An SCR like TYN612 cannot be turned off via the gate. It will turn off (commutate) only when the current through it falls below the holding current and the voltage across it is no longer forward biasing it (anode to cathode). In AC circuits, this happens every half-cycle as the current goes to zero, so effectively it turns off at each zero crossing unless triggered again. In DC circuits, you must force the current to zero (e.g., by diverting it or disconnecting the supply) to reset the SCR. Some circuits use forced commutation techniques, but these add complexity. Typically, plan that once TYN612 is triggered in a DC scenario, it will conduct until the source is removed or a fuse blows (as in crowbar overvoltage protection usage).

The core part “TYN612” denotes an SCR of 600 V, 12 A spec in ST’s lineup. The additional letters often indicate packing or minor variations. For instance, TYN612RG is essentially the same SCR but in tube packaging (the RG might stand for RoHS/Green compliance or packaging code). Some distributors list TYN612RG as the orderable code. There isn’t a functional difference implied by the “RG” in terms of electrical specs. So you can consider TYN612 and TYN612RG equivalent for usage (always double-check the datasheet footnotes). The key is to ensure you get the correct voltage and current rating – for example, ST also has parts like TYN812 (800 V, 12 A) or TYN1012 (1000 V, 25 A) in the family; the numbers change for different ratings. So “612” specifically corresponds to 600 V, 12 A standard SCR.

Yes, the TYN612 is designed for mains-voltage AC control. Its 600 V rating comfortably handles up to 240 VAC mains. Many applications include using it in a phase angle controller (like light dimmers, motor speed controllers) or in a controlled rectifier for, say, a variable DC supply. As for the gate: it’s not exactly “logic level” in the sense of a logic MOSFET, but the gate trigger current requirement is low enough (5–15 mA) that you can drive it with a typical microcontroller plus a transistor or even directly with a microcontroller GPIO if properly limited (some microcontrollers can source 15 mA, but often it’s safer to use a driver transistor or optocoupler). The gate trigger voltage is around 1 V, so even a 3.3 V microcontroller through a resistor can forward bias the gate-cathode junction easily. The critical thing is ensuring you have a common reference (SCR cathode typically goes to circuit ground if controlling with a microcontroller) and isolation if needed (since SCR will be at mains potential in many cases, an optocoupler like MOC3020 series might be used for isolation when controlling from digital logic).

A triac is effectively two SCRs in one package connected in inverse parallel, and it can conduct in both directions (AC). Using a triac (like a BT138, etc.) can simplify circuits for AC switching (only one device needed to control both halves of the waveform). However, triacs sometimes have limitations in terms of dV/dt and may not handle very high currents as well as a pair of SCRs. The TYN612, as a single SCR, only conducts in one direction, so for AC you’d need two of them (one for each half cycle) arranged back-to-back to replicate a triac’s function. The advantage of separate SCRs is more control in some cases and often higher surge current capability. In applications like controlled rectifiers (where you only put SCRs on the positive half cycles in a bridge), using SCRs is the way to go. If you specifically need to control AC in both polarities with one device, a triac is simpler. So the choice depends on the application: use TYN612 when you need the higher performance or when you are only concerned with one polarity (like half-controlled bridge or DC crowbar), use a triac for simpler AC switch needs when appropriate.

It is highly recommended to use a snubber network (typically a resistor-capacitor series across the device or across the load) in many SCR applications, especially if the load is inductive. The snubber limits the dV/dt and voltage overshoot when the SCR turns off (goes from conducting to blocking, the sudden stop in current through an inductance can cause a voltage spike). Without a snubber or other suppression (like a MOV or TVS diode for transients), the SCR could see a voltage beyond its rating or could re-trigger unintentionally. Also, a gate-to-cathode resistor (few kΩ) is often used to keep the gate from floating and to improve dV/dt immunity by bleeding off any induced currents. If the load is resistive (like heater, incandescent lamp) and wiring is short, sometimes you can get away without a snubber. But with motors, transformers, or longer cables, including RC snubbers or MOVs is cheap insurance for reliability. Another aspect of protection is ensuring the device doesn’t overheat – mounting it on a heatsink and possibly using a thermal cutoff or fuse for safety is wise in power circuits.

Yes, many. Within STMicroelectronics, if you need a higher voltage you might use TYN812 (800 V) or TYN1012 (1000 V), but if 600 V is fine, TYN612 is their standard. Other manufacturers have similar SCRs: for example, Littelfuse (IXYS) might have a part like MCR12DSMT (which is 12 A, 600 V in TO-220), or Vishay’s TS12 series. Another common part is BT151-500R (though that one is 500 V, 7.5 A). The exact naming varies. When substituting, ensure the gate sensitivity and pin arrangement match, because some SCRs are “sensitive gate” types meant for lower gate current but lower dV/dt tolerance, etc. If you need a more sensitive device, there are “logic level” SCRs but they typically handle less current. For 12 A class, most have similar requirements to the TYN612. EstiSource’s database can help find cross-reference equivalents if TYN612 is hard to find, so you can maintain second sources easily.