Rectifier question...

[windstuffnow]

 

  I'm using a pair of 50 amp rectifiers on my latest dual rotor project.  This is the first time I've used this type and am a little concerned.  I've been using the AC Delco type but my supplies have dwindled.   So here are the questions...

  1. How many amps will these handle total when using 2 of them.  ( typical 3 phase - 2 wires on 1 and the last wire on the other with one AC tab unused). These are standard square rectifiers rated at 50amps.

  2. Are they basically 2 pairs of 25 amp diodes in each ? or are they actually 50 amp diodes?

  The reason I ask the second question is because this is how AC/Delco does it, for a 40 amp 3 phase rectifier there are 3 pairs of 20 amp button diodes.  For the 60 they use 3 pairs of 30amp diodes.

  3.  If the diodes blow will they allow current to flow from the battery to the stator? or worse short circuit?

  I'm a bit concerned since the new turbine has been making 58-60 amps on windy days... I may just go and buy a new rectifier for a 100 amp GM alternator.  Or... would it be possible to double each output wire and run each wire to a single rectifier using both AC lugs.  This would double the current carring capacity... yes?

Just showing off my electronic component ignorance...

Thanks

Windstuff Ed

 

[commanda]

I'm not 100% sure how they rate the individual diodes in a bridge set, but each diode would see the full peak current for one half the cycle. I believe heat rise is the limiting factor with these. The spec sheet should give a range of peak currents & times, below which you should be safe.

I'd use 3 bridge's, and common the two AC lugs together for a three phase arrangement. This should be good for 150 amps; a nice safety margin over your 60-80 amp output.

Amanda

[Nando]

Alternator diodes, now days, run around 35 to 70 amps and some may go to 120 - 150 amps.

The diode current should not be greater than the rated current and, as well, put a good heat sink.

Regards

Nando

[ghurd]

Hi Ed,

1. and 2. The 50a rectifiers will handle 50a per diode.  It is 4, 50a diodes, with current going through 2 at a time. Don't exceed 50a... Ever... If you want them to last.  It is best to derate them by at least 30%.  Some standards show 40% derating (50a diode handles 30a).

"the reason.."  3 pair of 30a diodes = 90a. Derate 33%=60a.

Also- they are AC animals intended to be 'on' less than half the time. RMS math stuff (12v battery, no current flows below 12.7v).

3. I've seen them blow both ways. Open or short.  More often open.

"Or... would it be possible to double each output wire and run each wire to a single rectifier using both AC lugs.  This would double..."

I don't really get the question.  

Kind of sounds like all the current in 1 side would go through 3 diodes (2.1v) and all the current in the other would go through the last diode of the 3 earlier diodes, in a way. But it would all still go through the last diode of the 3 and neither of the first 2.

But don't put them in parallel if thats what you mean (one diode has a Vf of 0.6999v and the other 0.7001v. If they are too close to the Imax, the lower v will pop from excess current, then all the current goes through the other and it pops).

I think a 100a GM rectifier would be OK.  120a would be better if they make one. If in doubt, I would parrallel them (contradictory to above).  What I have seen, as I increases so does Vf, and that helps balance it out (0.6999Vd1 increases with I until 0.7001Vd2 is exceeded and begins taking some of the total current load).

Me, if I thought it would make 80a, I'd go for a 120a rectifier, or 2 parallel 100a rectifiers.  I'd heat sink the tar out of them either way.

I've seen 2.4a solar panels pop 2 parallel 2.5a blocking diodes (I did not put them in there- I just fixed it).

G-

[Flux]

Those bridges are rated for single phase resistive or inductive load.

The rating into a capacitor or battery is usually lower. Using them on 3 phase is probably somewhere between the two.

I would agree with Commanda about using 3 bridges each one connected with its ac terminals together.  The diodes within one bridge are more likely to be matched as regard forward drop than any others.  I think I would still only consider it a 100A rating but should be well adequate for this occasional 80A.

I would make sure that they are heat sinked to the extent that the point where the bridge is bolted on the sink doesn't go over 70 deg C.

Usually diodes fail to short circuit and if 1 goes it will overload the stator. If 2 go it could well stall the prop and be less destructive.  If 2 in one arm go down they will take the battery fuse or if there isn't one will usually blow themselves apart.

Flux

[hiker]

use 4 rect"s--wire the same as two--then you will have a 100amp rating.....

[windstuffnow]

  Thanks everyone...!  Since I have another pair of the 50 amp units I will add one more and common the AC pair to each of the 3 lines from the alternator.   This sounds like it would have a good current carrying capacity as well as add in a bit of a safety factor due to redundancy.

  This would also increase the voltage loss from 1.4 to 2.1 correct?  So my low wind performance would raise a bit... maybe from 6 mph to 6.5 to start charging... We'll see.

   So from what I gather here, the three pairs of diodes should handle up to 75 amps?  This could be why they haven't actually blown as yet.  They've gone 10 amps over their separate rating already.  In any case there will be a change made shortly... They are, however, mounted on a good heat sink.  The rectifiers are in a box but the heat sink is exposed to the cold weather outside so I'm sure this helps a little.

Thanks again...

Windstuff Ed

[Flux]

Ed

There is no reason why paralleling the diodes will increase the volt drop, in fact for the same current the drop will be slightly less.

[commanda]

Flux is correct. 2 diodes in parallel will have a slightly lower voltage drop, due to the dynamic Vbe of a PN junction.

Amanda

[richhagen]

Just some thoughts on this, take them for what they are worth.

Paralleling the diodes should reduce equivalent resistance seen by the circuit and hence the voltage drop across the diodes should be slightly less than a single diode.  

Paralleling the diodes may not result in a 50/50 split of the current across each diode due to slight differences.  The voltage drop will however increase slightly across a diode with increased current, so there would probably be a reliable derating percentage or number of amps when connecting diodes manufactured to reasonable tolerances in parallel.  (if the diodes are within .001 volts of voltage drop, then x number of amps of current would cause a change in the voltage drop across the diodes of this amount anyway)  

Rich Hagen

[RC in FL]

For low voltage applications there are two basic possible diodes. Regular silicon diodes or hot carrier (Schottky) diodes.

Silicon have voltage drop of between 0.5 vdc at low current and up to 1.0 vdc at highest current rating.  

Schottky range from 0.3 vdc at low current up to 0.7 vdc.  Schottky are limited to reverse breakdown voltages of 50 to 100 v max.  

For a 48 vdc system you need diodes with breakdown of 150 to 200 piv since the rating needs to be twice the output dc with some margin.  (+48 to +60 vdc while the non-conducting negative cycle from the altenator is going -60 vdc, resulting is about 120 volt reverse bias on diode).  This pretty much rules out use of the lower voltage drop Schottky diodes.

Basically larger current diodes are just bigger parallel structures like putting a bunch in parallel.  The diodes must be matched meaning same voltage drop for same current otherwise the lower drop diode will hog the current flow.

The current handling capabilty is ecentially when there is 0.8 to 1.0 vdc drop across the silicon diode.  Lower voltage is better because there is less loss in the diode and less heat to dissipate.

If you have 60 amp flowing through a silicon diode with 0.9 vdc drop you have 54 watts of heat generated by the diode.  That is a lot of heat to dissipate requiring a very large heatsink.  If you don't dissipate the heat the diode junction get very hot and eventually fails do to wirebond/pad metal failure or redistribution of the diode doping causing it to lose its diode characteristics.  

Keep um cool, and below 1.0 vdc drop across them.