Tomos A55 peak voltage level?

Adam Rodriguez /

Quick question, does anyone know what the peak voltage at max RPM on the lighting circuits for an A55 Tomos is? I'm interested because I'm looking to design a little box that can take in rectified and buffered AC and output DC to charge a lithium-ion battery and run lights. The basic idea is depicted below; use the ignition coil to drive a switching converter to get me a regulated 12V with no wasted power for the moped lights; when the engine is off a small lithium battery should get you an hour or so of lights before running out of juice:

To do this though I'd need to know the highest voltage an A55 can hit under load, so I can spec out the switching converter properly, and I was wondering if anyone on here knew.

Re: Tomos A55 peak voltage level?

Dirty30 Dillon /

Couple of questions: why not just go full-wave rectified to DC to charge a standard moto cell battery, than tap the lights off of that?

I'm not familiar with a switching converter? Getting hits for buck converter, which may mean you're trying to shunt regulate/step-down?

I understand the intrigue, but I am just curious why your DC conversion isn't just a full bridge rec/reg that's an off-the-shelf part. And also why you need to run the lights with the bike off?

Re: Tomos A55 peak voltage level?

Adam Rodriguez /

Here's the thing; I'd like to try to use small lithium-ion batteries for this; they weigh a ton less and are significantly smaller. The caveat is that if they're mishandled they tend to smoke and occasionally catch fire; the idea is to take the unregulated DC out of a full-wave rectifier with a capacitor on it, use a switching converter to get exactly the voltage the battery wants to see, and then another one to get exactly 12V for accessories. Apparently the existing TOMOS regulator is linear, so it just burns extra voltage instead of switching it.

Here's a link to switching vs linear regulators:

The gist of it is that a linear regulator is basically an adjustable resistor, so it gives you the voltage you want by burning the excess as heat; a switching regulator lets you go to a higher or lower voltage than what you have, and tends to be 90-98 percent efficient. On a system like a moped, where I only have 75-80 watts to work with and no ability to make more, linear regulators don't make much sense.

As far as why I'm looking at doing this, it's mostly an engineering challenge for fun; I'm trying to learn circuit board layout and a few other things, and designing something that I can use that other people might like strikes me as a good learning project.

Re: Tomos A55 peak voltage level?

So, speaking of engineering challenges....

The thing about a magneto is that it's not linear like a battery, voltage output is a function of resistance on a curve, and also rpm. Some Mopeds or old motorcycle use ballast resistors or choke coils to balance the load on the system.

I don't really know this stuff as well as I'd like to... Maybe post a more interesting subject and see if Maitland will chime in? It's definably a bit more complicated when you are trying to design for a nonlinear system, hence the suggestion to rectify first, then manage it as a DC source, throw a bunch of super capacitors on the output and accept the loss of efficiency.

Re: Tomos A55 peak voltage level?

Adam Rodriguez /

That's basically the plan; I'm mostly trying to spec out chips to build the DC-DC converter around, and while there are super wide-range automotive-qualified chips out there the more I know the less chance something pops when I plug everything in.

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