Yup, another diode question

[Jeff7]

I'm figuring on making large panels here, possibly close to 100W each. And if I get a grid-tie inverter, it seems that they like high voltage inputs. So bypass diodes might be a good idea. Would these things do the trick?

[drdongle]

well 100 watts divided by 17 volts ( open ckt volts) would be about 6 amps. Might be OK but I'd parallel two just to be safe. at that price and the hassel of possible replacement use two.

[Jeff7]

Well these are going to be "24V" panels - 72 cells, for a 36V open circuit output, yielding around 3 amps, or whatever the cells are rated. The panels would be in series then. For example, this Sunny Boy inverter runs on 250V to 600V. I don't know if this high voltage requirement is typical of grid tie inverters though.

[ghurd]

Bigger is always better.

More better is always much gooder.

I would go at least 8A, probably 10A, and 600 or 1000V.

It only needs one per string.

I do not like paralleling diodes, seen too many fail like that.

I do do it for my own stuff, just don't like to do it.

I think all the grid tie inverters use high, and strange (to me), voltages.

Helps keep the efficiency up.

Just 2 cents

G-

[Jeff7]

Ok, but why such high amperage? If the panels are 1) going to be in series, and 2) only going to output maybe 3.3amps max, why get more than 6 amps? I know I usually see it recommended to go double the expected amperage/voltage, but solar cells aren't exactly prone to power surges, unless there's like a massive solar flare. But of course, a flare powerful enough to produce power surges in solar cells would also probably cook off a good portion of the world's population, so I probably wouldn't be very concerned about much of anything then.

[ghurd]

If it may be grid tied eventually, the 600Vpiv is a good idea.

My tests show a lower forward V drop as the amps get lower.

Meaning a 10A will drop less Vforward than a 3A at 2A.

And there is some mailorder nut selling 2.4A PVs with 2 parallel 3A blocking diodes installed in the box.

Sounds OK.

But I keep having to go fix them when they burn up.

Giving solar a bad reputation in MY sales area.

I figure 1 goes bad, then the other.

They split open like a hotdog on the grill.

Maybe it is the heat. I don't know for sure.

But it is going up to $50 for me to fix his 25 cent screw-ups!

I used to do it for free.

Now it is just too common and a pain in the A$$.

So for diodes I go at least 2X, usually at least 4X for the amps.

G-

(Yep, I sell the stuff)

[Jeff7]

Geez, I wouldn't expect them to die like that. Maybe as you say, it is the heat. They're already getting cooked pretty nicely, and they're colored black too no less, plus they're passing 3+ amps.

Darn. The next highest ones that All Electronics has are 40 amp 600v diodes, which are getting a bit pricey. I guess I'll search elsewhere; not like there's a shortage of places to shop.

[ghurd]

Always include 'surplus' in the search.

Someplace had 8A Schottkeys 4 for $1 on sale a while ago.

The PIV was low, but it gives an idea of 'surplus' prices.

Even $5 per PV, or string, for a diode.

Thats cheap in the total cost.

[scottsAI]

I can help out here.

With a series string the blocking diode must handle the rated voltage of all panels in series.

The current is the short current of all the panels in series.

Now that said. To make it last, by last, I mean for it to survive more than a few times. The voltage rating must be at least 1.5x or better yet 2x.

The current rating must be 1.5x to 2x more than the max current.

The last thing to deal with is HEAT. Maybe the most important.

6 amp at 0.9v is 5.4 watts of heat. The die is rated at 175'c. Hot stuff.

Depending on the mounting, 40'c rise per watt. (each Watt will raise temperature by 40'c) 5.4 watts can raise the temperature 216'c above ambient. This can happen in just a few seconds. Bingo - fired diode. Wonder why the split diode cases?

To survive the diode must be heat sinked, the problem with the axial packages, the leads are the only way to dissipate the heat. I like TO220 package diodes, they cost more, but you can mount it and keep it cool. (electrical isolation is a problem here, there are solutions.)

Parallel diodes work, mostly by sharing the heat. The diodes must all be the same.

Failure to follow any of the above will result in a short lived diode.

Have fun.

[Jeff7]

Well, those 600V 40A diodes sound like they might do the job then. 1 per panel, maybe heatsinked somehow.

I'm planning on 8 of the 72 cell panels. So, let's even say 0.55v per cell - 316v, so that's close to 2x. And 3A - won't even get close to testing the 40A rating.

However, I will of course continue to shop. A TO220 package would definitely be easy to stick a heatsink to.

