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Old March 9th, 2011, 05:03 PM   #1
raul111
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1794 flex analog input power supply

I am using Flex1794-IF 4IXT. I have 2 wire transmitters connected to card. These transmitters are externally powered. I have connected as per the installation guide which says

+24 from ext power ---> + of transmitter ----> - of transmitter --->I input on module----> I ret from module ----> - of power supply.

My question is do i need additional power supply to the module.
Flexbus current is provided by the adapter 1794 ACNR15XT?
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Old March 9th, 2011, 05:17 PM   #2
Ken Roach
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All FLEX I/O analog modules require external +24VDC power.

They are not loop powered, and only the isolated communication section of the module is powered over the FLEXBus connector.

The XT version and the ordinary module are wired identically.
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Old March 9th, 2011, 05:25 PM   #3
raul111
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thank you Ken for your reply.

From what i understood, I do not have to connect power to module terminal base( c34 and B 16 on 1794 IF4IXT) since i have external power to transmitters, is that right?

Regards.
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Old March 9th, 2011, 08:00 PM   #4
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That is incorrect. 24V DC power to operate the module itself needs to be connected on the 2nd and 3rd rows (if you are using the 1794-TB3 terminal base).

The 1794-UM008 User Manual has complete wiring tables. Page 26 and 27 show the wiring for the 1794-TB3.

It can appear strange, hooking up +24V for the network adapter, then another +24V supply for the discrete I/O, then another for the Analog I/O modules, then another for the loop-powered transmitters. But FLEX gives you a lot of choices about the mix of devices, and a great deal of ability to isolate the different parts of your I/O system from one another. Some systems hook up everything to one big bulk power supply, and others provide separate power for all the functionally separate sections of the control system.
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Old March 10th, 2011, 08:48 AM   #5
raul111
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Thank you ken for your reply.

I saw the user manual and it does say i have to use 24 VDC to module.
Now I want to use separate power supply to all the analog and discrete modules and do not want to daisychain from adapter.
I also want to use Circuit breaker between power supply and each module just to isolate things from each other.
How much amps circuit breaker will I need?
all the modules are rated for 10 A max.
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Old March 10th, 2011, 02:31 PM   #6
Ken Roach
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That "10 ampere" rating is for the amount of power that could safely pass through the 1734-TB3 blocks. Often the TB3 blocks are used with discrete outputs with the "daisy-chain" arrangement and relatively large amounts of current could pass through them.

An analog module consumes very little operating power. The 1794-IE4I, for example, consumes just 80 mA at 24VDC.

If it were my system I would just provide fused terminal blocks for the separate Analog modules. Remember that fuses are just to protect the wire, so any ordinary hookup wire and the 1794-TB3 terminal blocks will be far heftier than an ordinary 1A glass fuse in a terminal block fuseholder.
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