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Keeeen
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Hello not sure if I've posted this in the right section. But could someone tell me the purpose of the zener diode and the diode in parallel with the resistor? Never seen this configuration before. Thanks!
.Scott said:The diode parallel to the coupling is to protect the coupling from reverse voltages.
The Zener and the resistor closest to it are to filter the input - specifically to keep transient spikes from tripping the coupler.
That type of filtering is needed when you are using a switch with mechanical contacts.
GrahamN-UK said:What is the coupling device?
The obvious choice would be an opto-isolator. However, if it is a relay then the parallel diode has the correct polarity to dump the back-emf and protect the zener diode.
Keeeen said:Yeah the coupling I believe will be an opto-isolator
Good Observation !AlexCaledin said:It looks like the Zener diode simply subtracts some voltage from the rectified voltage (or, which is same, increases the threshold), in order to make the "zero" time longer so the signal be better formed.
davenn said:no, because it isn't going to supply voltage as the image states
Averagesupernova said:You are all missing a key point. And that point is: Avalanche diode.
This is not an unusual electrical circuit for situations where PLC's are used. We are in a "process control" environment, most likely some sort of industrial setting - chemical process, manufacturing, machinery control. It is likely that only 230vac is ubiquitous outside of the electrically enclosed PLC. Not only may the button be tens or hundreds of meters from the PLC, but low voltage wiring may not be allowed in the machinery area. And ever if it was allowed, special shielded twisted pairs might be required to protect a low-voltage, relatively high-impedance line from picking up stray currents from light ballasts and other inductive sources.GrahamN-UK said:The circuit is rather odd. If all that was required was to check the status of the push-button switch it could be wired directly to the PLC and not bother with the dangerous directly-connected mains circuitry. The circuit could check the frequency, phase or presence of the mains but if you needed that, why would you only do it under push-button control? Perhaps the push button is remote from the PLC hence the isolation is useful?
Also, if I had a reason to work from the mains I'd look first to use a (mains class X, Y) dropper capacitor rather than the 3 resistors. Perhaps the presence of the anti-parallel diode implies that the designer started this way but changed their mind without removing the anti-parallel diode (which doesn't seem to be needed given the bridge rectifier and an opto-isolator).
.Scott said:This is not an unusual electrical circuit for situations where PLC's are used. We are in a "process control" environment, most likely some sort of industrial setting - chemical process, manufacturing, machinery control. It is likely that only 230vac is ubiquitous outside of the electrically enclosed PLC. Not only may the button be tens or hundreds of meters from the PLC, but low voltage wiring may not be allowed in the machinery area. And ever if it was allowed, special shielded twisted pairs might be required to protect a low-voltage, relatively high-impedance line from picking up stray currents from light ballasts and other inductive sources.
.Scott said:As far as dropper capacitors are concerned, the objective is for something very hardy that can be used in many situations throughout the industrial setting. The objective is not electrical efficiency, it's something that won't fail and won't confuse anyone. These systems are maintained by electricians, not EE's. Think "Ladder Logic", not circuit analysis.
On The Other Hand - with capacitive current limiting, should switch close at instant of peak voltage there's a large transient current .GrahamN-UK said:Resistors dissipate heat, which isn't good for reliability.
.Scott said:This is not an unusual electrical circuit for situations where PLC's are used. We are in a "process control" environment, most likely some sort of industrial setting - chemical process, manufacturing, machinery control. It is likely that only 230vac is ubiquitous outside of the electrically enclosed PLC.
yes indeed.Asymptotic said:It's been in the back of my mind since first reading this thread, but wouldn't a 230 VAC interposing relay be simpler?
sophiecentaur said:No one (not even the OP) seems to know that the "coupling" is or what it does. Can we really carry on a discussion about this without knowing what the black box does?
The values of the resistors is pretty relevant (Is R the same for all three? )to what actually goes on. That Zener could be any value, depending on what voltage drop there is across the Rs.
jim hardy said:yes indeed.
The rectified AC could be a timing signal, the full wave bridge sans filter gives 120 hz.
My microwave oven uses something similar - half wave rectified(gives 60hz) from another diode in the power supply advances the clock.
The zener could add the intelligence " there's enough AC present to start (something?) "
"What Evil lurks in the minds of programmers ? The Shadow knows..."
Yep - we're all guessing.
old jim
When relays are used, there is usually additional filtering placed between the relay and the PLC. And this is always the case when the relay is part of a PLC manufacturer supplied interface board.Asymptotic said:It's been in the back of my mind since first reading this thread, but wouldn't a 230 VAC interposing relay be simpler?
Diodes are used to control the direction of electric current flow. They allow current to flow in one direction while blocking it in the opposite direction. This makes them useful for converting alternating current (AC) to direct current (DC) in electronic devices such as power supplies and rectifiers.
The stripe on a diode indicates the direction of current flow. It is typically placed on the cathode side of the diode, which is the side that blocks current flow in the opposite direction.
Diodes are important because they allow us to control the flow of electricity in electronic circuits. They can protect components from reverse voltage, convert AC to DC, and are used in a variety of applications such as voltage regulation and signal processing.
If a diode is connected backwards, it will not allow current to flow. This is because the diode is designed to only conduct current in one direction, and when connected backwards, it is in the opposite direction and therefore blocks current flow.
Yes, diodes can be used as switches in electronic circuits. By applying a small current to the diode, it can turn on and allow a larger current to flow through the circuit. This makes them useful for controlling the flow of electricity in various applications.