[Psycogeek]

i aure am glad sombody asked some diode questions.

does anybody know of a diode that doesnt drop so much voltage?

it is HEAT that roasts em, the big differences between the higher amperage diodes are the heat sync, i HAVE the diodes that you listed, from the place you listed them, and if you ask me they are 3A diodes, they may run at 6A but they are getting quite hot.  a circular heat sync on them, or thermal epoxying them to the metal frame without shorting them , would make them 2X more heat resistant.

with my low voltage system, both the diodes, and the voltage regulator are killing my capacity :-(  there must be a better way.  for higher voltages its not a big issue, but still 1/12th of the whole Departing totally sucks (.9v drops).

on mine without the diode, the 9A of cells DRAIN back .1A when the cells are in the dark. that really isnt a big loss, when were getting 16 hours of sun, but during the winter, or multiple cloudy days, its a large drain. and any drain is unacceptable.  is .1A going to damage the cells themselves?

SO

i thought of stupid ways to stop this.  a tiny relay only uses 90ma, far less than a diode ever would.  i could trigger the relay 2 possible ways. a small solar cell set, from a bunch of broken pieces, or a Photo resister.

also i am trying to figure out how to replace the photo resister, with a piece of solar cell, beings i dont have a photo resister right now :-)

i wish each parts of my project were more simple, and easier.  like where would the entire electronics world be , if silicoln switching was improved to have JUST 1/2 the loss , and of course that loss is HEAT.

there has to be 101 ways to do this, other than going back to TUBES :-)

[commanda]

SO

i thought of stupid ways to stop this.  a tiny relay only uses 90ma, far less than a diode ever would.  i could trigger the relay 2 possible ways. a small solar cell set, from a bunch of broken pieces, or a Photo resister.

also i am trying to figure out how to replace the photo resister, with a piece of solar cell, beings i dont have a photo resister right now :-)

If you power the relay off the solar panel, it will only draw current while the sun is shining. So it won't flatten your batteries if you have multiple cloudy days.

Amanda

[Jeff7]

Standard diodes have a drop of 0.7V; I've heard that Schottky diodes have a drop of 0.5V. Not exactly a lot of difference, but it is at least something.

[Psycogeek]

that is what i thought at first , but without the diode (ahh) on the solar array, the relay gets powered by the batteries the same.

meaning without the diode, nothing knows the difference between the solar power, or the battery power.

each time i dream up some more efficient way to do these things, it gets complex fast , its no wonder why they just toss in a diode :-)

there is also no way to run a diode on Just the relay, as all there is without the diode is a line with both types of power on it.

i was reading more about MOSFETs (something i have never put in anything) they can move 12A with .1ohm of loss . if i was manufacturing cells , why wouldnt i use that method? something like a mofset, with a photoresister, sounds simple and less losses, if only i knew how to do that.

[Psycogeek]

that would be much better. i will remember that name

[ghurd]

A properly designed PV output is not going to be effected because of the diode drop.

Maybe a few ma, but not much.

Check the output charts. The amps do not really change much from 13.5 to 14.2V.

A good controller has a very low drop.

I only use Morningstars, so that's what I can talk about best.

The forward drop is often less than 0.1V, no matter what the book says.

And all of them, even the $30 plastic SG-4, is PWM, temp compensated and Mosfets.

G-

[scottsAI]

What I love about engineering is every problem has many solutions.

Yours is easy. Replace rectifiers with MosFets.

Takes work, but they can be used as rectifiers.

Much more costly, but with 10amps can have 0 .08volts drop. More expensive ones even as low as 0.04v.

MosFets have less reverse leakage currents to.

For reverse voltage protection, maybe not so good. Complex circuit.

Have fun.

[Psycogeek]

yes the morning stars have excelent specs.

http://www.mrsolar.com/pdf/morningstar/ps-eng.pdf

typical .6V TOTAL drop.  they must be using smart stuff there.

and because most of these "12V" charrging pannels are already 18+ volts, they already expect the losses that occur, and have the diode drop covered with more cells.

but i kinda doubt they make a Li-Ion charger for 3.6V , with 94% efficiency, so that dont help me none at all.

[Psycogeek]

Scotts, you wouldnt happen to know of any EASY, moronically simple online tutorials for how to use a mosfet?

this is the 3rd time i should have used them in my projects, and nobody tells how they are supposed to work.

how close to a Transistor do they work (even if i still cant run them either)

and how much loss would there be in the "gate triggering" currents?

[scottsAI]

Transistor and MosFet are much alike and nothing alike!

If you have something specific, it would make it easier to explain.

Have fun.

[ghurd]

"no matter what the book says"

I often have to reset the meter to even tell if I have a connection.

Less than 0.1Vf TOTAL drop.  PV to battery. Wire and all.

Seems the lower the battery the lower the Vf.

I guess it is PWM and my meter, but do not care really.

