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How many switches can I get out of a relay?


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Hi, I have a V130-33-RA22. Three of my relays turn on and off little indicator lights every two seconds; one second on, one second off. I'm concerned that I'll burn out these relays quite quickly doing this. So, how many switches are these internal relays rated for? Since I'm running less than .02 amps through the relay, should I use a digital output rather than a relay output to get more life out of the output?

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  • MVP 2021

The V130-33-RA22 doesn't have any digital transistor outputs, so you're stuck with using the relay.

Here's an article on relay contact life with more information that you can possibly need-

Relay Contact Life

The relay in this unit is a Tyco PCN-124D3MHZ, which is rated for twenty million mechanical cycles.

Since you're switching a resistive load, there won't be any appreciable arcing so you don't have to worry about contact pitting. An inductive load such as a solenoid, contactor, or even another relay will cause arcing and pitting which is what destroys relay contacts. An RC snubber across the load significantly reduces this, as the capacitor absorbs the voltage spike created when and inductor is disconnected and effectively eliminates the arc.

With a two second cycle time, you will reach twenty million cycles in 463 days of continuous operation. You will have to decide if this is acceptable.

Joe T.

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One more question: Are the digital transistor outputs rated for more mechanical cycles than the relay outputs? I'm wondering if I should use digital transistor outputs for my blinking lights instead of relays. Since the blinking lights are a low amp resistive load, maybe digital transistor outputs would be appropriate?

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One more question: Are the digital transistor outputs rated for more mechanical cycles than the relay outputs? I'm wondering if I should use digital transistor outputs for my blinking lights instead of relays. Since the blinking lights are a low amp resistive load, maybe digital transistor outputs would be appropriate?

Yes

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  • MVP 2014

Joe, when the relays wear out, can I replace them with replacement relays from Unitronics? Or do I have to replace the whole printed circuit board, or worse, the whole V130 PLC?

I'm not Joe, but usually the official line is that you would need to send the entire PLC back to Unitronics for repair.

However, Unitronics usually list the manufacturer's part number(s) of the relays in the PLC datasheets, and they aren't too hard to find in stock at general electronics suppliers.

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  • MVP 2021

Ditto to Simon's response on replacing the relays with Unitronics' blessing. They are soldered in. I have been known to replace internal relays myself, but this totally voids the factory warranty. You also need a soldering iron with a vacuum tip and a fair amount of experience to get something out of a double-sided board. Don't try it on the only unit you have - it's easier to re-program the PLC to use a different output.

The way I prefer to design a system is to start with a PLC that has transistor outputs and drive small external relays. There is no limit on the number of times a transistor output can switch- think about a solid-state audio amplifier switching thousands of times a second. It adds a few bucks and some more rail space to the project, but if a relay wears out or you blow it up you easily replace a single part. Use relays with indicator lights to give your project that sci-fi look.

Joe T.

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