AC to DC converter

I would rectify and filter the AC to DC, then use a DC/DC converter module to get your 6V. Choose a DC/DC that has a suitable input voltage range. 6V is not really a standard output - you might have to get a 5V converter and tweak it up. Be sure you establish your output current requirements.

I believe you mean that you have a two-stroke engine that drives a generator, producing a varying 2.4 to 6 VAC of electricity. You want to make it produce a constant 6VDC.

The easiest way I could think of is to step up your AC voltage by a factor of about 2.5. That would give you a varying voltage of 6 to 15 volts. Then you rectify it, use a zener diode to drop the maximum Vdc to around 8 or 12 volts and connect an ordinary 6V DC regulator.

This is why it's important to know the output current. A linear solution may be the best way to go for small amounts of power, but wasteful (and hot!) for larger amounts -where you may want a high efficiency switching converter.

I like the idea of the step-up transformer though.

Uh huh, but he didn't mention his output current. Perhaps he doesn't know. One disadvantage of the step-up is his current capacity goes down. Without the input current information, we can't determine the output current.

Another lesson in providing adequate information.

Interestingly enough, the linear solution has the same problem as a boost converter. The transformer increases the voltage while decreasing the available current from the secondary for any particular current through the primary. ie: The power in the primary is equal to that of the secondary (minus coupling losses). So now you have a transformer with imperfect efficiency followed a resistor loss followed by a zener which requires a minimum regulating current and is therefore lossy, followed by a linear regulator which is also lossy and whose losses depend on the current drawn.

A boost converter has 65 - 96% efficiency almost independent of Vcc and current drawn and does not require a transformer, resistor or zener (just the rectifier) so it is actually the best solution no matter what your current requirements are. Boost converters can be designed for currents lower than 10mA to currents of many hundreds of amps so the only issue is complexity and the knowledge required to build one with the desired efficiency. But, as everyone here agrees, you can't design one without knowing the current specification AND the source impedance of the generator so I'll stop here.

J

The disadvantages of this schematic are that you have to transform the voltage. If you dimension it for 6V input it will nor work for 2.4V, if you dimension it for 2.4V you`ll waste nearly 70-80% of your energy with this linear concept.

I would rectify and buffer the voltage in the range of 2.4V to 6V and complete the schematic with a switching regulator which is able to stepup/stepdown automatically depending on the input voltage, so you have the best efficiency all the time.

You find them at national.com, ti.com or linear.com.
Regards Uwe

Dear Muld:

to change your AC output to DC simply calculate the load and put 4 rectifiers together in a simple "bridge circuit"!

Everyone has given you good info in line with the limited infos you supplied.

How about detailing how much AC current the generator supplies and how much DC current you need for example.

I would also query as to why the output is so variable, is that at a steady speed or is that due to speed variations of the prime mover?

If the variations are there at a steady speed, I suspect that your generator will go to wherever generators go when they die.....then forget the whole project and buy something good.

Does this thing need X volts and X current all the time ? and is part of the problem in gas air ratio and a " phase lock loop" ( poor words I know consedering who im chating with) on fuel. carb problems fixed by exlectrical perfections?

The generator produces 750 mA. The varying voltage is due to the increase in engine rpm. At idle, the voltage is 2.4VAC and at full throttle the voltage is 6VAC. The output current should be about 750 mA as well.

Your right current across a transformer is the same - minimal IR and other generally accepted negligible losses. if you where talking .0001 voltage diff understandable but your not.

Why not control the engine RPM to obtain your required voltage. The elegant solution would be use feedback from your output voltage to control the engine throttle.

Ah well, 750mA is small indeed. Forget about the transformer thing. It'll reduce your current so much you won't have much to work with.

If you could somehow increase the rpm of your generator (and hold it there) so that you can get at least 4.24VAC out of the thing, you can forget about the transformer and just connect a bridge rectifier and a capacitor. That will give you a, no frills, 6Vdc output.

If the rpm varies by itself, you can try setting the throttle so that the minimum rpm will give you at least 4.24Vac. Then you can use a 6V linear regulator.

Putting a speed control would be nice if you can do it.

Not having full information still I suspect, it would appear that your fuel costs might exceed the produced voltage/current by a great margin, or is this a very, very tiny internal combustion engine?

For that tiny amount of current on the AC side, I would expect say a 5cc model aircraft motor for example.....

It does not sound like a worthwhile proposition to me at all......going by the limited detail available of course, but I could be wrong!