Internet PLC Forum

Trying to build an ampmeter using an LM324 op-amp to measure plus/minus 50 amps with a 1mv/amp shunt = +/- 50mv to the op-amp. The "-" of the shunt is also ground of the op-amp 12 volt supply. The ADC input is 0 - 5 VDC. Since the op_amp input goes negative, can I design the circuit with the +/- voltage in, and the op-amp input floats, with a range of minimum 0 to 100mv? The LM324 has 3 other sections in use.

Thanks for the quick relpy. Maybe I mistated, the first, and following 2 parallels are contacts of a coil, all three created with "1" contacts and selecting previous relay coils for the actuator.
     R        R
--!!-----!! -------??
        !     R   !
         --!!---

Had this before working, went to change the the dCUSF to CusFn, deleted that one thing and was unable to insert the change. Also deleted every thing on rung (not delete circuit) and got to same point.

With Trilogi 5.33, is there a way (like edit a config file), or can it be put on the wish list:
1: File>Open: Either remember last folder, or set default to a particular folder.
2: File>Print: Printer remembers last printing from Trilogi "options" like Fast Draft printing. Other windows applications do this.
3: File>Print>Ladder Circuits: Remember set default to 80 columns

With Trilogi 5.31b, using the "TAB" to move the insert point doesn't work.

I have been working on a UPS that goes the other direction. 120VDC (International Power IHC12-3.4) normal power, with a 12 VDC marine battery as backup)( See my post ot this forum some months ago for more details). To do the switching I use a "ice cube" relay (Square D 8501KP12P14V20) DPDT 120VAC coil). NC contacts to battery, NO contacts to 120VAC supply.  The coil is series with 12VDC coil DPDT DIP relay (Clare LX200000) the NC contacts. One of the PLC's outputs controls the DIP relay. When the 120VAC drops, the big relay coil power drops, and the PLC power is switched to the battery. The PLC is monitoring the 120VAC frequency and voltage already since this is a standby generator controller, and recognizing the 120VAC is out, will power the DIP coil, keeping the PLC power to the battery until the 120VAC power is good for a period of time say 15 minutes, to prevent chatter and the fluctuations that might occur as the power company is working on the lines. At this time I have bench tested the setup, and the 120VAC power supply bridges the gap, and even 12 VDC the PLC does not reset.  Hopefully in about a week I should have the installation complete and be able to verify the design, will keep posted.

I would like to measure say 0-15 vdc from a marine battery with a PLC (TRI-PLC T00MD-888+) that has 0 - 5 VDC ADC input. Concerned with developing a ground loop since there is already a common ground that the PLC is using to control other items that might have chassis ground. Thinking there should be an off the shelf transformer or something to do the isolation. I can handle voltage dividing resitors or OP-AMP selection, just the isolation got me baffled.

Thanks for the quick reply, but the number of spaces " " inside the quotes needs to be at least the number of maximum digit (characters) that might be displayed. With the HMI putting out a maximum of 4 characters, it looks like "    " (4 spaces). It would be nice that the HMI would take care of this detail automatically, and elimiate the flashing of the display. I guess a couple of "if's" checking for change of how many digits would take care of that.

The implementaion of the frequency and voltage sensing individually was stright forward as described, but trying to combine the 2 circuits from one 9 vac (wall wart) and rectifier bridge power supply created a problem, that probably requires an opto-isolator on the frequency side. The frequency side requires the rectifier bridge "+" output to go to the PLC's power in (12 VDC in my case). For voltage side, I prefered the bridge "-" output go to the PLC's ground terminal, assuming the ADC voltage in/output reading is fairly close to linear, then calibration only requires one voltage (120 VAC) checked with a voltmeter, and a simple equation in Basic to correct the readings. Then we have the rectified 9 VAC connected across 12 VDC which just doesn't work.  I have stock of a TIL 111 opto isolator that will try to interface.