another DIODE ?

[2windy]

I have read lots of different post here on the board about diodes. I am not very good when it comes to electronics as you can see from my previous post here, I'm always asking questions. Could somone tell me about the size of a diode needed for a DC motor thats being used for a wind genny that at its peak produces about 20 amps at 40 volts. I have some schottky diodes, there rating is 80 amp  45 volts. From what I have read these are not big enough. I have a 12 volt battery bank. Should the blocking diode being used be twice the open voltage? twice the current?

I have seen some for sale rated at 1000 volt 35 amp. Why would you need 1000 volt? I just don't understand this very well. If my genny is putting out 800 watts can I just use a diode that is capable of 1600 watts? [ 40 amps x 40 volts ] Or tell me diodes don't work like that.  Can I put more diodes together to get a higher rating, or should I just buy a bigger one? I now its hard to explain something like this to an idiot like me, but your help is really appreciated.

[RP]

These are two different parameters.

The voltage rating is the maximum voltage that can be placed across the diode when reversed.  Think of a check valve for water or air that has a maximum reverse pressure rating.  If you exceed the rating the valve blows the parts out of itself backwards and is ruined.

The amperage rating is how much current can you flow through in the forward direction before you burn it up.    Generally the amperage rating has to do with how well the diode can dissipate the heat that's generated by the minor inefficiency of the device.

Hope this helps

[stephent]

Looks like the max reverse voltage is a little close to what you are getting out.

Actually you will get a higher peak then you read on a meter--some meters a LOT higher.

IF your voltage stated is open circuit volts and you never let it spin fast without a battery clamping the volts--they should work fine. (but remember--doo-doo can--- xxxx the can--use will --occurr)

Usual recommended voltage or amperage is the old 2X values.

That gives you a little headroom.

Diodes "watts" aren't figured by that exact Volts times Amps like the output of a genny---diodes only have to dissapate/eat their voltage drop X the amps through (I x voltage drop + a teensy bit of R thrown in).

And Schottkey's are low voltage drop across types.

If you have some to spare--series a couple of them, doubling the reverse volts they will withstand. But then the total voltage drop is 2X just one diode--if you can spare the extra 3/10ths or 4/10ths of a volt dropping on the output.

[Ungrounded Lightning Rod]

Could somone tell me about the size of a diode needed for a DC motor thats being used for a wind genny that at its peak produces about 20 amps at 40 volts. I have some schottky diodes, there rating is 80 amp  45 volts.

You should be able to use that diode.  RP already explained most of it.

With a DC motor as generator the diode is blocking the battery current from driving backward into the genny and running it as a motor (and the mill as a fan).  Wind is unlikely to ever drive the blades BACKWARD at any appreciable speed (and thus generate a significant reverse voltage.)  So the voltage rating only needs to be as large as the battery voltage (including the boost from attachment of other chargers or pulse desulfators), plus a safety margin.  If you're using a 12 or 24V system, 40V should be  fine.

You'll have to consider the current at lower voltage loads and in wind gusts before furling.  But an 80 amp rating and a good heatsink sounds reasonable for a genny rated at 20 amps.  (Be sure your furling takes it out of the wind before it fries itself or the diode.)

Rectifiers on an alternator need a higher voltage rating than blocking diodes:  They have to block the generated voltage PLUS the battery voltage on the non-conductive half-cycle, and if the battery comes unhooked they also have to block the full output voltage of an unloaded, overspeeding mill.

[2windy]

How do you series diodes?

[Nando]

You have a DC GENERATOR -- NOT an AC --

The diode in your case is used to insure that when the Generator is OFF there is not a PATH to discharge the Battery bank.

You have a 12 volts battery bank, so the DIODE in your case needs to have a reverse breakdown good to cover the 12 volts, You can use a 25 volts or higher up diode.

You problem is the current capability of the diode, that should be at least 2 times the peak current to reduce the voltage drop

You have 80 amps 45 Volts = that is the solution for You -- use two in parallel with a good heat sink.

Your concept of:

>If my genny is putting out 800 watts can I just use a diode that is capable of 1600 watts? [ 40 amps x 40 volts ] Or tell me diodes don't work like that

is erroneous, the diode is not carrying 1600 watts, it is carrying the current ( 40 amps) and the voltage drop, that for a Schottky may run around 0.5 to 0.6 volts for a total of 0.5 * X 40 amps = 20 to 24 watts

Your heat sink should be capable of dissipating around 40 to 50 watts

Nando

[2windy]

Thanks for explaining this to me and not laughing out loud.

[GeeWiz]

There seem to be a lot of different possibilities for diodes.

I was not happy with the one that I did find, but it's working.

Does anyone know of any good suppliers ? maybe a couple of URLs ?

Thanks

Gw

[alancorey]

I'd look at general electronics suppliers like:

http://www.jameco.com http://www.digikey.com http://www.newark.com http://www.mouser.com

but I don't know what finding big schottky diodes is like.  I only needed 3 amp so far.  The quality is likely to be better than taking chances on eBay.

Just one comment on heat sinks: paint them black!.  A couple light spray coats of flat black paint almost doubles the rate at which they dissipate heat and doesn't cost much or take long.  Don't mount your diodes on top of the paint though.  I see pictures on here of naked aluminum heat sinks and I cringe.  I've got one bench power supply I built about 30 years ago and painted the heat sinks after one time I blew it up a few years later.  Haven't had much trouble with it in recent years.

  Alan