Garden tractor alternator question.

3v0

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Garden tractors often use a single diode on the alternator output to obtain DC. Am I correct in thinking that one could replace the diode with a bridge rectifier and get a few more amps out.

It could be that doing so might overheat the alternator coils but then large draws are normally associated with things turning on so maybe it would work to provide more peak current. They are constructed like the one below. Higher output units use a full circle of coils.

 
My air-cooled John Deere has the larger set of stator coils. I put a F.W. Bridge rectifier and home-brew shunt regulator on it.
 
A full bridge doubles the voltage but current is 50% compared to a Half-bridge with a center tap with the same power out.

But if you are only seeing a single diode then there would be a DC current causing more magnetization losses from asymmetry.

So check for a centre tap. and use 2 or 4 diodes. Depends on load/source impedance. which works better. The current limit is core saturation but a full bridge is also called a voltage doubler.
 
Tony Stewart said:
A full bridge doubles the voltage...
... but a full bridge is also called a voltage doubler.
Click to expand...

BZZZZZTTTT! You are mixing transformers with rectifiers. A given winding fed to a single diode makes a peak output ~1.4*Vrms of the winding. Same winding fed to a full-wave four diode bridge makes a slightly lower peak voltage due to the forward drop of the extra diode in the forward path...

If filtering it with a capacitor, the full-wave output is easier to filter because the current pulses come at twice the rate. The full wave rectifier will draw more power from a given winding that half-wave...

 
Last edited:
Bzzzzt half bridge has less ripple , less voltage ( but twice current at same ripple) . Full bridge has more. Half Wave has excess ripple at rated load.
Note the minimum voltage in each 1,2,3

 
Tony,

Do the comparison for the OP's alternator stator; not by adding a center tap to it..., or effectively doubling the number of turns in the stator winding as your analysis assumes.

The OP has a fixed number of turns on the stator winding with two wires. He cannot add or subtract turns or add a third wire center tap. 3vO is starting with a fixed AC voltage developed by one given winding.

The only variable he to play with is either using one rectifier or four.

Now rerun your sims with that constraint and see what you get...
 
I did that with configurations 1 & 3 . Look again pls. The fourth is the input sine wave using 1:1 transformers

Note that the ripple Vp-p and minimum voltage is worse with a single diode.

Different scales but see peak +- V on each waveform is indicated.
 
Last edited:

I'm not sure you what you are referring to. I already showed the comparison between hw and fw back in post #4
 
I was referring to my diagram with notation for 1 & 3 outputs.
 
3v0 said:
Interesting but my question is in regard current for a single diode as compared to a FW.
Click to expand...
Depends on the load. If primarily lighting (headlights) , then slightly lower voltage and current but with less flicker at idle with FW. Better battery charging at low rpm with FW.
 
I was just wondering how much more current one could get by switching a single diode to a full wave bridge. No need for the kind of power a car alternator would give. Not that it matters but if I was going to do that sort of work I would add hydraulics
 
Most of those types of unregulated PM flywheel alternators that are on the small engines were built to be as cheaply made as possible not as functional as possible.

Going to a full wave bridge will get you a little extra power but don't expect a miracle! If your lucky your 1.5 amp charging system might make it to a full 2 amps.
 
I don't see two wires exiting that alternator lash-up in the original post.
If it is completing the circuit through the chassis, then full-wave rectification is more complicated.
 
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