Re: 25A, 50v Full wave bridge rectifier
First, 50v unit will be more then enough for a outboard stator. 400v is over kill.
Just to clarify, rectifier will not prevent over charge. Rectifier is only used to convert AC to DC needed for your battery. In order to prevent over charge, you'll need rectifier/regulator unit. In case if you are wondering, RS doesn't carry such thing. You can get one here mentioned on
this thread if your stator output is less then 15A.
B
Just to clarify to you. Peak rms available on the average outboard stator is about 90V. That is limited by the load and the rectifier, if all is good. If the load should open up, the entire rms is applied to the rectifier, and 90V rms is about 225 peak to peak. The voltage rating of the rectifier is not the expected output voltage, but needs to be the worst case back voltage peak to peak applied.
Some ignition systems run off the battery. Most of the new ones do not. Once the engine is running, the battery, diodes, regulator, etc could disappear and the engine would keep running.
Now on to stator theory. The maximum current output of the stator is a function of the amount of iron in the core. When the iron core saturates, no more current can be generated. It does not limit the voltage.
Voltage (open circuit, no load) is dependent on speed. If the stator will put out the 15 V needed to charge a battery at idle, it will put out 90 volts at 6000 rpm. I tried an experiment once, and wired the two 20 amp stator windings on my V6 in series, and lit up 4 500 watt halogen work light to full brilliance at about 4000 rpm. (2000 watts total)
Now on to system theory. If the load pulls full amperage at say 14V, the stator will not put out any more than that. (remember, it saturates). If the stator output is low, and the battery acceptance rate is relatively high, you can use the battery as the voltage regulator. It just takes a little water, as the extra power is used to convert some of the water to oxygen and hydrogen. The output voltage can also be limited by a shunt regulator, which just routes the extra current to ground, or as in modern systems by a switching regulator, which just takes what power it needs, allowing the stator to run at whatever voltage it can.
Your question is
how does the rectifier dissipate all the extra voltage without overheating. It doesn't, it just blocks it, which is why you need a fairly high voltage rating. The blocked voltage is called back voltage, and must not exceed the breakdown characteristics of the rectifier, or it will fail. No current flows at that time, so no heat is generated. Heat is generated during conduction. The forward voltage drop of a typical silicon bridge rectifier is about 1 volt, so at full load 16 amps it will generate about 16 watts of heat, or about 52 BTU's per hour.
I suspect the unusually high failure rate of the stock Mercury rectifiers is a combination of not allowing for worst case, as in a loose connection, and the peak inverse voltage rating is exceed, shorting out the rectifiers, and the fact that it is a very small canned rectifier stuffed into a small plate heat sink and potted. The actual rectifier is about 3/8 in. diameter by 3/8 in. high.
Now, from my experience, most of the Rat Shack rectifiers do have a break down voltage greater than 200 V. They rate them at 50 so they can put whatever junk they can buy in the package and sell it with a straight face.
hope it helps
John
