Spark plug kill switch - Page 1
Forum section: electronics / projects

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: July 16, 2019, 08:18:12 pm
content: Hi everyone!I'm trying to figure out if there is a product that will hook in between the spark plug cable and the spark plug that contains a relay that you could control with a small voltage signal.I've searched around and did not find anything, so I thought I might build him some.Then I struggled finding a spark plug "connector", as in not the cable shroud that goes on it, but a connector that is like the plug itself so it can receive the cabled connector.It also seems like it may be challenging finding a relay that can deal with the high voltages on the spark plug (I've seen the internet refer to 10-40kV).The end goal is super silly: being able to shut off the generator that is outside and behind the cabin - without having to leave the comfort of the hearth.Thoughts? Ideas?Thanks!

author: ejeffrey
author_rank: Super Contributor
timestamp: July 16, 2019, 08:29:03 pm
content: Yeah, off the shelf relays are not going to handle interrupting a spark plug. After all the spark is intended to jump the gap of a spark plug even under the high pressure environment of a compressed piston. It is much better to shut off the 12V to the coil or otherwise prevent the ignition circuit from firing.

author: bdunham7
author_rank: Super Contributor
timestamp: July 16, 2019, 08:35:56 pm
content: You don't interrupt an ignition circuit as the spark can jump an inch under the right conditions. You either interrupt the primary power (12V coil systems) or short it (magnetos). And keep in mind that even the "low voltage" primaries will see several hundred volts at the peaks, so be careful how you set things up.

author: tautech
author_rank: Super Contributor
timestamp: July 16, 2019, 08:44:41 pm
content: Quote from: alexanderbrevig on July 16, 2019, 08:18:12 pmElectric start or pull start ?Electric start, insert relay between ignition switch and ignition coil.Pull start, insert some switch/relay to mimic the OFF switch operation and place it in parallel with the OFF switch. This will likely short the ignition primary wire to ground and disable the ignition spark but check the operation of the ON/OFF switch to confirm its operation.Be careful bringing any switching inside for fear of sparks starting an explosion.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: July 16, 2019, 08:51:32 pm
content: Thanks everyone!I think I'll just hack something inline with the on/off switch on the generator and tell him "No, this is not a viable product idea as its hard to make a one-fits-all and I don't feel like playing with HV DC".He likes thinking up products, not all of his ideas translate too well.Same as me!Probably true what they say about that apple...Thanks again!

author: kolbep
author_rank: Frequent Contributor
timestamp: July 16, 2019, 08:51:36 pm
content: For the Small Generators (5-8KVA) we have here, ones with an Electric Start Keyswitch,when the switch is in the off position, it does 2 things:Shorts the Magneto to the Chassis (Stops Spark),andConnects the Fuel Cutoff Solenoid to the 12v Supply the generator produces. When the Generator stops spinning, the 12v Supply goes away, and the Fuel Cutoff Solenoid (Under the carburetor ?spelling?) releases.If you have this start switch configuration, then just use a multimeter to check which pins are shorted when the switch is in the Off Position (Make sure battery is disconnected when you do these tests). Then put in a Double Pole Double Throw Relay, and set it so that when it looses power, it Shorts out the same wires that the Switch does (Use the NC Contacts on the Relay). That way the Generator will only run when the Switch is in the On Position, AND the Relay is Energised. If the switch is turned off, or the relay looses power, then those systems will be shorted, and the generator will shut off.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 16, 2019, 10:25:03 pm
content: If messing with the low voltage side is not an option, make a switch with a grounded bar swinging into a terminal. Basically like a distributor in old engines.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 16, 2019, 11:26:34 pm
content: One of the really old gas powered lawn mowers that I used to have had a simple metal tab that hinged to the engine shroud with an over-center arrangement so that it was spring loaded to move full up or full down. The spark plug connector was just a bare metal clip on the end of the wire with no insulating boot over it and when you pushed the metal tab down it touched the top of the spark plug and shorted it out. It was very simple and very reliable and had the added benefit that you could clearly see that the tab WAS shorting out the spark plug so there was no danger of getting shocked. The neat thing was that it just took one extra piece of metal to add it to the mower so it's cost was insignificant.

author: james_s
author_rank: Super Contributor
timestamp: July 17, 2019, 12:33:18 am
content: I had a mower once with that springy metal tab you'd push against the top of the plug to shut it off.Most of them just ground the primary side of the magneto which is safer and accomplishes the same thing.

author: soldar
author_rank: Super Contributor
timestamp: July 17, 2019, 10:09:44 am
content: Quote from: james_s on July 17, 2019, 12:33:18 amI remember those!Quote from: james_s on July 17, 2019, 12:33:18 amYup.

author: Wimberleytech
author_rank: Super Contributor
timestamp: July 17, 2019, 10:00:07 pm
content: Quote from: Stray Electron on July 16, 2019, 11:26:34 pmThe good old days!