high voltage triac circuit

[yougarage]

I want to build a simple "switch" to interrupt the supply to a moped ignition coil (Capacitive discharge ignition), it is fed with AC voltage, varying from 200 to about 500 volt depending on the RPM of the engine.
I need to drive this switch with a microcontroller, so i thought to use a triac and an optocoupler, but due to this wide voltage range, how do I select the gate resistor?
If I choose a low OHM value, the gate will melt at higher RPM, and a high value will not provide enough current to the gate during the engine start.

 

[Harald Kapp]

You would have to limit the gate voltage anyway. Even at 200 V the gate will punch through and finding an optocoupler that withstands 500 V is no easy feat either.

How fast has the switching to be? Could you use a high voltage relay?

Why do you wan to turn off the ignition on the hot side? What happens to the switched signal if the load (ignition coil) is disconnected? Will it still be in the range 200 V - 500 V or would it rise due to the missing load? Wouldn't it be easier to switch the low voltage (6 V or 12 V) side of the ignition?

Regards,
Harald

 

[yougarage]

Hi Thanks for your reply

Here is the schematic, I also posted some pictures of measurements made with my cheap scope
The voltage scale is on the bottom left side
The high voltage wire is the green one, the signal wire is the red one

The "cranking disconnected" picture shows the measurement across the green wire, disconnected from the coil during the engine start.

The "high voltage connected" shows the same wire, but with the engine running and the coil connected.
The voltage scale for the above is 80v for each row, the 0 is located at the central row.

The "pickup connected" and "pickup disconnected "show the measurements across the red wire, with engine running and engine starting respectively, here the voltage scale is 1 v for each row.
I need to find a way to stop the engine for a while for a rev limiter, and I'd like to find a solid state solution. Maybe it's better to work on the low voltage side? (the red wire)
I tested some commercial rev limiter, but they short the high voltage wire to ground to stop the engine, and sometimes burn the coil inside the generator.

 

[Harald Kapp]

If I understand the circuit right, it operates as follows:
If the voltage on the green wire is positive, the cpacitor in series with the coil is charged through the two diodes. Ignition is initiated by turning on the thyristor. This causes a discharge current to flow through capacitor. thyristor and coil. This current is transformed and generates the spark on the secondary of the coil.

So an easy way to disable the ignition would be to just disable the thyristor. From your measurements I take it that the voltage on the red signal wire is only in the range of a few volts. Since there already is a series resistor to the gate of the thyristor, a very simple disable circuit would be a switch connecting Gate and Kathode of the thyristor. Voltage and currrent rating of this switch are limited to what you see at the gate of the thyristor: some volts + unknown milliamps (check by measuring the voltage across the gate series resistor and calculating I=U/R).
Such a simple switch could be for example a small relay or a photomos relay (or even an optocoupler) driven by a microcontroller.
If you're not afraid of directly connecting your microcontroller to the engine, even a simple NPN transistor between gate and kathode of the thyristor could do the job - although I myself would go for the isolated solution.


Regards,
Harald

 

[yougarage]

I don't have access to anything inside the dotted line, it is a sealed box.
Anyway, I could try to connect a opto transistor between the red and the white wires, so when the led is on, it sinks the red wire voltage to ground, not allowing the scr to be triggered.
What if I use an scr to do this job? This way I could only sink to ground the positive side of the ac signal, reducing the short circuit load of the pickup coil. Could this work?

 

[Harald Kapp]

You may as well just interrupt the red wire. Without control signal, the thyristor should not fire.
To improve noise immunity, place an external resistor (some kOhm) between the red wire and GND (white). put the interruptor between the generator and the external resistor:

Regards,
Harald

 

[yougarage]

This would be the best solution I think, what device may I use as interruptor?

 

[duke37]

The Vespa has a switch to stop the engine. The engine can be stopped by paralleling this switch with some device. I suggest a relay connecting the green and white (frame) wires together.

Presumably shorting the output of the alternator will not cause problems since the stop switch does this.

 

[yougarage]

The point is this, it is ok to stop the engine because it shorts the wires for a short time, but at high rpm the current is higher, and the coil will burn (already tested...)

 

[Harald Kapp]

Sorrry I don't understand the point about the burning coil. From your schematic I take it that the coil is bypassed by the stop switch (shorting green and white). Therefore there should be no current at all. Why would the coil burn, then?
In addition, if the motor is stopped, the generator should stop, too (it used to do so in my bike . So there will be only a short period of ever decreasing voltage while the motor is spinning down.

There is one issue with duke's proposal, however: Such a switch needs to withstand the 200 V - 500 V during normal operation (switch open). There are mechanical relais that can do the job.



Harald

 

[yougarage]

when the switch is open the engine runs, when the switch is closed, the voltage coming out from the coil inside the generator is shorted to ground, so the ignition coil is not supplied anymore. As ignition coil I intend everything inside the dotted line.
For example if you run at high rpm, and press the engine stop switch repeatedly for a long period, the coil inside the generator will burn
So I thought to interrupt one of the 2 wires, the red or the white
Interrupting one of these will stop the coil sparking without damage.
because the green wire has very high voltage, It would be better to work on the red wire, that has a very low voltage.

 

[duke37]

The generator probably has a high leakage inductance to limit the current and compensate for frequency variations but you are right in that it will introduce extra stress.

You say that you could interrupt either the red or white wire to stop the engine. I would not touch the white wire since this is ground (frame), go with Harald's suggestion.

 

[yougarage]

yes you are right.
I did a test, I interposed a diode (1N4007) between the red wire and the ignition coil, and the engine runs, so I think I may replace the diode with a small scr.
Do you have a specific device to suggest me? The current is very low, but I need to choose one with very low voltage drop.

 

[Resqueline]

How about a small reed relay? They have no voltage drop, are insulated, are no larger than an IC, and can be had with a 5V coil with a current draw in the sub-10mA range.
I'm sure they can be had with NC contacts as well as NO. They're very reliable and quite fast but you may not be able to drop out specific individual firing pulses.