SLC Power Supply Specs and Calculations

Here's something I've never had a problem with before. Just put together a new 10-slot SLC rack with a P2, 5/05, and eight NT8 modules. When I power it up, all the module lights come on for an instant...then go out (including the power supply). If I kill the power and then reconnect, nothing lights up unless I wait maybe 5 minutes...then it briefly flashes and goes back off again. These are indications that the PS is overloaded. I've done the power calculations on the P2 for this hardware and it should be well within the limits. I also entered the hardware in the I/O config, and it tells me that the P2 should be fine. PS jumper is set to 120v, and I have also tried another P2 with the same result. I discovered that the rack will power up if I keep the number of NT8s at 6 or less, and I've tried swapping the modules around to see if the addition of any specific module causes the problem...but I can't conclude anything from that. Rockwell Tech support can only recommend that I replace the NT8 modules to see if that cures it, or install a bigger power supply. Unfortunately, I don't have $12000 to buy 8 more modules to try, and I'm concerned that there may be another problem, because I have never seen the specs be so incorrect, as they are suggesting may be the case here...so, I don't really want to just go with a P4. Anyone have any suggestions on how to troubleshoot this, or any experiences with the power consumption specs being wrong?

bluenoser337,

i would suggest going to the local ab rep and borrow some of his trainer equipment. i do it here sometimnes.

you might have a bad chassis.

regards,
james

James...only minutes ago, I had this arranged. A demo chassis will arrive tomorrow. Thanks for the post. I'll report my findings.

I ran it through the AB Integrated Architecture Builder and it said that you are well within the power requirements.
Power used: 22240 mW, Power remaining: 47760 mW.

Have you tried swapping the modules around to see if just 1 NT8 module is causing the problem.

Hi Mark: Yes, I have tried all possible combinations and all I can conclude is that the problem stays away until I have more than 6 NT8s in the chassis. Hoping it is the chassis. Switching to a bigger PS (P4) like tech support suggested just doesn't seem right. Thanks for the post!!

It's probably the inrush of the 1746-NT8's. You are right that the behavior of the power supply is consistent with the "crowbar circuit" that protects the power supply when it's overloaded.

I don't think the specifications are wrong, I just think that they're measured and written for one each of a type of module, rather than six or eight of them. The 1746-NT8 is based on hardware from Spectrum Controls, which I know uses some relatively high inrush components to power up their excellent isolation and signal conditioning circuitry.

I found an RA Knowledgebase document # 65226 that says that tests with as few as 6 of the 1746-NT8 modules can overwhelm the inrush capability of the 1746-P2 power supply.

That document says testing on hardware in 2008 showed that the 1746-P2 can start up a maximum of five 1746-NT8 modules, but that the 1746-P4 can run up to thirteen of them.

My gut feeling, based on what you provided so far, is that it is an inrush current problem, and might even be caused by having all of them perform calibration on power-up at the same time.

I would also bet that the P4 power supply would probably fix it, but I understand why you want to get to the bottom of it first.

EDIT: Ken seems to confirm my gut...

Ken...I knew you'd know! That knowledgebase document pretty much sums it up. The previously unknown thing was that "the 24V surge current is 287mA". Maybe the worksheet should have a column for the surge current, too...for situations like these. Guess I'm trading in my P2 for a P4. Thanks!!!!

Thanks also, to OkiePC for his post!