Hello,
I was reading the S-Series Hardware reference (D800040X012) and came across a formula calculating the 24VDC power input requirement from the DeltaV system power supply:
(page 149) 24 VDC bulk power to system power supplies = ( (LocalBus current * 0.5) + (# of controllers * 2 A * 0.24) ) /.75
0.75 shows that DeltaV power supply has about 75% internal efficiency (page 156).
The 12VDC LocalBus current gets normalized to 24VDC by multiplying 0.5, but I am not quite sure where "2 A * 0.24"came from?
The S-series controller requires 5VDC from the DeltaV power supplies, which has a power requirement of +5VDC at 1.4A maximum (7W) (page 167).
Considering a maximum consumption from the controller, I think the formula should be
= ( (LocalBus current * 0.5) + (# of controllers * 1.4A * [5VDC/24VDC] ) ) /.75 or
= ( (LocalBus current * 0.5) + (# of controllers * 7W / 24VDC) ) /.75
If one of you guys know the answer behind 2 A * 0.24, please advise.
Thanks very much,
Ken
I have to admit I've always wondered where that came from.
There was a 2A max on the combined 5VDC and 3.3VDC output, which I assumed is where the 2A came from. Then the mention of the 3.3VDC output let me know there was a "rest of the story" that might explain the 0.24.
So...yeah. Anyone at Emerson have the crib sheet on this one?
-Travis
In reply to Travis Neale:
Travis, Ken, I sent the thread to a few folks around to see if we could get this clarified.
In reply to Jim Cahill:
No answer yet... but I now know the engineers who are looking at this.
I wanted to close the loop on this one, but can't shed much more light. The DeltaV team relayed back to me that this equation uses proprietary factors that allow for margin.
They couldn't find the crib sheet, could they!
I made sure to share your response with the team!
One time in our office we verify the effective current need of MD controller with out any I/O's cards, the result is less then 0.7 A. 0.24*2A/0.75 = 0.64A is very close.
Maybe this is the answer .
Regards
Dawid
In reply to Dawid.Zieba:
The numbers may not appear to make sense, but the result is still 0.48 A / 0.75 per controller . Ask 5 engineers to come up with this equation and you'd likely have 5 formulas that yield the same result. Bottom line, the formula works and is backed by Emerson. It is a conservative number for sure.
Just for fun, I'd say the 2A number takes into account some worst case current draw at minimum voltages and the .24 number converts from 5 Volts to 24 Volts taking into consideratoin some efficiency losses happening in the 12 VDC to 5 VDC conversion. (the DC/DC supply first converts 24 V to 12 V, and losses here are accounted with the /0.75 at the end of the formula. The 5 V is derived from this 12 V source, so there is a second loss to account for).
That is one man's explanation. (Disclaimer: This does not reflect the views of Emerson or its employees, or of NBC, CBS, ABC, Fox or even the CBC and CTV or any of their affiliates.)
So thanks to Dawid for confirming that the result is consistent with a real world test. Though it doesn't explain the terms in the formula, we know they are giving us the right result.
Andre Dicaire