blocking diode size?

[Lennard]

hello all,  i am completing a 12v. 5 phase machine from Hughs plans and am in the process of wiring it up.  the rectifiers are no problem but he also shows a blocking diode in with the brake switch.  i searched the board this morning and saw a lot of information about these things and need some clarification.  in the book it says this machine is rated for 500 watts. in amps that would be 41.7.  a couple of the posts i read said to have the diode 2 or 3 times larger than the FLA of the generator.  some posts said that a diode wasnt needed.  

it seems to me that if i was to short out the dc side to stop the generator then without a diode in the pos. side then the batteries would also short out.  

my questions are

1. do i need a diode?  what size?  i am using 6 ga wire so how is it connected? (soldered, crimp connectors, etc.)
   
2. what size or kind of brake switch do i need?
   
3. what size of fuses or breakers do i need?
   
   

thanks for having a great site and thanks in advance for your help.

lennard

[richhagen]

If you have an additional diode in the DC side between the rectifiers and the batteries, you will be losing power in that diode when the machine is operating properly (voltage drop across the diode multiplied by the current)  I would think that unless your rectifiers are mounted up with the mill, I would probably short out the leads of the AC before it is rectified.  If the rectifiers were pole mounted, then I would rig a double throw knife type switch so that I would break the connection with the batteries before I short out the DC side of the mill.  That switch will close with a bang when shutting down a dual rotor axial flux mill on a windy day, so it should be very sturdy.  That way I wouldn't have the continual loss of pushing all my current through a diode that I didn't need.  Rich Hagen

[wooferhound]

I would wire the switch up like this . . .





This drawing was made up for another situation, but your genny would be on the right represented by a computer. And your batteries would be on the left represented by a power plug.

[Lennard]

thanks Rich,  the rectifiers are going to be on the tower.  this is my 1st machine and the 5 phase looked easier than a 3 phase.  my next one will be a 3 phase.  i thought the extra diode was a power waister.  im just learning about electronics.  im more of a mechanic/welder.  most of this electical stuff goes right over my head.

lennard

[Flux]

The blocking diode is normally shorted by one pole of a changeover switch. As you throw the switch the diode carries the current in the short interval before the generator side of the main rectifier is shorted. It is better than momentarily open circuiting the generator as Rich proposes, but that would no doubt work but would need a better switch.

The diode could well be a single phase bridge. You could use the positive as cathode and for the anode you could join the two ac leads together. It wouldn't matter if you used the pos and neg leads and ignored the ac ones. 2 diodes in series wouldn't matter.

Alternatively you could use a single diode with a rating of at least 20A. The normal ratings are for where the diode is always in circuit. In this case it is in circuit for only a fraction of a second and even a 20A diode will have an ac half cycle surge rating of about 200A and your time interval should be less.

A diode extracted from a car alternator would be ideal, you could parallel the 3 diodes of a car alternator half bridge.

Flux