# Capacitor commutation of thyristor and DC load

In my school notes I have been given a basic thyristor circuit. I would like to breadboard a similar circuit and have some questions about choosing a suitable capacitor type and value between points P and Q.

simulate this circuit – Schematic created using CircuitLab

Assuming the thyristor is latched on and S2 is open, the voltage at P will be close to zero and at Q should equal Vs.

When S2 is closed, I am told to expect Q to immediately drop to 0V.

To quote my notes: "However, there has been no time for charge to move. As a result, Q must still be 12 V higher than P. In other words, when the voltage at Q dropped by 12 V from 12 V to 0 V, the capacitor forced the voltage at P down by the same amount, from 0 V to -12 V."

The thyristor is thus reverse biased and switches off; the voltage at P therefore returns to Vs, but if S2 has not been released Q remains at 0 V. When switch S2 is released, both P and Q will be at Vs.

Bearing in mind the reversal of charge on the capacitor, could you suggest a suitable capacitor type to breadboard this circuit? I assume a polarised capacitor (e.g. electrolytic) would NOT be suitable. Also, how would I calculate a suitable capacitor value? I presume that R2 must be at least 1K to minimise current flow when S2 is pressed. I have not been given a formula to use.

I intend to use a 12 V supply and thyristor C106DG.

ON Semiconductor's Thyristor Theory and Design Considerations Handbook should give you some interesting bedtime reading.

Section 5, page 60, starts:

SCR TURN−OFF TIME $t_q$

Once the anode current in the SCR ceases, a period of time must elapse before the SCR can again block a forward voltage. This period is the SCR’s turn−off time, tq, and is dependent on temperature, forward current, and other parameters.

The information you require is, I think, on page 128.

$$C_c = \frac {T_q I_A}{V_c}$$ Where:

• $C_c$ = value of necessary commutating capacitance
• $T_q$ = turn-off time of the SCR
• $I_A$ = value of anode current before commutation
• $V_c$ = voltage of Cc before commutation

This relationship shows that to reduce the size of $C_c$, the capacitor should be charged to as high a voltage as possible and the SCR should be selected with as low a turn-off time as possible.

The article goes on to give calculations for a motor control application.

I am unable to find any $T_q$ specifications for your thyristor.

• Thyristors that are intended for forced commutation are specifically designed with short Tq. Manufactured lots are tested to determine the actual Tq and sorted into batches of known Tq range for sale as "fast turn-off" thyristors. It may be difficult to find a general purpose thyristor with published Tq.
– user80875
Mar 17, 2018 at 14:26
• ON Semiconductor's Thyristor Theory and Design definitely looks a helpful albeit very detailed read. @Transistor has given me a good start with those page references to read. I still need to find Tq (or an estimated value), but this reference states " This parameter is usually in the range of few microseconds to hundreds of microseconds" which will probably be good enough for my needs. stmicroelectronics.com.cn/content/ccc/resource/technical/…
– Pzy
Mar 17, 2018 at 20:51
• I found NTE 5424, 400 V, 5 A, TO220 case fast turn-off SCR with Tq = 4.4 microsec. Smallest available Tq increases as device voltage and current ratings increase. That may help with estimation.
– user80875
Mar 17, 2018 at 21:40