View Full Version : 24 Volt DC Inrush OverCurrent on PLC

October 30th, 2008, 04:58 PM
This has thrown me a curve ball so I'm looking for someone smarter than myself to provide me an answer.

I have a 24 Volt 1769-OB16 DC output card connected to 14 contactors. I am staggering when they pull in but unfortunately the inrush is 91 VoltAmps which correlates to 3.79 Amps given the system is 24 volts. The output card is rated to 2Amps for 50 milliseconds, but the inrush seems as though it could be more than the card can handle. How long does this inrush last? Anything under 10ms means the current set up will work, if not I need to either upgrade my output card or use a different network of contactors.



October 30th, 2008, 05:24 PM
I think that you will fry that sucker....it would be better (also read more panel space & $$) to put in sone small relays that then turn on those contactors....just my thoughts.


October 30th, 2008, 05:27 PM
I was thinking along the same lines; I was just hoping to save my panel builder from having to just about rewire the whole system.

Thanks for the input.

October 30th, 2008, 05:47 PM
There are some very small interface relays that only take up about 1.5 DIN rail TB spaces...those are the ones that I would look at...I'll see if I can get you some numbers...
SEE THIS: (http://ecommerce.altechcorp.com/Ecommerce/CatalogItemDetail.aspx?IID=11838)



October 30th, 2008, 06:02 PM
Just spoke to our other tech guy here, said he's seen this problem in the past and took that course to fix the problem as the inrush is roughly 100ms.

Which ever route we go we need 14 relays and pretty much the next day.

Any idea on the Amp draw these will have?

October 30th, 2008, 06:27 PM
About 200 mA. The Altech units above are .9 watts. That calculates to 37.5 mA at 24 DC.

October 30th, 2008, 06:30 PM
If that's for the whole lot I will suggest this to our engineer.

I really appreciate the help as we are up against the wall time wise.

October 31st, 2008, 10:47 AM
Take a look at these relays, I have a situation similar (3.9 amps at 24VDC) to yours and my output cards fail regularly. On the second machine that I did, I went to 240VAC coils with interposing relays. It is still hard on the relays but my output card is safe.

Hhttp://web2.automationdirect.com/adc/Shopping/Catalog/Relays_-z-_Timers/Electro-Mechanical_Card_Relaysave a look at these,

October 31st, 2008, 11:59 AM
I have these in stock from Phoenix Contact:


http://eshop.phoenixcontact.com/phoenix/treeViewClick.do;jsessionid=JLqJj6RxT1FtX4ppCp8nQ1 01Q4dK6PBjrpKLsdQ8Gp1s4q20RPzt!-324031627?UID=2967060&parentUID=null&reloadFrame=true

If you're in Northern Indiana, PM me and I can quote you.