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stanwich

Flex i/o outputs shorting

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Hello everyone! Im hoping someone could shed some light on a issue im having. Im having issues with a 1794 ow8 output module. This module is used to control 8 hydraulic cylinders that push biomass into a burner. The overall problem is that roughly once every couple months a output will short in the on state and force a coil on, activating the vavle full time and burning out the coil. Each seperate output is fused on the module with a 2a fuse and each modules incoming power is also fused at 2a. The output that is shorted did not blow its fuse. The 'ON' light is not activated but 120v still runs through it. Since each valve has 2 coils and one is forced 'on' the main fuse will blow once the second half of the valve is activated because the valve is fighting itself. It is not just this one module, this happens on other modules that serve the same funtion as well. This is a contractor installed system but they have been no help so im left trying to figure it out. Could my seperate inputs be fused too high at 2a for this type of card. These are parker valves 120v 60hz with a max 1500psi pushing wet hog/chips. Any suggestions would be really appreciated!

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Hi Stan,

The fuses are placed there to protect the contacts against a shorted solenoid. According to the module installation instructions the recommended fuse size is 3Amp, so my conclusion is that your 2Amp fuses are well sized and you have to focus on what is shorting your outputs.

I would check:
1st. If the terminal base is appropriate for the working voltage (see: Installation Instructions)
2nd. If the working voltage is really 120V
3rd. If the solenoid working current is bellow 800mA which the contact capacity for inductive loads

I would also consider:
1st. Installing a snubber circuit in parallel to every solenoid (not to the I/O contacts)
2nd. Interposing a higher capacity relay between the I/O contacts and the solenoids
 

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What we will typically do is install a relay between the output and the physical coil. This way the relay will absorb the inrush, or any higher then expected current rather then the output. 

I have seen where the output will get pulsed on and off quickly, and blow an output out. I again would suggest going the relay mode.

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Sounds like contacts are welding. Possibly from drawing inrush current for too long due to the valve solenoid sticking. Or possibly longer term overload due to solenoid not properly pulling in 100%.

I can imagine a program cause, but would need to know processor type and I/O setup.

 

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