Isolated 4-20 mA on a no iso-card

BillRobinson · Jun 3, 2009

Anybody know how to fix this:

I have a 4-20mA analog output from a VFD going into a non-isolated analog input card (AB 1756-IF16). The VFD unit is powered by 208V buy supplies 24V for the output. The analog input card is also 24V but is powered by a separate 24V power supply.

The card can't read the output from the VFD. I believe the problem is that both devices are trying to power the 4-20 mA signal. Since they are not both supplied by the same source I don't believe I can wire it like a 3-wire transmitter. Anybody know how can I make these two devices work together?

Mickey · Jun 3, 2009

Put an isolator between the two

http://www.miinet.com/products/sg_signal.shtml

http://www.actionio.com/

http://web1.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners

sthompson · Jun 3, 2009

The 1756-IF16 is a joy to wire. Check out the Manual carefully.
Did you tie the Channel Return (iRTN)to the RTN Terminal?
I have attached the Current Wiring Diagram in a Doc File.

rdrast · Jun 3, 2009

sthompson said:
The 1756-IF16 is a joy to wire. Check out the Manual carefully.
Did you tie the Channel Return (iRTN)to the RTN Terminal?
I have attached the Current Wiring Diagram in a Doc File.

Sthompson has it partially correct. The 1756-IF16 does not source loop power, but it can cause trouble if you have different grounds. The basic for current inputs that are referenced to the same supply ground, is to only hook up the In<channel> and the iRTN<channel>, then jumper the iRTN<channel> to the module common. This is the norm for 16 channel mode.

If you do, however, have a problem with commons NOT referenced to the common on the module, you have two possible choices. For 16 channel mode, make sure that the IF16 is the LAST input in the loop (closest to negative), and bond the iRTN, module RTN, and source common together.
For 8 channel mode (differential), in 99% of the cases, you still do not need an isolator for a foreign source supply, as long as it's maximum potential relative to the module ground is less then 100VDC or so (I've gone as high in emergencies as 230VDC, but that scares the heck out of me safety wise). In this case, the module is configured for 8 channel differential mode, the + from the current loop goes to IN-0, the - from the loop goes to IN-1, and you bond iRTN0 to IN-1.

The Plc Kid · Jun 3, 2009

What does the isolation do? Why is it needed? I see some have a 1500volt chanel to chanel isolation what is meant by that?

Mickey · Jun 3, 2009

The Plc Kid said:
What does the isolation do? Why is it needed? I see some have a 1500volt chanel to chanel isolation what is meant by that?

See link below

http://www.miinet.com/whatsnew/articles/interactive_signal_interface_ins_and_outs.pdf

BillRobinson · Jun 4, 2009

rdrast said:
Sthompson has it partially correct. The 1756-IF16 does not source loop power, but it can cause trouble if you have different grounds. The basic for current inputs that are referenced to the same supply ground, is to only hook up the In<channel> and the iRTN<channel>, then jumper the iRTN<channel> to the module common. This is the norm for 16 channel mode.

If you do, however, have a problem with commons NOT referenced to the common on the module, you have two possible choices. For 16 channel mode, make sure that the IF16 is the LAST input in the loop (closest to negative), and bond the iRTN, module RTN, and source common together.
For 8 channel mode (differential), in 99% of the cases, you still do not need an isolator for a foreign source supply, as long as it's maximum potential relative to the module ground is less then 100VDC or so (I've gone as high in emergencies as 230VDC, but that scares the heck out of me safety wise). In this case, the module is configured for 8 channel differential mode, the + from the current loop goes to IN-0, the - from the loop goes to IN-1, and you bond iRTN0 to IN-1.

Thanks, rdrast.

I am in 16 channel mode and I currently have 9 other devices on this card (channels 0-8) which all are being powered by the same 24V power supply, thus having the same ground. The 24V common is already connected to the RTN (terminal 28).

So you are suggesting that I connect the positive from VFD output to IN-15, and the negative to iRTN-15 and RTN (terminal 27)?

sthompson · Jun 4, 2009

The iRTN Jumper is required for Current Applications. It ties the open end of a 249 Ohm Resistor to the Common RTN.
I have several IF16 Cards with a mix of 2 wire, 3 wire, and self powered units. i.e. Temperature Sensors, Pressure Sensors, Flow Meters etc.
They are all set up for 4-20mA, and work as expected. I have seen miswiring blow the 249 Ohm resistor (By others).
I have had to replace a bunch of the surface mount 249 Ohm Resistors to get all the channels back up and running.
Stu.....

Isolated 4-20 mA on a no iso-card

BillRobinson

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rdrast

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The Plc Kid

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BillRobinson

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