Help replacing relay contact

[cachomachine]

I am seeking your help to find an electronic replacement for a NO relay contact.
I am using a 08M2 to energise a relay that shorts the ignition coil of a 1 cylinder electricity generator to turn it OFF.
The problem that I encounter is that the relay contact burns. (The contact is rated at 10 amps )
I have tried to replace the relay contact with a NPN transistor (2n2222) or a power mosfet transistor (IRF540) connected in parallel with the contact (collector-emitter or source-drain) but they prevent the generator to start even if the base or the gate is left floating or is grounded.
What am I missing?
Any help will be appreciate.

 

[westaust55]

From your schematic, you are shorting the supply that provides power to the coil.
Why not put the relay contact in series then use the contact closed to run and open to stop.

Many relay contacts are rated something like 10 Amps at 240 or 110 Vac but considerably less on a dc supply as the voltage and thus curent never go through a zero point to help extinguish the arc caused when the contacts open.

I may be off at a tangent . . .
Is this a coil fed from a 12 Vdc supply like an automotive cirucit or a magneto based coil as found on lawnmowers, etc ?
(I note you depict the On/Off switch as also shorting the coil)

 

[cachomachine]

You are right, it is when the switch is open that the generator can run.

It stops the motor when it is closed.

What i am looking for is an electronic part that can replace the relay contact.

 

[Goeytex]

You did not answer whether it was automotive or magneto type, so I will assume it is magneto type since it is on a generator. In that case, simply put a 10 ohm 1 or 2 watt resistor in series with the contacts and that should still kill the ignition while saving the contacts.

For an electronic , no contact solution you could use a (Logic Level) Sensitive Gate TRIAC.

See: US Patent # 5138996.

 

[cachomachine]

Thanks for your reply Goeytex.
yes this is a magneto type and i would like to addopt a electronic, no contact solution.
I am not very familiar with triac, would i simply connect mt1 and mt2 across the generator start/stop switch and connect the gate to the picaxe output pin through a 1K resistor?
Would a 2N6071 or a BT136 do?

 

[inglewoodpete]

Even though the kill switch is on the low tension side of the ignition coil, there can be many hundreds of volts generated. This is due to the reactive nature of the coil primary when the ignition points open.

If you are going down the solid-state-only path you should go for a high voltage triac or SCR. My suggestion is > 800 volts but I can't guarantee it will not fail in the long term. I would go for a relay + series resistor.

 

[Goeytex]

If you look at the link to the Patent I posted, A triac is used with a series resistor and a diode.
No specs are given for these.

I have measured the voltage on Briggs engines with the pointless "Magnetron" ignition) and it
seems to peak at about 200v max. I think a 400V Triac would probably be good enough.

I'm pretty sure they quit using points on these types of engine in the 80's or 90's. But I suppose
it is possible that some cheap eastern imports may still use them.

 

[cachomachine]

The generator in question is a Champion 3000 watts model 46555.
it uses an électronic ingnition. (no points)
I will try it with a 400V triac, a 10 ohm resistor and a general purpose diode and see what appens.
i might have to stay with the relay/resistor solution.
Thanks again for your help.

 

[Goeytex]

Be sure you get a four quadrant TRIAC that can trigger with about 10 - 20 ma. One from an old microwave
oven "MAY" work.

 

[fernando_g]

Use a high dv/dt Triac.

 

[cachomachine]

Following your suggestion and searching the internet, i found this.
http://www.vishay.com/docs/83627/il410.pdf
Is ir suitable for what i have in mind?

 

[Goeytex]

No,

The IL410 can only handle about 300ma. You need a device that will handle about 10 amps. You might could use it to drive a larger TRIAC but I would suggest a sensitive gate (logic level) TRIAC for simplicity.