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1

You don't need the R1/R4 divider. Eliminate those resistors and choose a value for R3 that limits the LED current. It looks like setting R3 to 2.2kΩ would give you about 10mA maximum through the optocoupler's LED. I didn't look at the 4N25's datasheet...you might be able to go down to 5mA or so. Note that the logic low voltage into the microcontroller ...


1

The rectifier 'rectifies' ('puts right' or 'corrects') the negative halves of the AC cycle to make the voltage always positive, but doesn't make it smooth. It still goes down to zero at the end of each half cycle. Here's the result of an LTspice simulation of your circuit:- The blue trace is the 220V AC mains (divided by 20 to make the other traces more ...


3

... does using optocoupler have any advantages rather to simple RC circuit? Yes, several. It's not obvious from the schematic posted but the circuit GND has to be connected to mains neutral. This then means that you have to consider the whole circuit live. That means there is a risk of electric shock and that you can't, for example, connect to a programmer ...


2

I select STTH20R04G-TR diode because they can handle more than 16 Arms This is an 'ultrafast recovery' diode which has a 'softer' V/A curve than most rectifier diodes. This will waste more power at high current, but still drop about the same voltage as other diodes at low current. The low current behavior is important because the TRIAC holding current could ...


0

I have placed a 0.47uF/450v cap across the 330R (cold side) resistor. Tested by repeatedly applying 240vac while heating (50 degrees C - the max temp my wife's hairdryer will go!) the triac. No miss-firing that I could detect.


1

This is just a pulse speed test circuit to minimize measurement errors with a common ground and simple DC positive pulse input. Other - - Although intended for line frequency it's max. toggle rate of 4kHz is not speedy by design in order optimize transistor parameters for high current gain yet low bandwidth. Yet with all the optical coupling loss factors ...


2

Step currents being full spectrum not only cause a broadband radiated EMI as well as conducted EMI and DC ground shift since that ground is not isolated. Thus the loop current radiates noise and the conductor path shifts by V=IR+L*dI/dt on the shared return ground path. Opto isolation will NOT reduce this EMI if you neglect the radiated noise on cables ...


1

IIRC, the reason that this particular board uses optos is to separate the logic and relay grounds. It is to prevent ground bounce issues when switching the relays (as the inductive relay coil can cause quite the spike on both power and ground nets, even with the freewheeling diodes across the coil). In addition, they translate the common 3.3V logic to the 5V ...


3

I think it's redundant. That said, I have heard from someone of experience on SE that although relays provide galvanic isolation, they can't be relied upon to prevent arcs from jumping from the primary contacts to the coil. I've run into more than one newbies online where they are using a relay to switch a large motor (always an AC motor if I remember ...


1

I think the issue is that M1 and M3 are N-channel MOSFETs in source follower configuration. When their Vgs (voltage potential difference from gate to source) is 0V they will attempt to have 0V at the source, which undermines the whole concept since you have the low side FET and high side FET driven off opposite phase pulses. When the high side FET has a Vgs ...


1

The voltage that controls a MOSFET is NOT the voltage at the gate. It is the voltage difference between the gate and source pins of the MOSFET. The voltage you are applying to the MOSFET gates are all relative to ground, but the MOSFET does not care what the voltage relative to ground is, and in fact cannot know what the potential at ground is if none of ...


1

I don't see much of a point for D3. R1 also seems really high. You actually need to calculate the current required by the opto LED and size the resistor accordingly to the voltage being applied. You don't need R3 because Q2 is not a MOSFET which stays on as long as there is a voltage on the gate-source capacitance (which would not be discharged if the opto ...


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