But no, they don't charge anything below 8V for safty.

If the 6% loss is a major problem,

maybe add an extra cell or buy larger cells.

Or just use 25 cent Schottky's.

Anyway,

For the mosfets,

Try a CDS cell pos to the gate, and a resistor gate to negative.

Adjust the resistor so the mosfet comes on when the PV reaches charging voltage.

Electronic goldmine has <50M to >1K and <1M to >200 ohms. Like 50 cents each.

Pick the mosfet carefully.

Try to get close to max out the amps.

And mosfets are very sensitive.

Just moving my hand under the bench it turned the LEDs on when the resistor was way to large.  Touching ANY wire shut it off.  Freaky really.

Mosfets are like a diode almost.

Just what is the resistance of a diode?

A 0.7Vf drop at 1000 amps is 0.0007 ohms.

A 0.7Vf drop at 10ma is 70 ohms.

Never tried the CDS thing with a PV,

but backwards works nice for LEDs that are only on at night.

G-

[ghurd]

I would love to see a 96% efficient 100w 12v windmill

with the Vopen under 20v.

[Jeff7]

Ok, another question: Searching on eBay, I've found some "Power recitifier diodes" and some "signal rectifier diodes". Both are in TO-220 cases. What are these things? Both are rated for 600V, 8A, but I've never heard those specific terms used before.

[scottsAI]

It's good that you mentioned Schottky diodes.

The good is the lower forward voltage drop.

The bad:

-Almost as costly as MosFet, (why I mentioned them)

-Reverse Voltage rating is limited, above 60v hard to find.

-Reverse current leakage is higher, when hot.

Rules of thumb for diodes:

0.7v forward voltage drop, doe not apply to power devices.

Voltage will be 0.9 to 1.4v drop at full load.

Schottky diode rule of thumb is 0.4v but under full load and as a power device 0.6 to 0.8v. A very lightly loaded  Schottky diode forward voltage drop may be below 0.3v.

I made an error in one of my earlier post. I said the reverse voltage must be rated to the entire series panels open circuit voltage. That is not correct.

If the diode was not there, the open cell could experience that voltage. But with the diode, it's conducting therefore will not see the voltage, the diode only needs to be rated higher than the voltage of the panels it's connected across. Not the entire string of panels. Each panel set will have the protection diodes.

Since this is the case, the  Schottky diode may be a good choice. But the panels get hot and that is when the leakage current will flow, for an 8 amp diode, I looked up a data sheet, said 5ma @ 25'c  to 50ma @ 100'c when hot.. A regular diode would be a better choice or the MosFet due to the lower leakage current.

Have fun.

[Psycogeek]

yup your going to need a Osillyscope to read accuratly with PWM, i have seen PWM (pulse width modulation) classed inefficient because of the lack of ability to read it properly with a normal meter. i think its VERY efficient.

PWM is great stuff, but hard to diagnose without $300+ type of electronics equiptment.

So what is CDS? is that some kind of photocell thing?

you see i have 2 photocell issues to solve in this, One at a Time though :-)

like you i want the leds to quit turning on during the day, they already are motion sencored.  and they are also going to be as much as 10-12A of them, or 30-40W. (the amps is the PITA when it comes to electronic switching stuff.)

[Psycogeek]

side notes:

i am getting 1A to my batteries, in full clouds, with it totally RAW (manually connected) that is much more power than i expected from my testing.

if i (insomuch) as put a slight DROPS in now, i lose almost everything, it would be desirable to keep the power during the total clouds times, because i only need a little to keep this thing charged.

[Psycogeek]

for example: these are the things i cant get off a SPEC sheet, it requires knowlege.

 when you trigger a mosfets gate, and you achive flow , genreally speaking do you then have ALL the flow?

meaning does it act like a switch when the gate triggers, or like a transister, which flows more the more juice you put at the gate?

if i knew that (at least) i would know how to expect what should happen, and what my losses should be.

to use it "as a rectifyer" i must trigger the gate for flow , is that correct?

nothing flows in a mosfets (input and ouputs), till you trigger the gate?

if i reverse the flow through its inputs outputs, by screwing up my Ps and Ns :-) will i destroy it?

if i reverse the power going to the gate, how much power will destroy the gate?

Can i trigger the gate with the FULL voltage of the circuit it is in, without destroying it?  or will i need to limit voltage or current to the gate?

just generally speaking, this will help me test with bench equiptment, so i can understand the part, before i apply it, and destroy a few of them :-)

[scottsAI]

Transistor compared to MosFet.

Simply stated a transistor is a current controlled current source.

The more current in the base the more current in the emitter and collector.

The transistor is a symmetrical device. If you reverse the E and C, it still acts like a transistor. The gain just goes to crap.

