Author Topic: New Bridge Rectifier  (Read 4388 times)

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taylorp035

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New Bridge Rectifier
« on: February 22, 2010, 05:50:09 PM »
For about forever, I have been needing a decently large rectifier for my windmill projects.  In the past, I have stripped every one I could find out of every scrap piece of electronics in my house.  Unfortunately, the largest I could find was probably only good for 2-5 amps.  


So I happened to be at the local Radio Shack and I found a 50V 25amp Bridge Rectifier.  It says on the back that it drops the voltage by 1.7 volts.  My plans are to use it first on my treadmill windmill.  I was wondering if anyone has any words of advice so I don't burnout my big investment (all of $3.29  :)  ).  It has an aluminum back, so maybe it is meant to be heat-sinked.  I plan on using it as a diode, not a bridge rectifier for the time being.


So, if the there is a no load voltage of say 70v, would it hurt it?

And, what do you think the 25 amp rating is good for (10 seconds, a minute?).


I imagine that 1.7 volts * the amps is going to turn into heat.

« Last Edit: February 22, 2010, 05:50:09 PM by (unknown) »

Rover

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Re: New Bridge Rectifier
« Reply #1 on: February 23, 2010, 03:27:45 AM »
It is what I do with my treadmill motor, just connect the output from the motor to the AC posts. Battery to the + and - post accordingly.


50v/25 amp should be fine, I can't imagine you burning it out based on a treadmill motor. I use 200v/35 amp bridges becaue I have them around pobably overkill


Please explain the scenario where you expect to see 70V no load?

« Last Edit: February 23, 2010, 03:27:45 AM by Rover »
Rover
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libra

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Re: New Bridge Rectifier
« Reply #2 on: February 23, 2010, 05:36:15 AM »
Look at alternator repair shops. It is possible to get 12v and 24v modules at 100 amps for good price, and they have a good heat sink.


Libra

« Last Edit: February 23, 2010, 05:36:15 AM by libra »

ghurd

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Re: New Bridge Rectifier
« Reply #3 on: February 23, 2010, 07:23:46 AM »
Yup.  They do get hot.

It does not take a whole lot to keep them cool.  Something flat and heavy, with good mass under the bridge, and lots of surface area.

I have used heavy angle AL, AL car parts, even a security light housing because it was in the truck. Yee-Haw. :)

http://www.otherpower.com/images/scimages/2050/FrigCntrlr.jpg


Heat sink compound helps a lot.


Using both AC terminals helps spread out the heat, IMHO.

No need for a negative connection on the bridge, just need to know the polarity coming out of the motor.

http://i701.photobucket.com/albums/ww20/ghurd1/Sketches/BridgeAsBlockingDiode.jpg


G-

« Last Edit: February 23, 2010, 07:23:46 AM by ghurd »
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taylorp035

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Re: New Bridge Rectifier
« Reply #4 on: February 23, 2010, 02:16:33 PM »
Maybe if I had it free wheeling and it was producing over 50v (no load).  Since there is no load, I am guessing it would be ok.
« Last Edit: February 23, 2010, 02:16:33 PM by taylorp035 »

taylorp035

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Re: New Bridge Rectifier
« Reply #5 on: February 23, 2010, 02:18:04 PM »
Thanks guys!


I should have some super large heat sinks for some big 90's servers.


As soon as there is wind, I will give an update.

« Last Edit: February 23, 2010, 02:18:04 PM by taylorp035 »

Rover

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Re: New Bridge Rectifier
« Reply #6 on: February 23, 2010, 03:10:56 PM »
I'm still curious on how you expect to see 70V ? I can't see a way that this could happen with the treadmill motor (similar to mine) through the bridges.


The battery should hold down your voltage way below 70, hopefully below 16. If not , Congratulations! :), you'll need a dump load of some sort.

« Last Edit: February 23, 2010, 03:10:56 PM by Rover »
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TomW

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Re: New Bridge Rectifier
« Reply #7 on: February 23, 2010, 04:09:07 PM »
In my experience over voltage kills more rectifiers than over current.


Doesn't take much to pop the junction and brick the part. Sometimes just a few volts will do it even static can do it on some.


