How to get a spark plug to spark continuously?
 

Title says it all i am making a bench spark plug tester but i can only get the spark plug to spark once how do i get it to continuously spark?

What are you powering it from?

You need a CONSTANT high voltage, and that's not easy to achieve and keep cool.

I've got a spare car battery so I'm using that, could it be the wires are too thin?

ignition coil,,live and earth to the coil and then coil strait to the plug

same thing as they use for a flamer kit lol

Thats the setup ive got except it only sparks once? do i need a igniter unit?

LOL, the coil only produces a high voltage from a BATTERY each time it is switched, on or off.

it's caused by the magnetic field from the primary coil expanding, or collapsing and, as it does so, it cuts the secondary coil, inducing a higher voltage in it (according to the ratio of primary turn to seondary turns), which, in turn, produces the spark.

To make it spark repeatedly, you need to turn the primary on and off quicky, so:

Either use and "make and break" circuit like is found in an electric bell/buzzer, https://en.wikipedia.org/wiki/Electric_bell or use an AC for the primary, but LOW VOLTAGE AC or you will end up elctrocuting yourself. You could do it via an AC transformer off the mains, such as are used for model railways, but no more than 12V.

Funnily enough I used the circuit for an electric bell as a kid to make a "shocking coil" that I took to school. It would floor ten kids in a row off a 6V battery, producing in the region of 20,000V.
Eventually I was caned and it was confiscated after one kid was thrown backwards and suffered a cut needing stitches, on the chalk rail on the form room's blackboard.:D

Could i use a make and break relay? if so how would that work or should i just use an igniter unit?

or should i just use a 555 switch?

If it were me, I'd purchase a cheap electric bell from somewhere like Maplins, remove the gong and wire it into the low voltage side, in series. That's what I did with my shocking coil.

Like this: http://www.mr-resistor.co.uk/item.as...7hoaAlXq8P8HAQ

A relay switched by a timer/oscillator circuit (741, or 555 wired appropriately with a output transistor to power the relay) to power a relay and turn on/off the primary current to the ignition coil "should" work on paper.

However the primary side can see a back-EMF voltage of a few hundred volts, this could cause the contacts on the relay to burn out or fuse prematurely from arcing or contact bounce, so you may need some suppression circuity across the relay contacts to keep it reliable. Basically just resistors and capacitors, but make sure they are rated to at least 500volts! Same goes for the relay.

This write up give an idea of is actually happening when you switch on and off the power to a ignition coil: https://www.picoauto.com/library/aut...imary-voltage/ (the whole picoscope website is a good learning resource for automotive waveforms).

Alternative idea to making a circuit is to go scrap yard hunting get the whole ignition system off a car engine made before 1980ish. Doesn't matter if its contact-breaker (points) or electronic so long as you get all the bits; coil, distributor, amplifier (if electronic), leads, ballast resistor (if its contact breaker). Assemble it all on a bench and turn the distributor with a motor or drill whatever.

Another alternative is to rob the ignition circuit off a gas hob. But it will work off mains power....do so at your own peril ;) :D