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Firstly I am sorry for this bad drawing but I just got it from the internet.

Secondly I'll give a brief idea about the project it is a static switch would be used to drive general loads (inductive and resistive mostly) unfortunately I have no idea about the loads types and values but mostly will draw 10 A maximum 220 Vac such as heating elements or motors.

Third I will explain the changes that I made:

I changed R1 from 220 Ω to 10 Ω because when I first connect the circuit to live power (without connecting the optotriac to 5 Vdc (Moc 3061 ZVC I am using)) the SCR gets triggered for a fraction of second and gives a pulse of 311 Vac peak (220 Vac RMS)when I changed it to 10 Ω surprisingly it became much more stable and done that for R3 too.

R2 got changed from 500 Ω to 100 Ω because when I changed R1 and R3 to 10 Ω R2 got burned so I lowered it to 100 Ω.

I added a snubber circuit too (R4 and C1)

Now the questions are:

  • Is this circuit considered to be sufficient for general using in the Market?

  • what is the use of these two diodes?

  • what is the general snubber circuit is used in Market (for general purpose I need it because I know it should be calculated for specific loads)?

  • Is it possible to just use a TVS diode instead of a snubber?

Note: I had tested it and it worked fine (I tested it for short duration).

If there is any information that you need please consider to ask in the comments.

Edit: Link of the circuit: Original picture

enter image description here

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    \$\begingroup\$ Sure, but why not use a triac? Also you reversed the SCR's anode and cathode labelling. \$\endgroup\$
    – 15 Volts
    Commented Oct 28, 2023 at 4:51
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    \$\begingroup\$ Did you add the snubber before or after changing the resistors? The resistors values are not permitted: check the maximum ratings of the MOC. \$\endgroup\$ Commented Oct 28, 2023 at 5:53
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    \$\begingroup\$ @OskarSkog Presumably an SCR fires, shorting out said voltage drop; evidently this didn't happen, after changing R1/R3 but before changing R2(!). (You've also assumed no load resistance, which isn't a great assumption, but, operation into a short-circuit load might indeed be a good consideration; one would hope for adequate fusing, for example.) \$\endgroup\$ Commented Oct 28, 2023 at 7:58
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    \$\begingroup\$ TRIACs drop little more voltage than SCRs do. Single units in common packages (e.g. TO-218, TO-247) are perfectly usable up to 30A. You need as much total heatsinking with a pair of SCRs, but twice the power components, and probably insulators (heatsink hardware in general). You can also consider a ready-made AC type SSR, which is the same circuit internally, and has a wide heatsink base to bolt down. \$\endgroup\$ Commented Oct 28, 2023 at 16:46
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    \$\begingroup\$ FYI to all - the original source of the schematic, with a little more background, seems to be here (and then adapted & copied to other places, including the Edaboard post mentioned by the OP). \$\endgroup\$
    – SamGibson
    Commented Oct 28, 2023 at 16:51

1 Answer 1

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I will attempt to answer your questions here. Firstly, some background context collated from the "comments" section (thanks to Sam Gibson, Tim Williams, & Russell McMahon):-

Source of the circuit:

  1. As provided by the OP:
    https://www.edaboard.com/threads/back-to-back-scr-driver-circuit.401988/

  2. Original source: Refer Fig 4 of this link, image copied below:
    https://www.bristolwatch.com/ele3/1d.htm

enter image description here

Responding to your questions:

Is this circuit considered to be sufficient for general using in the Market?

No. There are many things missing, just for starters: over-load and short-circuit protection (eg: fuses), over-voltage protection, EMC filters, safety eg: electrical isolation.

what is the use of these two diodes?

Referring to D1 & D2:
(a) They prevent reverse voltage being applied to the G-K terminals of each SCR.
(b) They ensure that the gate trigger current is determined by R2 & the load, and is not affected by R1 & R3.

what is the general snubber circuit is used in Market (for general purpose I need it because I know it should be calculated for specific loads)?

As you indicate, the exact snubber depends on the intended application, the type of load, and the details of the SCRs used. For example, if the mode of operation is to trigger the SCRs only when the applied 220VAC voltage is zero (zero-crossing) then then snubber required will have a lower rating than for random-trigger point-on-wave triggering (where the trigger can occur at any point on the AC cycle). Futhermore, if you intend to use this circuit for dimming lamps (where the trigger may occur at any point on the 50Hz waveform, and is repeated for each AC cycle), then this means the snubber will be working at 100Hz or 120Hz (once in the positive half, and once in the negative half) and this will cause the highest power loss within the snubber (and the SCR). It will also generate the most EMI - which needs to be carefully filtered, the snubber is a part of that EMI filter.

Loads can be resitive, inductive, or capacitive; a purely resistive load will probably have the lowest-rated snubber, whereas a capacitive load will require a different type of snubber compared to an inductive load.

