Internet PLC Forum
General => Technical support => Topic started by: cdenk on February 18, 2006, 05:36:34 PM
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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.
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There is two general ways to perform what you want. However, you won't be able to use such methods for AC frequencies above about 10 Hz due to the limited speed of the PLC. Your best line of attack for, say, mains frequency is to use an AC to RMS converter and then sample the resulting DC level.
Evan
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I guess I forgot to say, but the current is DC only, like an ampmeter on an auto, can swing +/- as the battery charges or discharges and as a result the signal can go +/- also, and the one side of the signal is near the PLC's ground, except for line and connection loses.
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You will need to connect your input as a differential amplifier with gain = 50 (so +/-50mV amplified to +/-2.5V) and the ground of your OpAmp is tied to a voltage reference = 2.5V with respect to the PLC's 0V. This way, the output of the OpAmp will be equal to 0 to 5V with respect to the PLC's ground. I am not sure if the LM324 is suitable for this purpose. An instrumentation amp should work better.
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Linear Technolgies www.linear.com makes a variety of current sensing amplifiers like the LT6100 which outputs a stable 0-5v signal from a shunt resistor. They are cheap, much easier to use than dual ended op amps, work well with the plc and can handle bidirectional inputs.
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Unless I missed something, my application is what Linear calls Bidirectional (the sense current can flow either direction as the battery is charged or discharged). Since the PLC can only use positive currents, Linear's example shows using an Op-Amp, and at this point I tend toward just using the Op-Amp and level shifting the inputs as discussed above. Thanks for the input, and still open to suggestions, but getting near implementing. :)