SNUBBER BOARD CIRCUITS

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PLEASE EXPLAIN WHAT THE PURPOSE OF THE SNUBBER CIRCUIT BOARD DOES IN THE FIRING OF THE SCR"S THANK YOU

-- G NESS (maint@nrthside.ca), October 03, 2000

Answers

The snubber on an SCR Firing Circuit helps to eliminate voltage spikes and keep a clean signal to the scr's and transformer. Its also a protection device for your SCR unit.

-- Brent Davis (wesco@iquest.net), February 01, 2001.

It protects from dv/dt.

-- shailesh kumar (shil_11@jsr.tatasteel.com), April 28, 2001.

it helps reduce overvoltage to the device during turn on and it also help reduce overcurrent to the device during turn off. The snubber also helps to reduce power loss.

-- Hoaji (Hoaji@hotmail.com), May 22, 2001.

The snubber, a series resistor and capacitor across the SCR, is needed to prevent self commutation of the SCR.

SCRs, a four-layer semiconductor device, have a sensitivity to high rise time voltage spikes on their anode, which capacitively couples down through the layers to the gate layer.

This spike, if of the proper polarity, amplitude and speed, can turn the SCR on.

In a welder, where the SCR may be quite large, and the inductance of the load causes a lot of current to flow after voltage zero crossing, we have a condition where the reverse line voltage can be a hundred volts or more at the time of SCR drop-out. This drop-out occurs when the stored magnetic energy has been depleted at the end of the half cycle.

When the SCR quits conducting, like a switch opening, it suddenly "sees" this large reverse voltage between anode to cathode. The companion SCR, used for controlling the other half of the sine wave also "sees" this spike of instantanious rise time, and may get gated on.

Capacitors and resistors from gate to cathode reduce the amplitude of this spike, but also interfere with how quickly we can turn on the SCR. It is important to gate the SCR on quickly, so that the whole wafer handles the current, and this swamping circuitry on the gate intereferes with this.

I concluded that the large SCRs used in the industry were a wrong reaction to the high faulure rate, as they failed because of voltage spikes, not overcurrent. The industry was inching up into the 1750 to 2000 amp range, and still losing SCRs. Toshiba made me an SCR with only a 400 amp rating, but 2600 volt PIV, that ended the failures. It was a hard sell to my customers, but they believed in me, and it worked. Most modern controllers are 400 amp units now, and they work just fine. Some claim they are larger, such as 1200 amps, I think to avoid the "hard sell" I had to go through. My point in all this, is that the smaller SCR has much better dv/dt characteristics, and are more immune to this spike. One of the leading weld controllers does not even have a snubber, and I have personally seen hundreds of these working just fine, although I don't approve...

-- David Bacon (dbacon@updatetechnology.com), April 20, 2002.


since one becomes I calculate of snubber RC and RL and for any tiristor?

-- juan carlos castillo (carlos20001_1999@yahoo.com), September 23, 2003.


respected sir, i want to know the design of r& c value in snubber circuits

-- ksheikmasthan (ksheikmasthan@hotmail.com), November 09, 2003.

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