In one of our skid we have one control loop (ie) gas temperature has to be controlled.The cooler louvers are inbuilt with the actuator. So we bought the I/P converter separately with the electrical load of 675 ohm. But the PLC will accept only <500 ohm. Due to this the loop is not working. Moreover the I/P requires a minimum of 13.5 V but the PLC supplies less than that. We checked the I/P separately, it works. Can anyone help me to solve this problem?

 

As you have mentioned load of i/p converter is more than 500 ohms, you are right power supply of controller is getting loaded.
To solve this problem you can use a isolator / splitter between controler o/p and I/P converter.
Loading problem will be solved.
Regards

Kapil
[email protected]

 

Bellofram's T-1000 I/P (part number 961-070-000) has an input impedance of only 180 ohms for the 3-15 psig (0.2-1.0 bar) range.

Change out the I/P.

Bud

 

True, if one can locate an isolator that can drive the stated resistance at the stated voltage of approx 13 volts. I couldn't.

I checked 3 different vendor lines and found numerous isolators that were limited to driving 500 ohms, and the few I found that could drive 800 ohms needed a full 24Vdc power on either the input side, output side or as power to the isolator itself. This surprised me. I thought for sure that I'd easily find an isolator that could drive more than 650 ohms.

As I read the original, it sounds like the PLC AO is active, driven by an internal 13V supply, not a passive loop powered output that could be juiced up with a higher voltage supply.

Bud

 

500 ohms is the standard for fixed range 4/20 mA instruments. Try a separate field rangeable signal conditioner between the PLC and the I/P that will give you a wider adjustment range. For example, a Wilkerson Instrument DM4380 A has a maximum output compliance of 24V (1200 ohms at 20mA) and can be set up for just about any input/output combination. I have used quite a few of these over the years for non-standard signals, and have had no problems with them.

 

Buy a signal isolator or amplifier from Applied Process Instruments or Phoenix Contact. This will take a separate power supply, receive the
information from the PLC in the form of the 4-20MA output, and relay an isolated or non-isolated signal of the same magnitude but with a higher load impedence rating.

--
Steve Myres, PE
Automation Solutions
(480) 813-1145

 

In reply to Bud Buyer - The original request appeared to be for a 4-20mA (or possibly 0-20mA) signal; nothing was said about it having to be loop powered. The description was 13.5V (min) and 675 ohms. The 13.5V would have come from 0.020 A x 675 ohms = 13.5V.

All he needs (according to his message) is to be able to drive 20mA through 675 ohms. The problem is that most signal conditioners (and PLC cards) can't handle more than 500 ohms (i.e. their output voltage has a maximum of 10V).
The 800 ohm models you said you found should do the job just fine. The reason they specify 24VDC power is because that is the next highest standard voltage (800 ohms would require 16VDC for 20mA).

 

That resistance is very high, usually you only see numbers that high for smart positioners. Is the I/P converter microprocessor based?

We (SMAR) make a converter called IS400 that will drive 4-20 mA through high resistance (750 ohm):
http://www.smar.com/products/is400p.asp

Usually we only use it when two smart positioners have to be connected in series to get split-range

Jonas