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Newbie here. please be easy on me. I have a spotwelder project where i need to sense when the secondary is shorted. The working of the pcb is as follows,

my circuit detects the zero cross on the AC sine wave and triggers two Thyristors in a back-to-back configuration which switches on a MOT(Microwave Oven Transformer) at the peak of the sine wave, which is the typical way to switch on inductive loads to avoid inrush currents. I also have two taps on the secondary to measure the voltage during a weld to calculate the Weld current. All these are initiated at the press of a button which am hoping to automate when the electrodes touch a conductive material like here, nickel strip. I've checked the OFF-State voltage at the transformer and it read 1.05VAC and when the secondary was shorted it was 0.5VAC.

So with the best of my knowledge i think using op-amp as a comparator would be the ideal solution.

I could add the op-amp input parallel to section of the primary (like a shunt resistor) and monitor the voltage variation and use the output of the op-amp to trigger the thyristor through a arduino. I am not sure how to add the input voltage to the thyristor.

Would appreciate if somebody could help me here.enter image description here

George
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  • that configuration doesn't measure anything, since the inputs are identical, 2) it is a gross safety hazard since it is connected to 240, 3) you want to switch on zero-crossing, rather than peak, and 4) why do you want to use an op amp as a comparator, rather than using a comparator as a comparator?
    • – WhatRoughBeast Dec 11 '17 at 22:09
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    Depends, where do you reference your secondary ground compared to the incoming phase and neutral? If I where you, I would try to sense it on the secondary or wind one more tap on the primary to get a low voltage, sense with that as Vcc and then optocouple that signal to the secondary. – winny Dec 11 '17 at 22:12
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    Why not use a current transformer and actually sense the current value? Trying to put a series resistor in such a high current application seems pointless. – Jack Creasey Dec 11 '17 at 22:18
  • Thanks for the response everyone. That is absolutely not the way to do i was thinking of going this way Circuit Image – George Dec 11 '17 at 22:54
  • I am using the primary because connection to the transformer goes to it from the PCB itself else i would have to connect a wire from the secondary. (trying to keep, things neat). The spot welding transformer is floating. The electrodes are on the Secondary and that is how it is used. – George Dec 11 '17 at 22:59
  • This is another design i had come up with.

    So normally with a DMM we probe the HOT and NEUTRAL to check the voltage. Here we just prone one terminal on the AC power which is connected to the inverting pin of the opamp via 1M resistor, so my question is, shouldn't there be another for the NEUTRAL side also to be connected from the load ? or since the Opamp is connected to GND it would use it and the neutral for the AC is not required.

    Would really appreciate if somebody could advise.

     – George Dec 16 '17 at 16:58