fast switch-off for transistor switch

[audioguru2]

Hi Trigger,
Of course your triac will eliminate the spark, because it eliminates the relay contacts. And the diac-triacs must be optically-isolated, so that the entire circuit isn't "live". Four diac-triacs will need to be connected in an H bridge, to reverse the motor.
But I think that a triac will create a problem where both the forward and reverse triacs will be conducting at the same time.
The logic signal disconnects its drive to the forward diac-triac, to turn it off, then activates a logic signal to the reverse diac-triac, to turn it on. But since the load is inductive, the current lags the voltage by 90 degrees, therefore the reversing diac-triac will turn-on before the inductor's current has reduced low enough to turn-off the forward triac. Don't forget that a triac is a "latch" that doesn't turn-off exactly when you want it to.

 

[trigger]

Hi Trigger,
Of course your triac will eliminate the spark, because it eliminates the relay contacts. And the diac-triacs must be optically-isolated, so that the entire circuit isn't "live". Four diac-triacs will need to be connected in an H bridge, to reverse the motor.
But I think that a triac will create a problem where both the forward and reverse triacs will be conducting at the same time.
The logic signal disconnects its drive to the forward diac-triac, to turn it off, then activates a logic signal to the reverse diac-triac, to turn it on. But since the load is inductive, the current lags the voltage by 90 degrees, therefore the reversing diac-triac will turn-on before the inductor's current has reduced low enough to turn-off the forward triac. Don't forget that a triac is a "latch" that doesn't turn-off exactly when you want it to.

Thanks audioguru, that takes my a lesson to learn
Actually, I know the problem as the circuits will gives me a blinking while the DC level is about the threshold level..... so finally I come up with a schemitt trigger circuit to have a voltage comparison, and give either a logic 1 or 0 to the diac. But for controlling the motor may be a big different from that of a light bulb..... so let see who will have a better solution... ;D