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Is it good to use simple 1n4001 diodes as fuses assembled between the TRIAC and the coupler or SCR?

Discussion in 'Robotics & Mechatronics' started by Yongbee, Jan 8, 2018.

  1. Yongbee

    Yongbee New Member

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    First of all, I'm a 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. 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, (also here is an article about electronmechanical technology for reference: http://www.apogeeweb.net/article/33.html )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?
     
  2. JimB

    JimB Super Moderator Most Helpful Member

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    No, it is a really bad idea.

    I have seen more 1N400x diodes fail short circuit that open circuit.

    When they do fail S/C it is often an explosive event with bits of diode flying all over the place.

    If you need a fuse, use a fuse.

    JimB
     
    • Agree Agree x 2
  3. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Agree! Use a fuse. You know how they will open. How fast/slow. They will not smoke or damage the board. They will open not short.
    You could use a resistor because they blow open. Metal film resistors open nicely. Carbon Comp resistors open very slow and "burn". Don't use them as a fuse. Even a "fuse resistor" is a guessing game unless you have done it before.
     
  4. dave

    Dave New Member

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  5. dknguyen

    dknguyen Well-Known Member

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    As a general rule, semiconductors will probably fail short rather than fail open. I once had a semi-conductor act (unintentionally) as a fuse by blowing the terminals off the device, but the main body of the device still measured as a failed as a short.
     
    Last edited: Jan 9, 2018
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I agree, it's a REALLY bad idea - where did you get such a silly idea from?. A diode isn't a fuse, and doesn't behave like one.
     

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