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adding va rated dvices to outputs #54204 07/21/05 11:02 PM
Joined: May 2001
Posts: 376
F
frank Offline OP
Member
I'M working with johnson controls these days automating everything from chillers and air handlers to boiler feed water.Anyway the question i have has to do with outputs.

For instance if you have an output rated at 15va and devices such as a 7va actuator and a 8va damper control wired in parallel to an output.Do you just add up the total VA?This is for a metesys M5 DX which is really just a specialized PLC.I would ask johnsons but they're programmers are asking me.thanks



[This message has been edited by frank (edited 07-22-2005).]

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Re: adding va rated dvices to outputs #54205 07/22/05 12:51 AM
Joined: Oct 2004
Posts: 55
R
Ray97502 Offline
Member
Kerchoff's Current Law (KCL)states that the algebraic sum of the currents entering a junction must equal zero. The sum of the currents entering a junction ie equal to the currents leaving the junction. With voltage constant, the VA is perportional to the currents. If the supply voltge is 24V the max supply current for a 15va output is is 0.626A. The load elements on each branch of the parallel circuit are 7va and 8va which would draw 0.292A and 0.333A. The total load currents accoding to KCL is 0.625A. So in theory yes the output va will support the load.
However, if the load elements have an inrush current at any point in the loading then the output limit may be exceeded.
Most of the information about the Metasys System can be located at this link.
http://cgproducts.johnsoncontrols.com/default.aspx?topframe.aspx&0

Good Luck

[This message has been edited by Ray97502 (edited 07-22-2005).]


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