sensing low voltage AC at varying frequencies

[Neil E.]

I am wondering if there is a solid state relay with low voltage AC input (output will switch DC). Presently the motorcycle has a single phase AC stator and uses AC for the lighting. It is equipped with a battery for electric starting. The full AC output is regulated to about 13.5 volts and a small portion is rectified for battery charging.
I like the fact that when the engine stops the lights go out. No fiddling with a key or switch to turn off lighting loads every time I stop on the trail. The LEDs in the tail light are able to run on AC with a bit of flickering. This is OK.
If I want to run the tail light on DC, but have it lit only when the engine is running, I need a relay that works on 12VAC. I called OMRON to learn that their lowest input solid state relay only goes down to 24VAC with a 15VAC minimum threshold.
I can get a mechanical relay with a 12VAC coil, but it is designed for 50 to 60 HZ operation. Since the bike can rev to 10,000 RPM I expect the frequency will exceed the capabilities of the relay coil and cause it to overheat. The fact that it pulls in at 7V and drops out at 1V is fine.
I plan to increase the headlight bulb to a higher wattage which will pull down the AC voltage at idle. This voltage drop will also reduce power to the LED tail light. I prefer to have the tail light operate at a normal brightness, so I would like to change the tail light to DC operation, but use AC to trigger a relay. The relay would then connect the tail light to the battery only when the engine runs.
Hope this is not too confusing.
Any suggestions?

 

[Phasor]

You could use a DC coil, together with a bridge rectifier, as shown (yes, you will lose two diode drops):

Else, you could connect the relay coil to the ignition circuit, so it activates with the ignition on (and connect the relay contact in series with the lamp. But you knew that, I'm sure )

 

[Neil E.]

I hadn't thought about rectifying first. Good idea, thanks.
There is no ignition switch on the bike and I prefer it that way (just have to keep my eye on the bike so someone doesn't hop on and ride away).

 

[FRIED]

Or solidstate

 

[pebe]

I am wondering if there is a solid state relay with low voltage AC input (output will switch DC). Presently the motorcycle has a single phase AC stator and uses AC for the lighting. It is equipped with a battery for electric starting. The full AC output is regulated to about 13.5 volts and a small portion is rectified for battery charging.
I like the fact that when the engine stops the lights go out. No fiddling with a key or switch to turn off lighting loads every time I stop on the trail. The LEDs in the tail light are able to run on AC with a bit of flickering. This is OK.
If I want to run the tail light on DC, but have it lit only when the engine is running, I need a relay that works on 12VAC. I called OMRON to learn that their lowest input solid state relay only goes down to 24VAC with a 15VAC minimum threshold.
I can get a mechanical relay with a 12VAC coil, but it is designed for 50 to 60 HZ operation. Since the bike can rev to 10,000 RPM I expect the frequency will exceed the capabilities of the relay coil and cause it to overheat. The fact that it pulls in at 7V and drops out at 1V is fine.
I plan to increase the headlight bulb to a higher wattage which will pull down the AC voltage at idle. This voltage drop will also reduce power to the LED tail light. I prefer to have the tail light operate at a normal brightness, so I would like to change the tail light to DC operation, but use AC to trigger a relay. The relay would then connect the tail light to the battery only when the engine runs.
Hope this is not too confusing.
Any suggestions?

Why not just feed the LED tail light from the alternator via a bridge rectifier. Fit a reervoir cap if the flickering bothers you.

 

[FRIED]

BTW

I plan to increase the headlight bulb to a higher wattage

If this is a permanent magnet alternator, this will not work.

 

[Nigel Goodwin]

BTW

I plan to increase the headlight bulb to a higher wattage

If this is a permanent magnet alternator, this will not work.

True!.

The lights are already going to be really crappy, fitting a larger wattage headlight bulb will only make them worse - the alternators are specifically designed to limit their output, so as not to blow your existing bulbs, whilst providing a slight glimmer of light at low speeds 8)

Why on earth trail bikes can't just have a 'proper' lighting system, with everything fed off the battery (like a real motorbike!) I don't know?.

 

[Neil E.]

I could mount a 3 phase stator and have a fully rectified system like a road bike, but it's not needed for my riding.
A higher wattage headlight bulb (45W instead of 35W) can be used on a permanent magnet system no problem. It's just dimmer at low rpm and better at high rpm.
I am concerned about being overtaken at night. When I slow up on a tight section of trail, I need a bright tail light. A bright tail light is a better bet than relying on a following rider having a good headlight.
This is for occasional night riding in the woods.