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#1 |
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![]() ![]() Join Date: Sep 2012
Location: ontario
Posts: 69
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Hello,
I need to control an Acid pump at a water treatment plant, Using an analogue output of a micrologix 1400. This PLC only has 0-10V analogue output card. So I installed a 500Ohm resister across the pair of my instrument cables, to get my 4-20Ma. So at 2Votls will be my 4mA and 10 Volts will be 20mA. PLC Resolution (0-4095) Acid pump 1 0-60 L/H Acid pump 2 0-12000 strokes per hour In The PLC. I'm using RXLogix micro developer, and I'm using the SCP instruction to scale I tried using one SCP block, but the input number that is entered is much lower than what is display in the pump Example: F8:1 input: F8:1 --->> entered 7 l/h in the HMI input min: 0 input MAX: 60 scale min: 819 Scale max: 4095 Output O:0/0 --->> Pump said about 12 l/h I tried using two SCP block per pump, to scale input number from the HMI to 4-20 and the second would scale the 4-20 to the PLC 12 bit analog card (4095) but I got similar results... SCP 1 Input: F8:1 ---->> entered 15l/h in the HMI input min: 0 input MAX: 60 scale min: 4 scale max 20 Output F8:2 ---->> 8 mA SCP2 Input: F8:2 ---->> 8 mA Input min: 4 input max: 20 scale min: 819 scale max: 4095 Output: O:0/0 ----->> 1638 The results for example today I entered 30 L/H in the HMI the pump said 41.8 for the other pump I entered 6000 strokes per hour, pump would say 12000 strokes per hour. PLEASE HELP, if anyone has ideas.... Thanks everyone Last edited by richleva; February 9th, 2018 at 10:29 AM. |
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#2 |
Member
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The 1400 analog output can only source into 1000 ohms or greater. The 500 ohm resistor is loading down the output so you will never reach full scale.
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#3 |
Member
![]() ![]() Join Date: Jun 2007
Location: Barcelona
Posts: 345
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You can't convert 0-10V to 4-20 mA output by simple adding a resistor, you need a converter like this:
http://new.abb.com/products/1SVR0117...v-4-20ma-24vdc and probably it will solve the scaling too In addition a the voltage output need to be connected to high impedance input, if not the voltage drop across the cable generate a significant error. Last edited by lfe; February 9th, 2018 at 10:47 AM. |
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