enter image description hereI have used a TRIAC and an MOC3021 opto-coupler connected to a PIC16F887A to simulate a a full wave rectifier. I am now to design the same using an SCR h-Bridge. My question is, in order to trigger the SCRs, do i need 4 separate triggers for each SCR? or could i use two trigger pulses, one for each pair of the SCRs? I would appreciate any advice. am fairly new to the field. Thanks. The following is the layout of the circuit. Specs:220VAC, 60 Hz. Load 1Kohms. These were provided by my senior supervisor.

  • \$\begingroup\$ not what you asked, but you probably want a resistor in series with the optocoupler diode \$\endgroup\$ Jan 5, 2016 at 6:52
  • \$\begingroup\$ Thank you sir, though not what i asked for...i could use any info i get. id rather be a fool for a minute than forever so i must ask, what effect would such resistor have on the Zero cross detecting? \$\endgroup\$
    – Noble
    Jan 5, 2016 at 7:45
  • \$\begingroup\$ it would stop the first pulse of AC from destroying the optocpoupler LED the resistor should be chosen to limit the current through the led to a safe value. it will not have measurable effect on the phase of the zero cross signal, but higher resistances decrease the amplitiude of the signal thus making it narrower. \$\endgroup\$ Jan 5, 2016 at 7:57

2 Answers 2


Answer for How to Trigger the SCRs.

You have to trigger U6 and U5 simultaneously. This causes current flow in the direction of u6-load-u5-neutral (return path) during Positive half cycle of input.

During Negative half cycle of AC input SCR U6 and U5 turned-off. Due to Natural Commutation.

And then you have to trigger U3 and U7 simultaneously. This time current take path U7-load-u3.

You can control the output Voltage by adjusting the Firing angle of SCRs

And also UJT Relaxation Oscillator with Pulse Transformer is good choice for firing this SCRs

  • \$\begingroup\$ thanks for replying....i had understood the basic idea but implementing it proved a little too complex...am using a PIC to generate the trigger pulses through an optocoupler to the SCR gate. now m supervisor demands that i figure out whether to use four pulses from the PIC to trigger them or two pulses on for each pair of the SCRs. am still stuck so any suggestions will be welcomed... \$\endgroup\$
    – Noble
    Jan 6, 2016 at 10:18
  • \$\begingroup\$ You have to use two pulses from PIC. Each pulse will trigger the each pair(u6u5, u3u7). No need of four Pulses. If you prefer four pulses mean connection may become complex. \$\endgroup\$
    – Photon001
    Jan 6, 2016 at 10:24
  • \$\begingroup\$ yea i preferred two pulses but then some common cathode issue came up on some articles i was reading...though i did get the wave in simulation, i thought perhaps in real implementation it wont work...so will it work? from the PIC to optocoupler to both the SCRs? \$\endgroup\$
    – Noble
    Jan 6, 2016 at 10:35
  • \$\begingroup\$ Try it will work. If it is work mean you have done!!, otherwise you will get something which may unexpected; from that you can learn still more. \$\endgroup\$
    – Photon001
    Jan 6, 2016 at 10:50
  • \$\begingroup\$ haha...sure i will try, the easier the better...if it blows up, i hope they don't cancel my internship... \$\endgroup\$
    – Noble
    Jan 6, 2016 at 10:54

it looks to me that U6 and U7 could share a common drive (via a pair of resistors to split the current.

U3 and U5 will be harder to drive and will probably need individual drive - could you replace them with ordinary diodes instead?

  • \$\begingroup\$ thanks for helping...i appreciate it with kindest regards.... well it would be easy if i could but the instructions i am given require SCRs. so i cant use a diode there...also U6 and U7 will need to be triggered at separate timings...right? how would a common drive work? i mean if m to trigger U6 at 1ms, and U7 say at 5...a common drive will drive them togather...or m i missing something? \$\endgroup\$
    – Noble
    Jan 5, 2016 at 7:47
  • \$\begingroup\$ the instantaneous polarity of the AC will determine which one turns on when the pulse is sent, \$\endgroup\$ Jan 5, 2016 at 7:59
  • \$\begingroup\$ ok great i will try implementing it...thank you again...will update here... \$\endgroup\$
    – Noble
    Jan 5, 2016 at 8:09
  • \$\begingroup\$ If U6 and U7 shared a common drive you'll have a fire. \$\endgroup\$
    – Andy aka
    Jan 5, 2016 at 8:50
  • \$\begingroup\$ @Andyaka no you won't (assuming a CORRECT SCR driver... isolated, pulse XFMR etc...). U6 & U7 are the two upper SCR's so they share a COMMON Cathode. \$\endgroup\$
    – user16222
    Jan 5, 2016 at 10:39

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