Tom

« Last Edit: February 23, 2010, 04:09:07 PM by TomW »

Ungrounded Lightning Rod

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Re: New Bridge Rectifier
« Reply #8 on: February 23, 2010, 05:13:16 PM »
if his battery comes unhooked while the mill is spinning in wind the voltage could go up quite high.


If you're going to use rectifiers with such a low voltage, hang an "alternator protector" across the DC output side of the rectifier.  This is a varistor that holds down spikes (and gets REALLY HOT and/or eventually burns out) if the battery comes unhooked under drive.

« Last Edit: February 23, 2010, 05:13:16 PM by Ungrounded Lightning Rod »

scoraigwind

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Re: New Bridge Rectifier
« Reply #9 on: February 24, 2010, 12:03:37 AM »
If the battery comes unhooked then there is no voltage across a blocking diode.  It's only in the case of an alternator rectifier that you would start to see high voltages with a disconnected battery.
« Last Edit: February 24, 2010, 12:03:37 AM by scoraigwind »
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Norm

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Re: New Bridge Rectifier
« Reply #10 on: February 24, 2010, 06:21:40 AM »
I see what you mean by 'as soon as I get some wind'

  Right now I look out and the little yellow

windmill isn't turnng....so you must be experiencing about the same.....

I look at the weather site and it says

 N  0 mph.  ??? how do they know which direction

when it isn't blowing  ???


   (yeah yeah....I know last time it was blowing

...it was probably coming from the North)

« Last Edit: February 24, 2010, 06:21:40 AM by Norm »

TomW

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Re: New Bridge Rectifier
« Reply #11 on: February 24, 2010, 08:26:09 AM »
Norm;




   (yeah yeah....I know last time it was blowing

...it was probably coming from the North)


By golly, Glen was right! Youy are smarter than you look.


Hehe. Just could not resist.


Tom

« Last Edit: February 24, 2010, 08:26:09 AM by TomW »

taylorp035

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Re: New Bridge Rectifier
« Reply #12 on: February 24, 2010, 02:54:59 PM »
Good point!
« Last Edit: February 24, 2010, 02:54:59 PM by taylorp035 »

taylorp035

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Re: New Bridge Rectifier
« Reply #13 on: February 24, 2010, 02:58:37 PM »
This winter has been very, very calm.  I bet we are averaging 9-10mph instead of the normal 14-16 mph.


Since I put my windmill up 3-4 weeks ago, it has only spun on two occasions.  :(

« Last Edit: February 24, 2010, 02:58:37 PM by taylorp035 »

Ungrounded Lightning Rod

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Re: New Bridge Rectifier
« Reply #14 on: February 24, 2010, 07:40:49 PM »
I missed that it was a DC generator.


So the voltage just fine (because the mill is not about to be spun BACKWARD at high speed).


Also:  You can use just one of the diodes in the bridge (or two in parallel for higher current) and cut your voltage drop in half from the 1.7V rating:


Tie the two AC terminals together with a piece of wire and connect the MIDDLE of the wire to the + side of the genny.  Tie the + terminal of the brick to the + side of the battery.  (Or connect the - terminal to the genny and the middle of the AC jumper to the line to the battery + if that's more convenient.)


This will bring your current rating up to 50ish amps or more.  (Diode imbalance means it is less than double for a given heatsink temperature, but having only two of the four diodes conducting means the brick runs a bunch cooler than was contemplated in the ratings.  The trick with tying the middle of the wire to the external connection makes the wire act as a pair of small balancing resistors which helps the balance a tad, too.)

« Last Edit: February 24, 2010, 07:40:49 PM by Ungrounded Lightning Rod »

ghurd

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Re: New Bridge Rectifier
« Reply #15 on: February 25, 2010, 08:12:52 AM »
I am not completely convinced connecting to the center of the jumper is important at this scale.


If both wires are connected to the first AC terminal, the jumper is 1.25" long, and carrying 20A, then the imbalance is only about 0.0026V.

Probably less on a well soldered jumper.


That's when feeding about 40A through a 25A bridge, which is always a bad idea.


With 15A total current, imbalance is only about 1mV.

Change the jumper to #10 and the imbalance is about 0.6mV.

G-

« Last Edit: February 25, 2010, 08:12:52 AM by ghurd »
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