When I connected a silicon control rectifier by applying positive voltage in anode and negative voltage in cathode, after that I triggered the gate by pulse voltage and it connected, after that I want to connect gate with ground and I want to know what happened.
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1\$\begingroup\$ Have you read: electronics-notes.com/articles/electronic_components/scr/… ? Realize that an SCR is also called "thyristor" and that it is either in an on state or an off state (somewhat simplified but good enough for now). Then note this sentence at the bottom of the page: Once switched on, the thyristor can only be turned off by removing the supply voltage. \$\endgroup\$– BimpelrekkieCommented Nov 20, 2018 at 12:17
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\$\begingroup\$ It really depends on what you refer to as ground and more specifically what the Cathode potential is. \$\endgroup\$– user16222Commented Nov 20, 2018 at 12:20
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\$\begingroup\$ yes I read it , I connected this circuit in Proteus 8 Professional to show what happens I showed that when voltage is very large SCR would be turn on without triggering voltage on gate , I think that it happens , because the applying voltage larger than VB for SCR and when voltage is small , don't turn on without triggering voltage and when I connected gate with ground SCR became turn off , but I don't know why \$\endgroup\$– Eslam HaSsan UWkCommented Nov 20, 2018 at 12:51
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\$\begingroup\$ Beware of high slewrates on the anode of the SCR. These cause (via I = C * dV/dT) large transient currents inside the SCR, and may be adequate to trigger the SCR into positive-feedback and thus into permanent SCR behavior (at least until the current thru the SCR is taken to zero) \$\endgroup\$– analogsystemsrfCommented Nov 20, 2018 at 18:26
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\$\begingroup\$ Think of it as cattle in a fenced in field with a gate. You unlatch the gate, and cattle will pour through. The gate will remain unlatched until the cows stop coming through, at which point the gate closes and relatches, until it is opened again by unlatching it. \$\endgroup\$– Scott SeidmanCommented Jun 25, 2019 at 13:28
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2 Answers
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If the SCR is passing sufficient current (latching current or greater) then no matter what you do with the gate, the device will continue to be "latched": -
Image from Wireless World. However, if the current is taken to below the "hold current" value, the SCR will unlatch and the gate (if desired) can be used to re-activate it.
answered Nov 20, 2018 at 12:53
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If you connect the gate to the cathode directly, or apply a negative voltage, and current above the holding current is flowing from anode to cathode, it may commutate (turn off), especially if the anode current is close to the holding current, however this behavior is not guaranteed.
In GTO (Gate Turn-Off) devices, the behavior is guaranteed (and the design is different to make behavior turn-off reliable and more sensitive).
See the accepted answer here for the difference in construction.
If you consider the two-transistor model of the SCR, it looks like it should always turn off, however that model does not exactly correspond to reality (image from Wikipedia.
answered Nov 20, 2018 at 13:24
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\$\begingroup\$ I think that i have answer , when I apply voltage between anode and cathode make (VAK ) >(VB) so the SCR will turn on without triggering voltage from gate to turn on,so when i connect gate with ground some current will pass through gate but the current between anode and cathode will not decrease about current holding \$\endgroup\$ Commented Nov 20, 2018 at 13:58
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\$\begingroup\$ If one connects gate and cathode directly when the SCR is on, most likely the gate current is too high and the gate is destroyed. \$\endgroup\$– JankaCommented Nov 20, 2018 at 14:04
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\$\begingroup\$ @Janka No. Maybe you're thinking of gate-to-anode? \$\endgroup\$ Commented Nov 20, 2018 at 14:06
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\$\begingroup\$ But when the voltage between anode and cathode is smaller than VB , so the SCR need triggering voltage from gate to turn on and after that when i connect gate with ground , the current will pass through gate to ground make the current between anode and cathode decrease from current holding ,but if the current doesn't decrease from current holding the SCR will still turn on my description is correct or no , please answer . \$\endgroup\$ Commented Nov 20, 2018 at 14:09
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\$\begingroup\$ @EslamHaSsanUWk The SCR will stay on.. yes that sounds essentially correct. \$\endgroup\$ Commented Nov 20, 2018 at 14:11