[SOLVED] Do thyristors work as they supposed to?

[stevenmahoney]

I am a bit confused about a bridge with 2 controlling thyristors in it (see attached).

Both thyristors receive controlling impulses (yellow graph). What I expected to happen is that when first impulse comes, thyristor suppose to open till the voltage potential goes through zero (fed AC signal is in pink).
Yet, it looks like when impulse triggers thyristor, it opens for a short period of time and closes (see blue resulting graph). Then several more impulses trigger it again, all within same half-cycle. Every time it closes right after.

Is that how it suppose to behave? I thought it suppose to stay open till voltage drops to 0.

Funny thing, by changing the number of impulses controlling circuit is able to adjust output voltage. So, it looks like thyristors work as they suppose to be, but I still have some doubts.

BTW, green graph is the current through the load.

Please help.

 

[pinout]

Thyristor should turn on with the gate pulse, (i.e closed or near short circuit) and remain in this state until the current drops to zero.
For resistive load this will also be the same for the voltage phase, but with reactive loads the voltage waveform can be some totally different point in the cycle when the thyristor switches off.

I couldn't relate this to your scope display, so there may be other factors....

 

[Tunelabguy]

Was a load connected when you made these measurements? A load would draw current to keep the thyristor on.

 

[chuckey]

Yes I was going to question that. Thyristors have a minimum HOLDING current, below that they just switch on and off with the gate pulse.
Frank

 

[stevenmahoney]

Thank you very much, guys! The load that I connected was about 1.3kOhm, but thyristors are quite powerful, capable of delivering over 15A of current. The current through the load was not enough to keep them open.

THANK YOU very much for fast reply!!!!!