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Registered Member #8497
Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
I've done some research on SCR commutation and most successful CG builds use 'V-switch' commutation where two parallel SCR's dump current into a single coil - where the second SCR releases a higher voltage than the first to commutate the conducting SCR.
However, many higher-powered SCR's do not like the inductive load placed after the SCR.
What is the process to commutate a conducting thyristor? Is it shut off by a higher voltage seen on the emitter side? Or does current need need to drop to zero.
In most CG circuits, Voltage and Current are 90deg apart, so the method to commutate is critical.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
"They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed)."
From Wikipedia:
Wikipedia says they continue to conduct while there is a forward voltage difference across them.
Say we add a second SCR whose cathode intersects at point (x) above. Say the main SCR is open-conducting from a large capacitor with an initial charge of 100V. The second SCR is activated, which dumps from a capacitor charged to 300V, but of a very small capacitance.
Will not the circuit act as a voltage divider momentarily and allow the small cap at a high voltage to discharge through the same wire resistance to ground. This would make the main SCR stop conducting, effectively commutating the current.
For V-switch you need an another cap with 1/10-1/15 capacitance of the main cap (at minimum), so consider this as an ineffective option. But if you are still interested, then your circuit must be reassigned this way: Electrolytic Cap + --- SCR --- (x) --- Inductor --- GND The idea is to make cathode of the main thyristor to become over it's anode in potential - then it will not be able to conduct. So the secondary cap should be charged to at least the same voltage as the main cap so by the time you need to switch SCR off it will be higher in voltage. Anyway, the difference it makes is that just another cap discharges instead of the main one, that's all. Would be good to use a really small cap for closer, but then it will be discharging that fast, that main SCR will not have time to understand what happened and will continue to conduct.
Registered Member #8497
Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
Yandersen -
I understand what you are referring to - as we're both looking at the same theory here.
However, I have noticed that SCR's do not like to place the inductive load on the cathode side of the SCR, Instead, the inductive load must come before the SCR.
I'm not sure how the successful CG's implement the 'V-Switch' without destroying their main SCR.
Where did you heard that BS?! It doesn't matter, really. In my recup gauss every even stage is connected one way, every odd - the other. Thing still works.
Successfull CG doesn't implement V-switch. Successfull CG is either halfbridge or non-polar recuper. None of them use V-switch, as it doesn't shut down the current - it just stops it from rising in a coil. And after that, dempher diode works returning such gauss to the monkey design age.
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SCR Thyristor Commutation
