What are the Functions of the Resistors and Diodes in this Circuit?

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Discussion Overview

The discussion revolves around understanding the functions of resistors and diodes in a specific circuit, particularly focusing on a bridge rectifier and a zener diode. Participants are exploring the roles of these components in the context of an AC supply and an optoisolator, with an emphasis on current limiting and voltage regulation.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant identifies the presence of a bridge rectifier and a zener diode but is uncertain about the function of the two resistors before the rectifier and another resistor after it.
  • Another participant suggests that the resistors may not limit current effectively due to power loss, proposing that using one resistor in series would be more efficient.
  • Some participants propose that the resistors could serve as a safety measure for current limiting, especially since the circuit interfaces directly with mains voltage.
  • There is a suggestion that the two resistors may function similarly to a line dropper circuit, reducing voltage and current to safe levels for the optoisolator.
  • One participant expresses uncertainty about the zener diode's role, questioning its ability to stabilize voltage across the load.
  • Another participant emphasizes the need to consider the entire circuit when determining the function of each component, including the optoisolator LEDs.
  • There is a discussion about the zener diode's behavior, with some participants noting that it allows current to flow once a certain voltage is reached, while others question how it interacts with the rest of the circuit.
  • Participants discuss the necessity of current limiting for LEDs in the optoisolator, raising questions about what components are responsible for this limitation.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the functions of the resistors and diodes, with no consensus reached on their exact roles. There are competing interpretations of the zener diode's function and the overall circuit design.

Contextual Notes

Some participants note that the circuit design may not be efficient and could be a low-cost alternative to transformer circuits. There are also mentions of potential missing elements in the circuit that could clarify the roles of certain components.

  • #31
Relaced by #34
 
Last edited:
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  • #32
Which one? the blue one? or the brown / blue one? or the brown green one?
 
  • #33
cjm181 said:
Which one? the blue one? or the brown / blue one? or the brown green one?
I think Nidum is suggesting that you sketch the voltage waveform that you expect to see across the optoisolator's LED. You haven't sketched it, yet.
 
  • #34
NascentOxygen said:
I think Nidum is suggesting that you sketch the voltage waveform that you expect to see across the optoisolator's LED. You haven't sketched it, yet.

@cjm181 :

I understood from your posts #28 and #30 that you had decided that the waveform shown in diagram C in #26 was the correct one .

I've removed my diagram . You have a go now and we'll see if we can all agree on what that waveform looks like .
 
  • #35
PS: Looking up ' Pulse Width Modulation control of LED's ' might give you some useful background information . Quite an interesting topic anyway .
 
  • #36
Hey up, ok i will have a look tonight at some waveforms and feed back. I did assume the graph C in post 26 was correct. I am guessing it is not.

I know a little about PWM control of LEDs. I have had a dabble at the arduino, and also fly rc helis where i believe the servo position is obtained by PWM.

Are you saying the zener produces a square wave in this setup?I understand it will output a steady voltage, and i know it will resist flow at a voltage less than its knee voltage. Let me draw some stuff out later tonight.

I didnt estimate the brightness of the led was an issue with a normal DC suppy. I know from athe arduino if you give it full DC, or 100% pwm, then the led is on full. (edit, almost 100% PWM, significantly more on duty time than off)

Also as the opto isolator is only reading high or low, there will be a threshold to an amount of light it will call as on, and an amount of light it calls off?, so as long as when we want a high output the light from the led exceeds this then we are good?

I will come back with some wave forms later tonight
 
  • #37
cjm181 said:
I did assume the graph C in post 26 was correct. I am guessing it is not.
In post #26, voltage waveform (c) is seen on the circuit in (a), provided the filter capacitor is not present. That is correct.
 
  • #38
so is that waveform in C present in our circuit. Going to have a look at this tonight and draw some waveforms
 
  • #39
cjm181 said:
so is that waveform in C present in our circuit
In post #27 I pointed to the similarity of the two circuits, so, yes, waveform (c) is found in your circuit.
 
  • #40
This is what i reckon! After D1 might look a little strange. The reason i have drawn a small 0 inbetween is because the zener does not allow anything thru until the voltage hits the knee voltage, or at least very little. This drop may be instantanious, or atleast very quick, so the zero sections may well be not to scale. The zener will allow the voltage to increase after the knee voltage is hit, as the voltage goes up from the rectifier. After this, the zener will only permit so much voltage through, in other words hold the voltage steady. This is when the curve goes to a flat line. Once the supply starts to drop, below the voltage output, the curve will come back and the voltage will drop.

My drawing is poor, but the output is almost a square wave.

IMG_6533.jpg
 
  • #41
It sounds good, though I'm not sure what you mean by "after D1". Your final sketch, is this a voltage across a component?

Most helpful would be the voltage waveform you'd be expecting to see across the optoisolator LED (with that LED between the zener and ground).

If you are to convince an examiner that you deserve high marks because you are able to show a good understanding of circuit operation, you would sketch your waveforms either superimposed or at least accurately one underneath the other.

Read again helpful hint in my post #25.
 
  • #42
Hey

Yes 100% agree with the graphs. I quickly scribbled them down. I understand exactly what you mean with that.

Not sure what u mean by "If component values are well chosen, it should do so---though by "squaring off" you may be thinking of something quite different from what I mean!"

Right, so if we take different values of Vz, say 5v and 50v as you said. With the larger 50v example, i would expect the gradients either side of the flat at the top to be longer. But the flat at the top would be shorter. So i would guess that this would mean the led would not be as bright as a 5v one, as the pwm signal would be longer, and have less 'duty' time on.

Kr
Craig
 
  • #43
cjm181 said:
ok, so to summarise there are 4 things,

1. what's limiting the current going through the circuits for the led and D2
2. Whats the zener doing?
3. what is r1 doing
4. what's d2 doing.

1) The are no current limiting devices for the LED in this circuit. The opto-coupler would have a maximum input current rating, which if necessary would require a resistor, but depending on your circuit may not always require one. The LED is contained within the coupler, its not an LED you can see, and this activates the phototransistor. So the two circuits are only connected by light and nothing else. Helps prevent damage/ground loops/interference etc.

2) There is no zener. Google avalanche diode.

3) R1 limits the current flowing through D1. Google avalanche diodes.

4) D2 prevents the circuit from momentarily going to a negative voltage as the AC polarity shifts.

p.s. I think the reason for two resistors on the inputs to the bridge rectifier are to ensure that the current is the same entering both sides of the rectifier - however I'm not 100% sure on this one.

I found these websites very useful

http://www.electronics-tutorials.ws/diode/diode_6.html
http://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/rectifier-circuits/
 
  • #44
cps.13 said:
2) There is no zener. Google avalanche diode.
These days no one draws a distinction between the Zener and the avalanche diode, in general low voltage devices.
 

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