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SCR pulses

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#1 taknavaz


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Posted 12 October 2005 - 10:12 PM

I have a question. in some factory and in SCR blocks for SCR gate there are more than one pulse under the half cycle. what is the matter? because we need a pulse for turn on the SCR.

#2 marke


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Posted 12 October 2005 - 11:14 PM

Hello taknavaz

Welcome to the forum

It sounds to me like you are describing picket fence firing of the SCRs.
There are three methods used for controlling SCRs.
1) Hard Fire
2) Picket Fence Firing
3) Trigger Pulse firing.

In theory, with an SCR, you can apply a single pulse to turn the SCR ON and it will remain ON until the current through the SCR falls to zero.
In practice, there are a number of constraints in using the Trigger Pulse firing.
The duration of the trigger pulse must be longer than the time taken for the current through the SCR to increase to a current higher than the latching curent of the SCR. If the current falls below the holding current of the SCR, the SCR will self commutate. This can cause premature commutation if there are disturbances in the current flow due to supply or load disturbances.
The use of Hard Fire techniques overcomes thses potential issues. Hard fire techniques requires that the gate current is held ON for the duration of the conduction angle of the SCR. This ensures that the SCR can not be prematurely commutated. This requires gate energy and presents a higher load to the trigger circuit and extra heat in the SCR. It is a little difficult to generate a trigger pulse of 10ms (6.66ms for 60Hz) that has a fast risetime and requires more than just a pulse transformer.
Picket fence firing comprises a series of pulses applied to the gate for the duration of the expected current flow. These short pulses are easy to generate with a pulse transformer and ensure that the SCR can not commutate OFF for a period of time greater than the pulse repetition rate. It is a compromise between the Trigger pulse technique and the Hrd Fire technique.

Hope this answers your question.

Best regards,

#3 jraef


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Posted 14 October 2005 - 12:17 AM

Well stated Mark, but then again, what else would I expect form you?
"He's not dead, he's just pinin' for the fjords!"

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