Here is the circuit we used (BT169 thyristor):
And this is the output:
In theory it should be a straight line when AC source goes negative.
The question is "Why does it go into the negative area? Why is it not a straight line?"
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\$\begingroup\$ Dear op - I believe you have accepted the wrong answer - see my comment below what I believe to be the correct answer and note that thyristors are not normally noted for their ability to block reverse voltages and therefore your circuit is faulty. Some will but, as far as I am aware, the BT169 isn't capable of doing that very successfully. It's the breakdown of the gate causing the negative voltage in your waveform. \$\endgroup\$– Andy akaMar 18, 2015 at 20:10
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2 Answers
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From memory, the GK junction of an SCR, in reverse mode, acts as a Zener diode of 7V or so (much like the BE junction of a transistor). A current flows through RL, this GK zener diode, and R1, creating the voltage across RL observed. The zener thresold explains why there are two periods with almost no current during a cycle.
If that effect is undesirable, inserting a diode in series with R1 (cathode connected to G, anode to R1) should reduce it (but the SCR will turn on a little later).
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2\$\begingroup\$ This is the correct answer and I'll point that out to the OP - the peak verse gate voltage for a BT169 is 5 volts and one the input supply has gone negative by about 6 volts the gate will breakdown and conduct as you have said. \$\endgroup\$– Andy akaMar 18, 2015 at 20:08
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Any non-ideal diode has some reverse current. If you look at the datasheet of your part you might find some charts of the reverse voltage/current dependency. The reverse current is inducing the negative voltage drop on \$R_L\$, and this is what you are observing.
answered Mar 18, 2015 at 17:03