12V Jacob's Ladder circuit introducing noise on input and messing with PSU
Due to the simplicity of the circuit, the power supply current it takes from the supply will be extremely irregular. It will take a current pulse from the supply each time the darlington transistor is switched on.

When powered from a car battery that should be of little concern as a car battery has an extremely low internal series resistance so the battery's voltage will barely drop when a current pulse occurs. Most of the drop is likely to be in the wires you use to connect to the battery (unless you're using very thick wires).

A switched mode power supply relies on its output capacitors to handle current pulses. Most switched mode power supplies aren't designed for and/or don't work well with a pulsed load as their output filtering capacitors are dimensioned for filtering out the switching frequency of the switching powersupply itself, usually these are switching at around 50 kHz to 200 kHz. That needs a (relatively) small value capacitor but with a low series resistance (low ESR).

What I would do is add some measure to "isolate" the circuit from the supply so that the supply will not be bothered by the pulses. I would try this:







simulate this circuit &ndash; Schematic created using CircuitLab

For the diode use any diode you have available which can handle at least 3 A, it can be a Schottky diode but does not have to be. For the capacitor a 4700 uF 16 V electrolytic capacitor is what you need. The 4700 uF is just my guess, maybe 1000 uF will work as well.

If this still gives issues then you could try adding a resistor in series with the diode, start with a 1 ohm, 5 Watt one similar to the one in your schematic.