Snubbers are used to:
(a) protect the SCR from over-voltage, & over-current (particularly at turn-on into capacitive loads);
(b) prevent false turn-on (usually caused by high dv/dt across A-K), and
(c) limit power dissipation within the SCR for non-resistive loads.

Please note that different types of SCRs have different values and limits for all these parameters, and these parameters vary with temperature, so the snubber must be designed for the specific SCR being used (and the manufacturing tolerances to be expected), and the operating conditions - ambient temperature, and internal temperature rise.

Is it possible to just use a TVS diode instead of a snubber?

Possibly, but it depends - see the answer above.

Snubber design:
These links may be a good starting point:

Snubber design for switching an inductive mains load with a relay?

How to design an RC snubber for a solenoid relay driving an inductive load??

Designing an RC Snubber for TRIAC motor control?

Update 30-Oct-2023:
Responding to this part of the OP:

Third I will explain the changes that I made:

I changed R1 from 220 Ω to 10 Ω because when I first connect the circuit to live power (without connecting the optotriac to 5 Vdc (Moc 3061 ZVC I am using)) the SCR gets triggered for a fraction of second and gives a pulse of 311 Vac peak (220 Vac RMS) when I changed it to 10 Ω surprisingly it became much more stable and done that for R3 too.

R2 got changed from 500 Ω to 100 Ω because when I changed R1 and R3 to 10 Ω R2 got burned so I lowered it to 100 Ω.

I added a snubber circuit too (R4 and C1)

What you experienced here is known as "the first pulse effect": how the circuit behaves when power is first applied. This behaviour becomes more complex as the number of power sources increases; in this case there are two: (1) the AC supply, and (2) the voltage at the LED side of the opto-coupler. The RC snubber you added has a large effect on this behaviour, and you may find that this snubber has corrected the false triggering that occurred on the first pulse with the resistors R1, R2, & R3 set to their original values.

I would suggest changing resistors to higher values; 10Ω & 100Ω seem too low to me, and may result in excessive gate currents. Check the SCR data sheet. Don't forget also that the optocoupler output stage has to carry this current.

I would also suggest that you take into consideration the "first pulse effect" for the opto-coupler, and consider using resistors to ensure the LED is kept off during the start-up phase; in particular, you must ensure that currents caused by capacitive coupling between nodes with high dv/dt cannot cause unintended turn-on.

Update 02-Nov-2023:
Responding to part of the OP's comments under the OP:

@FabioBarone I have a question mate if you may, now I don't understand why did you suggest that we increase the resistor values of 10 and 100 ohms, I mean are not they safe or would they have low lifespan? and about the Opto-coupler the LED side is already turned off when I connect the circuit to AC power. – Hazardous Voltage 9 hours ago

My response: Because the current flowing in the opto-TRIAC may be too high. May I suggest looking at the datasheet and application notes. For example, assuming the opto triac you are using is the MOC3061TVM, and its datasheet is here:
https://www.onsemi.com/pdf/datasheet/moc3163m-d.pdf

Page 6 shows a typical application for firing two back-to-back SCRs (image below):
enter image description here

This suggests making R1=R3 (using the component IDs of your diagram, not those of the diagram above) = 330ohm (or not installed at all, these are "optional"), and R2=360 ohm, and note that this is for a 115V AC supply, a 220VAC supply would need higher values again to keep the current to the same values.

Suggest consider the two cases:

  1. When the opto LED is OFF (no current). Look at the voltages present on all the components of your circuit.
  2. When the opto LED is ON (conducting forward current). Look at the currents that are flowing in all the components of your circuit.

The other cases to consider are:
3. The transition between case 1 (OFF state) and case 2 (ON state).
4. The transition between case 2 (ON state, when the main SCRs are conducting load current) and case 1 (OFF state).

For cases 3 & 4 you need to consider in small time steps how the circuit behaves as it makes these transitions. This is when the snubber, and how it interacts with the load, becomes very important. A simulation program, such as LTspice, will be very useful for that.

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  • \$\begingroup\$ unfortunately mate, when I used the snubber with a higher value resistors( 330 ohm instead of 10 ohm) the capacitor of the snubber used to charge and discharge in the circuit causing more pulses so for that I needed to decrees the resistors and it worked and I don't know if it would be safe or not \$\endgroup\$ Commented Oct 30, 2023 at 14:16
  • \$\begingroup\$ Fun fact: you are right when I lowered the resistor value and drew excessive load from the triac wheres the temperature raised more than 140 degrees, then I turned off the circuit and turned it on again the optotriac blew out with a quite big spark however I don't know if the resistors were the main reason for such behavior. \$\endgroup\$ Commented Oct 30, 2023 at 14:18
  • \$\begingroup\$ If the opto-TRIAC you are using is the zero-voltage-switching type (the output turns on only at the zero-crossings of the mains voltage, then beware: driving an inductive load with this can cause problems. Suggest drawing up the expected load current waveforms for a purely inductive load to see what I mean. Cheers. \$\endgroup\$ Commented Nov 1, 2023 at 21:46

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