LED as warning light on a Dynastart.

[VicS]

A question recently appeared on our owners association message board about the possibility of using a (12v) LED as the warning light on a Dynastart. Can any body see any problem. I wondered if there would be any large voltage spikes associated with an electromechanical cut out and control system that might be a problem. Any comments or more to the point has anyone tried it.

 

[William_H]

You do not say whether you wan the warning light to inndicate if it is charging or in start mode. Perhaps both would be usefull.
The dynastart circuitry can be a bit tricky but I don't think that voltage spikes would cause much of a problem to an LED. They can and often are run at high current for a short period ie pulsed mode without damage providing average power dissipation is not exceeded.

If there are contacts which have a voltage ie 12volts when it is charging or possibly not charging then a cheap LED or even a high brightness type could be connected across using an appropriate resistor ca. 120 ohms in series.

Another approach would be to use a magnetic reed switch with several turns of wire wrapped around it which carries the current. either starter or charging current. The switch operates in the magnetic field which is created by any wire but intensified by multiple turns of wire around the glass tube. It may be possible to use excess wire of the supply cable or use specific wire which must be able to carry the current without overheating. The switch is wired to power a LED with series resistor from 12volt supply or an incandescant lamp if you prefer (or even a buzzer)

I hope this gives you some ideas. PM me for more ideas. olewill

 

[VicS]

Sorry Will I have no more details. I asumed it was a warning light that worked just like the ignition warning light did with dynamos. That IIRC was connected across the cutout and I wondered if there would be a high voltage surge produced when the cutout opened that would destroy an LED. A bit like way in which diodes are blown in alternators if the battery is disconnected while the alternator is running. The person who asked would not be interested in making things more complicated by using reed switches etc. His Dynastart panel is rather exposed and he felt that by using an LED he could make a more weather resistant set up. Personally I am sure an bulb could still be used but he has a bit of a love affair with LEDS. He would be using a 12v LED with its own series resistance.

 

[roger]

The Dynastart control relay system gives continuous voltage spikes which give an average of 12 volts or so. Ina disastrous and unintended experiment on my boat it fried the regulator on a parallel connected alternator so that the alternator then gave continuous max output and so boiled the batteries. This is not something I'm proud of.
Much electronic kit can be fried by high voltages even over microseconds - think of the precautions needed when fitting computer memory chips.
My guess is that an LED would die very quickly - however its likely to be a cheap experiment.
You might get away with it if you fitted a CR circuit with a longish time constant. For design of such a circuit you will have to find someone who still has his/her textbooks

 

[roger]

Dynastart power o/p

Dynastarts have notoriously low outputs even at max. revs.
I think the alleged max is about 7 amps. This is pretty marginal for night passages IMHO.
However it may be possible to fit an alternator - thus giving a reasonable current output and use the Dynastart only to start the engine. That's the solution I adopted with an MD2 and it worked reasonably well. It does depend on you having sufficient room to get an alternator in your engine compartment. I have more details if needed.