Building a HV transformer

I built this circuit recently though it did not work for me

https://www.instructables.com/id/Variable-voltage-ignition-coil-power-supply/

Using this coil

https://www.autosurplus.com.au/brand/delphi/

What effect would using a 5nf capacitor have on the circuit? Also, since the coil did not create an arc, does this mean the voltage would have built up within the coil, therefore damaging it? I ran the circuit for a combined total of around 2 minutes...

• Possible duplicate of Transformer Welfare – Andy aka Oct 11 at 8:37
• CONSIDER ALL PARTS OF THIS CIRCUIT AS BEING AT MAIN VOLTAGE . As you were already told - and as should be obvious to you from the "instructions" - the 5 nF is much too small. 5 nF is 1/200th (1/2 a percent) of the specified value. If the dimmer functions at that load level you WILL get SOME output but it will be very low. Someone suggested using the capacitor from a microwave oven. You could try that. – Russell McMahon Oct 11 at 22:39
• Your lack of basic electronics is concerning when dealing with high voltage and high powers. – MadHatter Oct 11 at 22:44
• The lack of spark damage the coil due to voltage buildup internally? – Steve Oct 13 at 8:06

THIS MACHINE KILLS

CONSIDER ALL PARTS OF THIS CIRCUIT AS BEING AT MAIN VOLTAGE.
It can kill you even when there is NO spark.

As you were already told - and as should be obvious to you from the "instructions" - the 5 nF is much too small. 5 nF is 1/200th (1/2 a percent) of the 'specified' value.
If the dimmer functions at that load level you WILL get SOME output but it will be very low.

Using "any old capacitor" is liable to cause capacitor breakdown.
Consequences are liable to be not marvellous.
Or worse.

Someone suggested using the capacitor from a microwave oven.
You could try that.

Do you think the coil could have been damaged due to the lack of spark?

Unlikely. If you at any stage managed to connect the dimmer to it without the cap then it could easily be dead.

Test:

DO NOT use AC mains for this test.
Do not use a capacitor FOR THIS TEST.

Connect a spark plug to the EHT output.
Connect a 12V OR LESS battery to the inputs then disconnect.
At the moment of disconnection you should get a spark.

Also, without the cap [& using AC mains], would the light dimmer also be damaged due to the back emf?

Trying it without the capacitor is akin to the
"intercepting the speeding bullet without the bulletproof vest test".

THIS CIRCUIT IS LETHAL.
It will try to kill you if you do everything "right".
It is a kluge / hack / heath robinson / ... lashup that happens to survive well enough (apparently) when done "right". Doing it without the cap is way way way beyond the pale. (ie even more lethal - it may kill you 2 or 3 times at once). |

YES - without the cap it may well kill the dimmer - and back emf is just one of the ways it may do so.

• Ok thanks. Do you think the coil could have been damaged due to the lack of spark? – Steve Oct 12 at 1:56
• @Steve Unlikely. If you at any stage managed to connect the dimmer to it without the cap then it could easilt be dead. | Test: Connect a spark – Russell McMahon Oct 12 at 8:44
• @Steve Unlikely. If you at any stage managed to connect the dimmer to it without the cap then it could easily be dead. | Test: Connect a spark plug to the EHT output. Connect a 12V OR LESS battery to the inputs then disconnect. At the moment of disconnection you should get a spark. DO NOT use AC mains for this test. Do not use a capacitor FOR THIS TEST. – Russell McMahon Oct 12 at 9:21
• Ok thanks. Also, without the cap, would the light dimmer also be damaged due to the back emf? – Steve Oct 12 at 9:44
• @Steve Have you tried it without the capacitor? That's akin to the "intercepting the speeding bullet without the bulletproof vest test". THIS CIRCUIT IS LETHAL. It will try to kill you if you do everything "right". It is a kluge / hack / heath robinson / ... lashup that happens to survive well enough (apparently) when done "right". Doing it without the cap is way way way beyond the pale. (ie even more lethal - it may kill you 2 or 3 times at once). | YES - without the cap it may well kill the dimmer - and back emf is just one of the ways it may do so. – Russell McMahon Oct 12 at 12:06