power electronicsscr
Can anyone tell me why are Thyristors(SCRs) used in power electronic circuits? How are they more adept at handling power as opposed to other active or passive electronic components?
UPDATE:
As per the request made by jippie i am uploading the circuit diagram for a simple full wave rectifier to add a context.
(source: circuitstoday.com)
why should some one use SCRs in this circuit when the same action can be achieved using diodes?
Why are the wires of thyristors always twisted in pairs?
For the same reason that any twisted-pair cable is used: to reduce interference.
Thyristors such as that you have photographed are used in high-current circuits. The thyristor gates are relatively sensitive and an untwisted pair of wires running off to the control circuit will create a loop which will be susceptible to picking up induced currents which may trigger the thyristor.
By twisting the pairs a series of small loops are made and each one is in opposite orientation to the one before it. The result is that the induced interference is, to a very significant extent, cancelled out.
It appears that the device in your photo has four terminals: the stud and the heavy red lead for the high current circuit and the twisted pair for the control circuit. The reds are connected internally to eliminate problems trying to attach the control wire to the bolt on the large ring terminal.
Ok assuming your drawing is of the mosfet drain voltage, are you sure that waveform is correct? You claim the waveform is the same as this one from the question you linked, but that's not the same as your drawing.
A spike right after the mosfet shuts off is expected. That's the "boost" voltage. The signal you've drawn... I don't know what that would be. Is it maybe the gate voltage and not the drain? idk.
I'm going to guess that your waveform does look like the one in this picture.
I think what's happening is your mosfet is not being driven with enough gate voltage, either that or the switching time is too slow. A dead giveaway is that the mosfet is getting hot. You should probe the gate and update the question with your results. I think you're going to want a different mosfet with a lower Vgs(on).
You should also post the complete circuit including the optoisolator, because there could be a problem there. For example, you might be able to lower the 24 ohm gate resistor, but I can't say because I don't know the part number of the optoisolator.
Best Answer
Using Silicon Controlled Rectifiers it becomes very easy to control the output voltage by changing the phase angle for firing the SCR's. With the phase angle the average voltage and power can be controlled without dissipating energy in series resistors. Before SCR's became common, huge series resistors were used to control output power.
Using diodes does not allow for controlling the output voltage.
Image found on Wikipedia