Hello everyone, I work as an electrician but have limited knowledge of PLC circuits. Normally, I come across simple 2/3/4 wire terminations for transmitters on electrical blueprints, where the 4-20mA signal passes through a resistor before reaching the PLC analog input. However, recently I have encountered more complex input circuits that involve multiple resistors and capacitors. My initial thought was that it might be converting the 4-20mA signal to a voltage, but I'm unsure of the purpose of this conversion. I have included some photos of these circuit diagrams for reference. If anyone could guide me on relevant resources to further educate myself on this topic, I would greatly appreciate it. Thank you, Thomas.
Typically, 4-20mA sensors do not include resistors in their inputs. The exception is when a sensor's output voltage needs to be converted for a 4-20mA input, or vice versa. However, capacitors are not commonly found in these setups, unless they're being used to reduce noise interference. Please note that no images were included.
Pull-down resistors are essential in driving LED indicators due to potential mismatches in current requirements with PLC outputs. While capacitors are also commonly used in electronic circuits, their specific applications in this context would be worth exploring.
Utilizing a precision resistor to run a 4-20mA or 0-20mA signal across input terminals can convert the signal into a voltage. For example, a 20mA current passing through a 2.5 ohm resistor will result in a 50mV output, while 20mA passing through a 3 ohm resistor will give a 60mV output. Similarly, a 20mA current passing through a 500 ohm resistor will result in a 10V output (0-20mA corresponds to 0-10V, while 4-20mA corresponds to 2-10V). Contrary to popular belief, analog inputs receiving mA signals are actually converting them into voltage signals internally via a resistor. Adding a capacitor to an analog signal may be uncommon, and could potentially alter the signal.
I anticipate that the images will display properly this time. Hopefully, this will provide a clearer understanding of the question at hand. Cheers!
The second image appears to show a low pass filter in action. Nowadays, many Programmable Logic Controllers (PLCs) come equipped with filtering features either integrated into the input module or through logic programming. Which type of PLCs are compatible with these filtering mechanisms?
Hi Thomas, the complexity you're encountering in these circuits could be for signal processing, such as filtering noise or scaling the signal. To convert a 4-20mA current signal to a usable voltage range, it's common to pass it through a resistive shunt, creating a small voltage drop that can be read by a low-voltage input on the PLC. Cascade resistors and capacitors might be used to provide protection against overvoltage, as well as filter out any frequency-related noise. If you want to understand this better, I'd suggest digging into some electronics textbooks or online resources, such as Khan Academy’s electrical engineering lessons or MIT's OpenCourseWare. They do a splendid job of breaking down these topics. Also, online forums like All About Circuits could be your go-to place for practical, on-field advice.
Hi Thomas, your assumption is pretty accurate. The purpose of converting 4-20mA to a voltage signal indeed depends on the system requirements. PLCs usually prefer voltage signals. The additional resistors and capacitors you're seeing in these more complicated circuits likely serve to filter out noise or suppress voltage spikes, which can help to improve the accuracy and reliability of your measurements. I recommend checking out some online courses or textbooks on electronics - there should be quite a few covering advanced topics like signal conditioning. Coursera and Khan Academy have some well-structured courses that you might find helpful for your continued learning!
Hey Thomas, glad you're eager to learn! Converting a 4-20mA signal to voltage in PLCs is indeed common - especially if the input module doesn't inherently accept current-based signals. The purpose can be to standardize the signal type across the system, since voltage signals are less prone to noise and can be transmitted over longer distances. The resistors and capacitors you're seeing might be apart of a RC (Resistor-Capacitor) circuit, often used for signal filtering or delaying to achieve more precise signal measurements. Look into resources that detail the operation of basic electronic circuits, RC circuits, and signal conversion in PLCs, that should give you a solid grounding in understanding these concepts. Keep inquiring and growing your knowledge - it's a great attitude to have!
Hi Thomas, you're on the right track! These circuits are used to convert the 4-20mA current signal to a voltage signal, usually because voltage signals are more immune to noise and interference, especially over long distances. Additionally, the components in these circuits are also doing the important function of signal conditioning - filtering, amplifying, and isolating the signals before they reach the PLC. Regarding resources, a book I'd recommend is "Programming of Computer Numerically Controlled Machines" by S.K Sinha. Also, some online courses on Coursera and edX can also be extremely helpful. You might especially find the "Introduction to Electronics" course on Coursera as a good starting point. Keep learning!
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Answer: Answer: The complexity in some PLC analog input circuits with multiple resistors and capacitors may be for tasks such as signal conditioning, voltage conversion, noise filtering, or impedance matching.
Answer: Answer: Converting a 4-20mA signal to voltage in PLC circuits can be beneficial for compatibility with voltage-based input modules or for interfacing with devices that require a voltage signal.
Answer: Answer: To further educate yourself on complex PLC analog input circuits, you can refer to technical manuals, online resources, training courses, or seek guidance from experienced professionals in the field.
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