Transistor Output Tolerance

Specing Point IO to drive 3w 24vdc pneumatic solenoids. Switching frequency is approx every 3.5 sec. Math puts that at .125 amps.

Debating between 1734-OW4 (relays rated at 2 amps) and 1734-OB8 (transistors rated at 1 amp).

I've read on here a rule of thumb that if the relay is rated for 10x the current of the solenoid it can generally handle the spike on opening. My question is because a transistor does not physically break the connection, can it manage the inductive spike better than relay contacts might?

If you were designing this would you choose one over the other? I'd prefer not to add interposing relays in the junction box this point IO is going to be in for simplicity and neatness, but will if the consensus is neither of these cards will last long.

I would not use relays in this application, based on the 3.5 sec frequency of switching. If this is some kind of production machine that runs 16-24 hours per day, the contacts will eventually wear out regardless of contact amperage.

We've been driving MAC valve solenoids (6W @ 24vdc, 0.25A) with PLC transistor outputs for decades and have rarely had any issues. Nowadays we use solenoids with built-in suppression diodes, but failures were infrequent even before we made that improvement. Replacing relay output modules is way more common.

kolyur,


I agree with the previous post.
ALL relay outputs have a cycle life before they break or the contacts weld (stick) together. 3 watts @ 24 volts = .125 amps with an inrush current of 6 times that you have .75 amps. it's 1 amp if the inrush 8 times.
I would do two things. #1 look at the solenoid specs and see what the inrush current is and also make sure that there is reverse bias protection at the solenoid to help prevent output damage.
james

I also agree with post #2, although I have used Mitsubishi plc's with relay outputs, these drove solenoids without suppression at a rate of over 10,000 actuations per machine in a 12 hour shift and 8 machines, these ran for over 12 years, way above the rated operations of the relays in the PLC cards, We replaced only 2 relays in that time and on one occasion this was due to a shorted coil splattering the relay, pretty impressive, I would still use Transistor outputs though. on another note, MAC solenoids are bomb proof, used them many times by the hundreds, only ever had a couple of problems due to dry air (food environment so lube was not allowed by the site on oilers not even food safe) this was easily rectified by a small addition of lube & cleaning the shuttle (yes they allowed us provided it was cleaned before re-assembly).

Most transistor output cards have a suppression diode built-in to each output, which probably helps contribute to their long life even when driving inductive loads. Not 100% sure about Point I/O- you may want to check the manual.

I go exclusively with Solid State Outputs.
If I must use a relay contact, I'll just drive a mini, easily replaceable, relay module from the Solid State Output.

Are the relay output cards usually cheaper than the solid state? I noticed a lot of our relay output points go out, and I have to change the output, or if I'm out of space, change the entire card. Why use relay cards if SS will last longer? Even if the output is pulling on a relay, would it not be better to use an SS card?

alive15 said:

Are the relay output cards usually cheaper than the solid state?

Click to expand...

I can't speak for everything but at least on some series, (Looking at you, 5069 CompactLogix), the relay output cards are significantly more expensive.