there is a cost vs. ease of maintenance decision to make (the diagnostic outputs are cheaper than regular outputs plus fuse blocks). Of those two, if your downstream device is only available in 120 VAC then unless you add an interposing relay (expensive), the better choice is probably 24 VDC. 24 volt outputs are much faster since you don't have to deal with zero crossings (switching time on a triac output is a random variable up to 16 ms, the slowest transistor outputs are usually 3-4 ms), there are no switching transients to deal with (no diode protection), damage in the event of a line-to-ground or line-to-line fault is almost nonexistant, and with long leads, you don't have to deal with inductive crosstalk causing spurious switching without the use of heavy relay coils to suppress it. You will also find that in most cases, the IO cards are slightly more expensive but the devices themselves are slightly less expensive since they don't have to have AC-to-DC power supplies and can use much lower cost DC-DC power supplies in the devices. This is of course USA-centered. In European systems, then 220 VAC has to be considered as well. When interfacing to TTL logic, it is usually easier to just get 5 VDC cards (if available) then to try to interface to it through an interposing layer. And once in a while you run into something strange. I have heard of folks using 48 VAC coils in starters simply to get around the newer arc flash rules. You just have to find a compatible IO card and recognize that all of your starters and coils will be special order.

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24vdc is more common since most I/O devices are 24vdc themselves. (At least from my experiences) If you need to actuate 120V outputs then you could use isolation relays of some type such as cube relays. Are your input devices 120V?

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Am I missing something? Aren't the 1762-OW8 (NO) and OX6I (NO-NC Type C) just dry contact modules that will work with AC or DC?

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One thing to also consider with the "relay" output is mechanical switching. The contacts are rated for hundreds of thousands of on/off's but with certain applications that require fast switching or pulses you would be surprised at how short that time actually is. The triac or transistor (solid state device) is not a mechanical output so the physical switching limitations do not apply.

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