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First of all, I'm a simple mechatronics technician student first time posting here, so please don't mind if I say something that seems completely stupid, since I consider myself to be a total newbie at this. do feel free to correct me if you find anything wrong with my thought process.

With my limited knowlege of eletronics, and in an attempt to consolidate the knowlege aquired in the course I'm doing at the moment, I've been trying to learn by studying and developing a missing phase solid-state relay for a 3-phase engine (star-delta startup), as a viable replacement for the more usual mechanical relays. The objective is to be able to handle currents in the order of 30-50 amps and small enough so that it can be assembled on top of the engine's connector box, and able to only trigger if one of the phases goes off for 1 second at most, half a second minimal.

The engines where the relay should be placed are 3-phase 400V 50hz engines, capable of developing 5bhp+ of power.

The issue here is that I've been head deep on simulators (mostly proteus) and investigating components. after extensive trial and error, I've managed to come up with the following:

  • 3 quadrant TRIACs are mandatory, as to avoid snubbers, thus driving costs down
  • SCR or optocouplers as TRIAC drivers, any of these two options are fine
  • 3 input AND gate as trigger
  • On the command part as a whole, to aquire the signal I've been using a simple resistor (to drop voltage down) + retctifying bridge combo, where the capacitor acts as the "sensitivity" of the trigger. The signal is aquired after the TRIAC. FYI, the command portion is supposed to run at 5VDC at most and a consumption of about 300mA tops
  • there is a need of adding a small power cell to feed both the AND gate IC and to serve as a "starter".

With all this I've been spending a good part of of my time investigating components, searching datasheets and attempting to make it actually work, but after the realization that a 3-quad TRIAC is needed, I´ve hit a wall. The TRIAC needs the gate to be at negative (working at quadrant II and III), and for all that's holy, I simply can't make it work on the simulator, even if the logic of the circuit is correct, so yes I'm pretty much asking help here.

Small considerations:

Nearly all the components are overspecced for the job at hand, mainly to have enough redundancy so that it requires little to no maintenance except to change batteries. Also, been thinking about using simple 1n4001 diodes as fuses assembled between the TRIAC and the coupler or SCR, is that a good line of thought?

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  • \$\begingroup\$ A schematic of what you have so far would be helpful because there are things like "using simple 1n4001 diodes as fuses assembled between the TRIAC and the coupler or SCR, is that a good line of thought" which are not hitting home. It doesn't matter if you have come up with a bad idea electronically. \$\endgroup\$ – Andy aka Nov 14 '13 at 20:33
  • \$\begingroup\$ The idea showed up at first when I needed a cheap DIAC to serve as the TRIAC driver. But since I realized that I only need DC on the TRIAC gate for a snubber-less setup, the thought has started to get less and less space in my head. As for a schematic, once I get on the laptop where I'm keeping it, I'll post a proteus screenshot. \$\endgroup\$ – Grimpak Nov 15 '13 at 6:50
  • \$\begingroup\$ sorry about taking this long to answer. the first "prototype" I managed to come up with in the simulator was this: dropbox.com/s/p1zwpo84wi0dq3w/… it's before I arrived to the conclusion that the best way was to go 3quad. at the moment that design is pretty much thrashed. \$\endgroup\$ – Grimpak Nov 18 '13 at 19:56

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