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I have a Honda CG125 - kickstart CDI motor cycle and I am adding a security system in it which has a relay to cut off the engine remotely.

The ignition coil has two cables coming to it, one green and other black/light green. one of the cable is coming from CDI and the other is ground.

I am planning to install relay to one of these wires. I connected the Voltmeter to find out the voltage, when I kick the starter the voltage seems to go very high like 10,000 V etc.

Is this OK to install relay to one of these cable to stop engine - the relay 12 V / 40 A ?

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  • \$\begingroup\$ In a word, "no". Among other options, you could use a high voltage relay but, more importantly, how do you plan to turn it on and off? \$\endgroup\$ – EM Fields Dec 25 '14 at 19:39
  • \$\begingroup\$ The security device has a GSM module, when I send an SMS to power off, it will activate the relay.. \$\endgroup\$ – Ahmed Dec 25 '14 at 19:49
  • \$\begingroup\$ OK, now that you can turn the relay ON and OFF remotely, can you use its contacts - on the low voltage side of the CDI - to keep the spark from being generated in the first place? If you can do that, or something like that, it'll save you the $$ for the high voltage relay. \$\endgroup\$ – EM Fields Dec 25 '14 at 20:02
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A CDI usually has two inputs. The first receives several pulses of some 100V to charge a capacitor, the second is a trigger signal of just a few volts to discharge the CDI via the primary side of the ignition coil.

So, if you want to place your relay here, you should switch the trigger. Remember that the charging and trigger pulses come from a generator like setup, if you open these circuits, you may get high voltage spikes. So, you may ground the trigger signal via an appropriate resistor.

However, the motor is stopped by the key, so why don't you switch the wire from the key?

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It's probably easier to interrupt the input power to the CDI than it is to interrupt (or short to ground) the high voltage output.

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