I'm building a xenon flash unit with a power of 50Ws (300V 1000uF capacitor).

I need to trigger it from a microcontroller. For that I use a MOC3041 to opto isolate my µC from the HV circuit and command my thyristor (MCR12DG). This one trigger a HV coil to light up my xenon tube.

Here is the schematic : Schematic xenon tube trigger

The Igt of my MCR12DG is 8mA, so I set a resistance of 1.6K, VTrans is 14v.

Here is the spec of TCP3308 trigger coil: Trigger coil specs

It's the first time that I use an SCR, and may be something is missing ?

Is my SCR correctly chosen for my application ?


Here is a correction with a MOC3023 as suggested. The main term is now connected to the SCR in order to turn it off. A 2N3904 is used to trigger the led from the uC and a 500ohm resistor to limit the current through the MOC3023.

New schematic

But now as I connected my main term to 200v.I use 2 resistor and a capacitor connected to the main term as recommended by the application note AN-3003 for inductive load.

Application note inductive load

Is this correct ?

  • 1
    \$\begingroup\$ "... with a power of 50Ws". Power is measures in watts (W). Power by time (Ws) is energy and can be converted to joules (J). 1 J = 1 Ws.. \$\endgroup\$ – Transistor Nov 28 '18 at 22:53
  • \$\begingroup\$ You seem to be using a triac to switch a DC voltage. You will not be able to turn it off. \$\endgroup\$ – Edgar Brown Nov 28 '18 at 22:58
  • \$\begingroup\$ R1 current should be less than T3 holding current to turn it off. It looks sub-optimal but the Bulb will act like an SCR once it fires and those details are missing. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 28 '18 at 23:04
  • \$\begingroup\$ @TonyEErocketscientist that’s to turn the SCR off, but the Triac has no limitation. \$\endgroup\$ – Edgar Brown Nov 29 '18 at 0:51

The circuit you have will not work for a couple of reasons:

  1. The MOC3041 is a Zero crossing configuration that will NOT trigger if the T1-T2 voltage is above 20V (this is the maximum spec). Typically these TRIACs will not trigger at 10-15V, so it is very likely that the TRIAC will not trigger at all. You need to use an MOC3023 (or MOC3020 with higher gate current requirements) to ensure you can turn the device on.
  2. The MOC3023 main term should not be connected to a separate power rail. Once you trigger the device it will simply stay on. Connect the selected main term to the MCR12DG Anode terminal This allows the MOC3023 to turn on and be turned off by the MCR12DG once it is on. Now once the voltage on the cap C1 turns negative (it rings with the inductor) it will ensure both devices are off.
  • \$\begingroup\$ I have updated my question with a new schematic. But I have seen that I may require a 0.1uF capacitor connected to the main term (pin 6 of the MOC) and to the cathode of SCR due to my inductive load. \$\endgroup\$ – TGuerin Nov 29 '18 at 10:48

As already pointed out, the opto you have chosen incorporates a triac, which once triggered may remain on if there is sufficient holding current. If you want isolation, perhaps use an opto with a transistor output. Do you really need isolation? Both sides share a common ground so it's not really isolated. The SCR looks a bit large for the application. Consider something like X02 from ST. Only needs 200 uA to trigger.

  • \$\begingroup\$ There is an error as you point, the ground should not be common if I want isolation. Now the ground are separated. I have changed the opto-triac by the MOC3023 as recommended by @Jack Creasey. I will consider the X02 SCR as suggested. \$\endgroup\$ – TGuerin Nov 29 '18 at 11:03

the optocoupler is not needed lust use a transistor.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ You need to proof-read before hitting "Post Your Answer". \$\endgroup\$ – Transistor Nov 29 '18 at 6:51
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    \$\begingroup\$ I've made a mistake in my schematic by connecting my capacitor to the same gnd. I need isolation with my HV circuit because LV circuit must absolutely be protected. \$\endgroup\$ – TGuerin Nov 29 '18 at 9:03
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    \$\begingroup\$ that changes things.... \$\endgroup\$ – Jasen Nov 30 '18 at 3:04

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