SCR activation question

[max_torch]

Is there a difference with the SCR behavior when it is activated by a pulse and when a constant voltage is applied to its gate? I know that it will start conducting and then latch as soon as a pulse is sent to its gate. But what if it is not a pulse but rather a constant voltage sent to its gate. I know it will conduct, but will this affect the quality of the load current going through it? Will this result in an anode-to-cathode resistance that has to be overcome?
Or is there really no difference between the way it switches whether it receives a pulse or a latched high signal to its gate?

 

[Daniel Robertson]

You normally supply a pulse to a thyristor or triac gate to trigger it at a certain time, IE. in phase control circuits like dimmers for use on AC. You can however trigger a thyristor by applying a constant voltage to it's gate, IE from a logic device, to keep something turned on (AC). When the gate voltage is removed, the thyristor will cease to conduct when the AC cycle goes through zero. On DC, once triggered, the thyristor will cease to conduct when the voltage across it's load is disconnected.

Daniel.

 

[Arouse1973]

Good answer Daniel.
Adam

 

[KrisBlueNZ]

Daniel, that's not what he's asking. He knows how an SCR works. He wants to know...

I know it will conduct, but will this affect the quality of the load current going through it? Will this result in an anode-to-cathode resistance that has to be overcome?

I'm afraid I don't know the answer either. My guess is that it wouldn't make any difference. Someone else here may know though.

 

[Arouse1973]

Sorry Kris and Max I didn't read the question well enough. The SCR is like two diodes in series the PN junctions should act just like a diode. So the dynamic resistance will vary depending on the current the device is supplying. So for a silicon diode the resistance with 1mA of current is 0.026eV/0.001A = 26 Ohm. So for an SCR it will be double this as you have two PN junctions. So as you can see if you increase the current the resistance will go down. You will get voltage increase across the diode due to the increased current but this won't change a great deal versus current. If it did the SCRs wouldn't be able to supply the large currents they can.
Adam

 

[Daniel Robertson]

Hi Adam and Kris.

Maybe I did not read it correctly as well. I put it down to age. Thanks for explaining before I got back here Adam. Also one thing to bear in mind is that because an SCR is in theory a diode during conduction, It will when controlling an AC load act as a half wave rectifier. Therefore will conduct only during half a cycle. On our high power dimmers (up to 20Kw each), we use two SCR's back to back and trigger them with a 1:1+1 transformer or we use triacs on the smaller dimmers (2.5kW) and trigger them using an opto isolator. The reason for using SCR's on the large dimmers is that they are a more robust device and in stud form, can be bolted to large heatsinks.

Cheers guys.

Daniel.

 

[Arouse1973]

No probs Daniel, nice to hear what your doing with them. Dont see them much now.
Adam