(something <1) Saturation voltage <0.1v

Transistor has saturation limits on the EC, 0.3 is common but under a load 0.7 or higher.

The Base does not conduct until the voltage is >0.4 volts. Most use 0.7v

The C B junction is a very low leakage diode. NOT a high current diode.

The E B junction is a 5-7 volt zener diode. In the past used in a voltage regulator.

Lots more to a transistor than you thought?

MosFet. Voltage controlled resistance, sort of :-)

The Gate control's the resistance inversely. More gate voltage the lower the output resistance. If the load 'overcomes' the resistance then it behaves like a current source.

Take a look at a data sheet, in the back are a set of curves. Starts out at 45' then flattens out. This is when it behaves like a current source.

A MosFet can be used as a switch or linear. The gate controls how it works.

MosFets come in two types. Regular, these require 10v on the gate to fully turn them on. Turn on starts much lower, 2.5 v or so. Take a look at the curves.

Logic level turn on at 1.5v fully turned on by 4.5 v or so. These cost more.

A MosFet is a symmetrical device. Reverse the S, D, still acts the same. Only the substrate diode gets in the way. (The way to use it as a rectifier). Most MosFets have an enhanced substrate diode. (actualy a diode in parallel)

Most MosFets can handle 15 to 20 volts on the gate. (relative to S)  Voltage above that will damage the device. Even for a very short time. Micro seconds. The higher above the faster the damage.

Out of time. How is this as a start? Help any?

Have fun.

[Psycogeek]

yes help lots.

i will now try and pull a rabid out of my hat :-)

so there is a diode, and a resister , in parellel, and the resister gets smaller AS the voltage on the gate rises.

why didnt they just say that then :-)

i think i now know enough to burn up the first one, and actually know WHY i burnt it :-)

thanks.

[ghurd]

The Morningstars when in PWM, are all on, or all off.

Square wave. About 358Hz by my meters (not like the book says).

A simple meter with Hz will test it.

A CDS is like a light sensitive variable resistor I guess.

The CDS only controls the Mosfet. 12A not a problem.

Except thats like 1800 white LEDs!

FOR 12V ONLY.

Don't try it with CFLs or other sensitive loads.

The LED circuit kind of slowly go on or off as the sun comes up or down.

The LED circuit still needs its own current limiting protection.

Total load should be 2A or less. A #1156 should be OK.

This is more to play with, so I left out heat sinks etc. that should be in there for permanent use.

To try it with Radio Shack parts...

Get their $2.79 CDS pack ("various ratings"), 276-1657.

IRF510, $1.99, 276-2072.

And resistors, 10Kohms 1/4w, 99c, 271-1335.

(or between 5k and 20k if the selection is poor- like here)

A resistor battery pos to gate.

A CDS cell gate to battery neg.

Battery pos to LED circuit pos.

LED circuit neg to the drain.

Source to the battery neg.

If it comes on to soon or too late,

Try 2 resistors in series, or parallel.

Haven't made one in a while but will again soon.

[scottsAI]

LOL!

Have fun!

[Psycogeek]

Can you explain mosfet Latching?

does the thing lock its positions untill its delatched or something?

i have a protection curcuit i am trying to mod so it will controll 10a instead of 1, by tossing in a relay, and the mosfet seems to be latching in states of on or offedness.

i did test the Photocell driven relay, and its working :-) i get 2 more amps out because of higher voltage, by replacing the diode (that should be there) with a relay that is triggered by a photoresister.

its just loosly wired in, un-calibrated, and not waterproofed, or aimed well, but the IDEA was functional.

the relay latches as soon as there is 2 things

enough power from the solar cells to power the relay

enough passage of that power through the photo resister from light hitting it.

it UN-latches only after the photoResister is VERY dark.

because a relay only unlatches when the power drops VERY far.

so right now, a "porch light" could potentially mess with it, lukily the porch lights are automatic motion sencors already.

[scottsAI]

MosFet = MF getting tired of typing!

MF do not latch. If your seeing this, you must have some sort of feedback. Or things are really screwed up. Or you have an SCR not a MF.

Here is how to use a MF as a diode in a low voltage application.

N-chan goes on the low side or NEG.

S pin goes towards the solar panels neg terminal.

D pin goes towards the load.

The built in diode should be conducting with light on the panel.

Now to make it a low voltage drop rectifier.

Connect the G pin to the Pos wire. (voltage must not exceed gate voltage limit).

When the voltage from the panel gets above 6v or so the MF will turn on.

If this not good. Then extra circuitry will need to be added to keep it off until some voltage level. Then tie it high..

P-chan: S to panel, D to load, G to ground.

Does this help?

Have fun.