How does a SCR turn off ?

[ze]

one thing that i did not understand was the way to turn off a scr!
you simply "wait" for to turn off?
when a gate current it given, the scr turns on, then i still behaves like an open circuit for how long?
it's until the sine wave reaches a certain value of voltage that the scr stops??

 

[hock]

Re: SCR way of working

the device has to be turned off externally as the internal PNP and NPN structure latches. the emmitor of one is driving the base of the other transistor and as the overall loop gain is more then one u have to turn it off externally.
read M. H. Reshid or nedmohan for this.
hock

 

[naoda2055]

SCR way of working

Yeah.So who can tell the details?

 

[mohrsh]

Re: SCR way of working

you must apply a reverse voltage accross anode to cathode .
when Vak becomes negative scr stops conducting .

 

[edonor]

Re: SCR way of working

Look **broken link removed**

 

[yeechyan]

SCR way of working

You may try to apply a negative voltage relative to the Cathode at the Gate.

 

[s.b.f]

Re: SCR way of working

you can refference:
Power Electronics by Rashid

 

[epp]

Re: SCR way of working

To turn off SCR you have to met two conditions at the same time:
1. Turn off current through it.
2. Aplly negative gate-cathode voltage for a finite interval of time.

This time depends of SCR model (look at datasheets).
If you let this time to be low SCR will start conduct again as it got direct voltage G-C.

 

[rambus_ddr]

Re: SCR way of working

SCR turn on which must turn on pnp or npn, which need a current through the resistor and build a Vbe voltage.

 

[happy2005]

SCR way of working

A small gate current can make SCR turn-on. It stays at conduction mode if it carrying a current through it. When the current falls the below a certain value (holding currrent, ex. ~1-10 mA), the SCR immediately turn-off.
For example, when SCR is turned-on by gate pulse, it turns off automatically at zero cross of current. (not voltage!)
Negative gate voltage is applicable only GTOs (Gate turn off thyristors)

 

[_]

Re: SCR way of working

well after the gate pulse, for an SCR to remain switched ON, it requires a minimum current going from A to K (IAK). if current gets below the minimum limit, then the SCR will turn OFF. if you are feeding you SCR with AC, then basicly as you also sed, if the AC supply voltage (Vs) gets low enough (this is offcorse in the positive semi-cycle, in negative semi-cycle a SCR is OFF) then the current throught the SCR would be:

IAK = (Vs-VAK)/RLOAD

so when the AC is at the low voltage instance, the IAK will be low enough to switch the SCR OFF.