My circuit: enter image description here

Signal coupled in power supply (3.3V) of STM32F103 during spark:

enter image description here

enter image description here

Is there any benefit to using this diode (circled in red) or not (without this diode the snubber circuit will start working)?

What is a better solution/topology to avoid noise and maintain the spark efficiency?

My doubt is:

If I use this shown configuration, is it more difficult to cut-off the SCR (maintain hold current for a long period) and can it disturb another part in circuit?

I use an automotive sparkplug resistive cable with 4K ohms and a earth strap to connect the engine with ignition coil GND (frame).

The diode D4 is relevant during the C3 charging, is it a necessary solution or does it not make a big difference in this circuit?

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  • \$\begingroup\$ How can we know if your probe ground length is picking up noise? You can show noise with probe on nearest ground \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 8 '19 at 18:57
  • \$\begingroup\$ To do this measure I put the osciloscope GND in the negative terminal of Battery and microcontroller VDD (3.3V), how should I have done? Thanks for your attention! \$\endgroup\$ – Fabio CRJ Jan 8 '19 at 19:42
  • \$\begingroup\$ Directly across D5 electronics.stackexchange.com/questions/298657/… \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 8 '19 at 19:46
  • \$\begingroup\$ Move probe tip to same ground used by probe and see how big the glitch is. THis is measurement error. That's why it must be short to reduce loop AREA \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 8 '19 at 22:40
  • \$\begingroup\$ In this video: youtube.com/watch?v=kfKosUKUt-A , you can watch in channel 1 (referent D5 - anode of SCR) and channel 2 Microcontroller VDD. Diode circunded in red are present in circuit (connected)! \$\endgroup\$ – Fabio CRJ Jan 8 '19 at 22:46

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