Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

[SOLVED] Commutation in Power Electronics Devices (thyristor vs diode)

Not open for further replies.


Member level 3
Jul 18, 2015
Reaction score
Trophy points
Activity points
Can anybody differentiate between natural or line and forced commutation with reference to power electronic devices such as diodes and thyristors.....

If you have a DC supply for a motor being controlled by a SCR or thyristor then switching the device off would have no effect until the current through it falls to a low value (Ihold). This would occur naturally if the DC supply was unfiltered since the voltage would fall to zero between every half cycle.
If you connected a second device across the first via a capacitor, triggering the second device would cause the current to be diverted to it via the capacitor so the first device would switch off, once the cap was charged the second device would switch off. This is forced commutation. Its complicated with multiple drive pulses, but in its time it was the only way of controlling the current in large traction motors.
I am a beginner in the field...but why do you say that the DC supply would fall to zero every cycle....and what does cycle have to do with DC?

For high power work , like train electric motors, the overhead supply is 22KV AC from a transformer. So in the locomotive, there is a rectifier, so while the voltage is DC, there is no smoothing on it as the components would be huge and expensive and unnecessary, so DC is pulsing.
Have you looked at :- The site has a search function so you can search it for any terms you are unfamiliar with.
thyristor vs diode switches

Which one is better and why?

Re: thyristor vs diode switches


AFAIK diode is often used as a switch with high frequencies.
Thyristors are used for high power switching with AC mains applications.

You are in the power electronics forum, so I assume thyristor is more suitable for you.

Please remember to ask specific questions and give related informations.
Your question is almost as specific as asking "what car is the best?"


Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to