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SCXI chassis wants its own power cord - can't splice into rest of system

Hello All,

 

I have a computer & DAQ hooked to a SCXI chassis. One of the cards in the chassis is a 1161 relay card, and I use it to actuate 120V coil solenoids. This has worked fine for weeks.

The SCXI chassis and the rest of the system had their own AC cords going to the wall, like the schematic at the top.

 

The other day, we “simplified” the wiring of our system, and the SCXI chassis power was spliced into the rest of the system, like the bottom part of the schematic.

 

With this new wiring, when the SCXI-1161 actuates the solenoids, it often sends another card in the chassis into a tizzy. That card (an analog input card) goes to rail (but throws no errors).

I unwired the splicing and hooked the SCXI chassis back into its own AC outlet (although it’s on the same circuit), and now all is well again.

 

Schematically speaking, the two connection methods are the same. Any ideas of what may be causing the issue? Why, when actuating the solenoids, does the

 SCXI system want to have a separate cord to AC? The solenoids are 10 watts each, and there’s 5 on at a time maximum.

 

Thanks for any insight.

Richard






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Broken Arrow:

 

Are you sure the line and neutral connections are matched up?

Does the lower outlet used in your original configuration have a good earth ground connection (or lack of)?

 

-AK2DM

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"It’s the questions that drive us.”
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Thanks for your response AK2DM. If line and neutral were not matched up, I'd have blown fuses right? As far as ground, that has been what I suspect, that the system (the rack) has a ground loop, and by plugging the SCXI into a "real" outlet, I get better ground.

 

I read a while back about someone having issues with surge suppressors and/or GFI's on the power source of a chassis that sourced inductive loads. I also suspect that may be the cause, since the rack has both. But even bypassing those devices didn't help. The SCXI just want a cord of her own.

 

It comes down to "if it works it works" but this type of thing drives me crazy.

Richard






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Darn editor ate my post, arghh.

 

Regarding blowing fuses if line/neutral swapped- not necessarily, unless the equipment tied the neutral to ground which is not common practice.

 

Other than the splice, has the physical layout of low voltage and AC wiring changed? Keep low level analog lines away from high voltage lines.

 

I suspect the analog card going to the rails may be happening when you turn the solenoids off. That is when they create a back EMF spike. Snubbers across the coils may help. There are also zero-crossing solid state relays that only turn on/off whan the AC power waveform is at the zero-crossing point.

 

Without being there first-hand, it is hard to guess what may be going on with the splice. I would check that you have a good earth ground on your outlet.

 

Good luck, let us know if you get it resolved.

 

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~
"It’s the questions that drive us.”
~~~~~~~~~~~~~~~~~~~~~~~~~~
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You're right about L1 & L2, there's no connection, but if you have GFI's in line, that'll cause issues.

I need to place the snubbers just for good measure, but I can't capture a decent noise spike on a scope, so I thought maybe the solenoids had internal MOV's. I need to check that.

Thanks for your suggestions. My thought at the moment is that sending AC cords to the wall is more of a star ground than splicing, which is anything but a star ground. Even though the wall is 3 feet away from the original splice, it has made a difference.

 

Serves us right for cleaning up the wiring. The more things change, the more they stay the same. Smiley Wink

Richard






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