Assertion (A) : A number of thyristors operating in parallel cannot share a common heatsink. Reason (R) : For simultaneous firing of the thyristor opto-isolator may be employed in the gate driving circuit.
(A) is false, but (R) is true.
Paralleled thyristors are not merely able to share a heatsink — they are deliberately mounted on one, so the assertion is false while the reason is true. Option 4.
Why a common heatsink is the correct practice. Thyristors in parallel do not share current equally, because their forward characteristics differ slightly. The device with the lowest on-state voltage takes more current, dissipates more power and grows hotter. Now the crucial point: a thyristor's forward drop falls as it warms, so the hotter device takes even more current — a positive feedback loop that ends in one device carrying everything and failing.
Mounting all of them on a common heatsink couples them thermally, so they are held at nearly the same temperature and the runaway loop is broken before it can develop. Sharing the heatsink is the standard cure, not a prohibition.
| Measure | What it does |
|---|---|
| Common heatsink | Equalises junction temperatures |
| Series reactors or resistors | Force current sharing electrically |
| Matched devices from one batch | Minimises the initial imbalance |
| Simultaneous gate firing | Prevents one device taking the whole load first |
The reason is true, and is one of those measures. If the devices fire at different instants, the first to turn on drops to about 1 V and can rob the others of the anode voltage they need to fire at all — leaving one device carrying the entire current. Gate pulses must therefore arrive together, and since the paralleled cathodes may sit at different potentials, the drive circuits must be electrically isolated from one another and from the controller. An opto-isolator does exactly that: it transfers the pulse by light across an insulating barrier, giving several kilovolts of isolation with no common connection. Pulse transformers serve the same purpose.
Note that R is true but is not the explanation of A in any case — gate isolation concerns firing, not heat. The answer is determined by A being false, and R happens also to be a genuine and related fact.
Flagged only because the assertion could be read as concerning electrical isolation of the mounting tabs, which do differ in potential; but the standard practice of a shared, insulated-mounting heatsink makes the assertion false as written.
Hence, (A) is false, but (R) is true.
Similar Questions
In thyristor \(\dfrac{di}{dt}\) failure is prevented by
Match the following :
List – I List – II a. Snubber circuit i. SCR b. Inverter ii. High efficiency c. Phase control iii. \(\dfrac{dv}{dt}\) protection d. SMPS iv. UJT Codes :
An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. It has
A. one cycle surge current = 2121.32 A
B. One cycle surge current = 1121.32 A
C. I2t rating = 45000 Amp2
D. I2t rating = 1257358 Amp2
Choose the correct answer from the options given below:
Important Questions from Thyristor Protection
Match List-I with List-II and select the correct answer using the given lists:
List – I
(SCR rating)
List – II
(Protective element)
A. di/dt limit
1. Snubber
B. dV/dt limit
2. Heat sink
C. i2t limit
3. Series reactor
D. Junction temperature limit
4. Avoid runway speed on no load
Which of the following is the function of an R-C snubber circuit connected in parallel to an SCR?
For series connected SCRs, static equalization is obtained with the help of
The Snubber circuit is used to thyristor circuits for -
In thyristor \(\dfrac{di}{dt}\) failure is prevented by
