Diode Help needed

[velacreations]

I am looking at some diodes as a blocking diode.  I found the following at DigiKey:

DIODE SCHOTTKY 45V 40A TO-220AB  Is this what I need?  It looks pretty good, and at $1.50 a piece, not too bad price, either.

I won't be seeing much more than 10-15 amps, and volts are staying below 30, for the most part.  If someone has a good source, please let me know.

Thanks,

Abe

[jimjjnn]

If you are positive that you won't go over that rating even in short bursts. Go for it. Price is reasonable. Those diodes will go higher in current if they are heatsinked. Most mfrs rate their parts at a lower rating than they will actually operate at. That way they can guarantee to the customer that the part will meet their ratings.

[velacreations]

Well,

What do you suggest?  I am new to diodes, so what I am trying to ask is will a Scottky Rectifying Diode work?

[scottsAI]

15 * 0.8 = 12watts, good idea to use a heat sink.

If power loss is of concern you can use a MosFet as an active blocking diode.

The advantage is 0.1watts power loss Vs the 12w the diode will have.

Have fun.

[scottsAI]

Scottky diode will work, will have less power loss; 15 * 0.4 = 6w.

Other wise Scottky diodes usually cost more than a reg diode.

A MosFet cost more than a Scottky diode, But, with the lower power loss MosFets are my choice, if it can work in the application.

Scottky has a lower voltage operation, make sure the voltage rating is not exceeded.

I like to use 2 x normal even 4 x.

Have fun.

[velacreations]

What is a good MosFET to look for?  How do I wire it in?

What about a regular blocking diode, like from a solar panel?  What is the voltage loss, etc.?

Abe

[velacreations]

Is there a type of diode I should be searching for?  If a regular one-way diode will work, what do I need to search for?

[picmacmillan]

 here is a store that sells all kinds of rectifiers...most take a good size one from 25- 40 amps and danb has used up to 9 of them ... http://ca.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch

 i use the 25 amp  3 of them for my small mills and will get some 40 amp ones for my piczilla machine in the works  have fun..pickster

[picmacmillan]

to add to my post above..once your on the digikey site..type in bridge rectifiers and then on the next page type in part # gbpc3501 ..this will get you where you want to be, then just pick the ones you want ie. 35 in the number stands for 35 amp..if its 25 then...well, you get the picture good luck...pickster

[scottsAI]

First of all is the 12w important?

For solar panels it may be, but a wind gen maybe not?

Using a MosFet requires a more complex circuit. (three wires vs  diodes two!)

A diode looses (or drops) 0.7v to 1 volt under full load.

So the power lost is P = I * V, V = 0.7v from diode times the current.

Scottky diode drops half the power

So what you got?

Have fun.

[ghurd]

My belief...

A solar panel, 17v is designed to account for the diode V losses without current loss.

A wind gen kind of makes 'so many' watts, and I don't want to lose any more than necessary in the diodes!

The real high current diodes have less V drop at lower amps, my experiance.

And I always go for a PIV way, way over what I would expect in the circuit, they cost about the same.

G-

[ghurd]

How about that mosfet circuit?

Couldn't that just be Gate and Drain together to the mill, source to the battery?

I mean in place of each diode.

No other parts.

A 12v 10a machine could use something like a IRF530 or IRF540.

Not sure how the internal diode is going to screw up the works.

Especially if the mill is standing still.

I am going to try it, have the parts, didn't bother with it yet.

Gain, if any, depends on which chart I take numbers from.

Some come out worse than regular diodes.

G-

[scottsAI]

Gate is not connected to source or drain. See below. If connected your way, you are just using the built in diode, not a cheap way to get a diode.

Several Circuit design considerations:

The IRF530 is Not a good choice. It's 0.115 ohm.

A 10a load, the MosFet drops 1.15v, worse than the diode.

At 40a much worse. Actually way beyond it's 14a rating.

If you already have this MosFet, Parallel several. until you get the ohms desired.

(source of each connects together, same with drain and gate, they must all be the same device, do not mix).

For the plus 12v wire the MosFet must be P-channel.

Neg wire – N-channel.

Many MosFet would be a good choice, Look at the ohm's the device is rated at.

Surplus stores have 10 power MosFets for a low price...

Power = I * I * R, the smaller the R, the lower the power losses.

Your goal in useing the MosFet is to get the P loss much lower than the Diode.

I mentioned Solar before, at $4/w, an active rectifier can save several watts, the cost is much lower then $4/w, so as long as you can make use of the power savings it is a good value. Even using the Scottky diode at half the power loss is a good value. The reverse voltage rating on Scottky diodes are much lower than regular diodes.

To use a MosFet as an active diode, connect the MosFet Source and Drain pins as if it was a diode, so the built in diode conducts with the direction of desired current flow, this is considered backwards for most applications. The gate is tied to ground for P channel, N-chan goes to positive. The voltage between the gate and source MUST NOT ever exceed it's rating. Most are rated 16v, some 20v. The gate is very fragile, easily damaged by any over voltage, No mater how short. Protection of a zener across Source Gate with a series resistor on the gate circuit can be used (recommended). The series resistor must keep the current low so as not to exceed the power rating of the zener diode, when dealing with the highest voltage expected.

I have used this method for years. I design battery powered electronics. We want the max power out of the battery. So I use this active diode as a reverse battery protection. Only on the pos wire. Works very well. Adds considerable run time to a battery pack.

If you have any questions about how this works, draw up what you think I mean, we can all look it over, as we have been.

Have fun.

[ghurd]

For solar, the amps will not really change with or without a diode.

The amps at 17V is almost the same as 0V.

The amp change from 14 to 14.7 is trivial.

Have to go over the mosfet thing more.

The PMG makes AC, so both halves of the wave need taken care of.

On a 12v system the gate will be OK, as long as the battery is in the system.

We use quite a few mosfets in 12V systems as 'power logic AND' circuits, no zeners, resistors, etc. Just the mosfet.  Sometimes even in place of relays with no protection.  Always with the +/-20V gate rating, but no failures we are aware of.

The IRF mosfets are left over from some starting dohicky we were building for diesel powered saw mills.  The 12V windmill so far peaked at about 750ma.

G-