Thyristor SCR

Here are a couple of links that explain it nicely.

Link 1

Link 2

I've not looked at "Jack of all trades" two links but he's very good at his research. I suspect that his links include my explanation.

A Silicon Controlled Rectifier is a solid state member of the Thyristor family of components. Thyristor devices can only be controlled when they turn ON with a minimum current for an extremely brief minimum period of time applied into the Gate to Cathode junction. The maximum ON state current (330A) is the maximum current that can operate from Anode to Cathode. Once current starts to flow from anode to cathode, the SCR will remain ON until the Anode to Cathode current drops below the maximum holding current. The SCR then turns itself OFF. A SCR cannot be turned OFF with the gate control. The brief interval of 0 current condition that an alternating current source provides is sufficient to turn OFF a thyristor's current flow.

Thank you "redfred" and "jack of all trades",

The information you have provided has been very helpful. I am a little confused however. Hypothetically if I have the SCR in forward bias, (controlling high current DC) with a small current present on the gate, the SCR will conduct. If the high current across the SCR never drops below the threshold 600mA, it will continue to conduct regardless of the presence of gate current??

Yes, that is precisely what will happen. Another type of thyristor is a device known as an ignitron. An ignitron is a device that conducts by deliberately forming an arc between two conductors. As long as the current between the two conductors maintains the arc this current will continue with no added assistance. These devices are used in the amusing, explosive and very dangerous world of coin shrinking.

Incredible! I had no idea coins could be "shrunk" in this manner, nor that anyone would want to. Very interesting all the same. Gives new meaning to "consolidating your assets". Thanks again for your help.

You have found the very reason why they are not quite as practical in DC applications.

What you have to do is make a short across the SCR with relay or switch contacts, to divert the current away from the device....I personally find this impractical, but that is how its sometimes done....

May I suggest that you try and use AC as much as possible, or use the SCR where AC is still present, maybe earlier on in the circuit perhaps? As you haven't told us exactly what you have, I am only guessing as to the possibility.....

I think it should read "minimum holding current".

Yes. You're correct. I meant "minimum holding current".

Nope, it is "maximum holding current".

Holding current is the least current (A to K) that will keep the SCR conducting.

If the manufacturer states that max holding current is 600 mA, some parts will switch off at say 550 mA, some at 400 mA and so on, but never at a value above 600 mA.

brgds

Snel

According to the Tektronix Semiconductor Device Measurement handbook, on page 96, the holding current is a boundary condition for the part, and is only available in a condition that is NOT a normal operating condition. Specifically, no gate voltage is to be applied to make this measurement with a curve tracer.

Hi WoodwardDL,

Made a lot of such measurements with the Tek 576. You have too choose a low "collector" resistance, to allow for a large anode current at some volts. Use the "DC" mode, then inject some gate current just to switch the SCR on. Reduce voltage slowly and read collector current at the moment it jumps to zero.

brgds

Snel

Right on! I have made the same measurements too, but with a 370A Curve Tracer. I created a course for the 370A, and taught it to hundreds of users over a decade in the 90's.

Tek has dropped their curve tracers now, due to low sales caused by the SPA (Semiconductor Parameter Analyzer) devices which were way more sensitive. That may have been a poor move, because now the PV devices need Curve Tracers!