testing short circuit current

[john p]

Sunking yes he did get an answer in your first post but then he got to wanting to find tiny differences on posts 4 and 10 and 28. . and my last few posts have tried to show thats what he wants to know, he wants to know about the tiny differences and he feels if he finds them and knows how to exactly measure them than all will be ok with him..
but during this whole thread it has wandered all over the countryside like browns cows. with everything form mike telling him to use arc welding rods to crimping connectors with vice grips(will stop doing that in another 10 yrs) to NEC rules to ???? the only thing left out was how to build your own Ford Edsel from a kit of parts.

I love to go a wandering along the mountain track

[Sunking]

So ohms law does not apply for current sources, please enlighten me

Sorry you do not like my answers, but of course Ohm's Law it applies as it will determine a voltage drop across the resistance. But it is irrelevant with respect to the Isc test. At the other extreme if this were a voltage source and you shorted it like say a battery, you got a huge problem on your hands, well if you have any hands left after shorting it out.

What I am trying to convey the resistance is so low it is insignificant and can be ignored, and not something you have to concern yourself with for the Isc test.

For example using you numbers of 8 amps for the cell or panel and overall resistance of .002 to .008 ohms works of to .016 volts to.064 volts, and .128 .512 watts. It does not matter if it is a single cell of .5 volts @ 4 watts (8 amp test), or 100 cells in series for a 50 volt @ 400 watt ( 8 amp). The Isc test will give the same exact result if everything is working correctly.

In an Isc test all you are looking for is if the current meets the specs or not. Let's say if one of your cells had a bad hairline fracture which results in high resistance, the Isc test will fail every time. Toss it in the trash heap and move on.

Just remember the Isc test will read low (about 9 %) if testing a single cell using a Shunt current method.

[Sunking]

Somehow you have all missed Rif rafs need..

John what did I miss that was not covered in the very first reply? He asked if it was safe or not to perform a short circuit test.

[greenHouse]

John,

Never mind. I think there's something I'm missing. Or something like that.

[john p]

Julie my point to you was I know you have a good electrical technical knowledge. Ive seen other posts of yours on other forums , but sometimes making a joke (that I didnt see any point to) does not help the person trying to learn more.. ok?
I also say I dont always agree with your posts but have same problem with good on line friend Sunking . and on this site Mike. but thats life.
They sometimes think my ideas agricultural..(farmer joe.. John).
Despite being farmer joe sometimes I do work where we have one of the best equipped design and testing of solar and battery related equipment and other exotic things facilities,, we dont do commercial things for sale.
Rif Raf has electrical knowledge he just wants to learn more.

[greenHouse]

John,

I don't think any who can't grasp why Ohm's Law is Just Fine has a grasp on the basics. I might have missed him pulling Sunking's leg(s) or something, but there seems to be a major disconnect going on.

There definitely seem to be cliques around here and in jokes and a few other things. I'm pretty boring and my son insists I can't get jokes.

[john p]

Somehow you have all missed Rif rafs need.. He knows all about ohms law you dont have to teach him that.. we do have a quite good education system in Australia at technical colleges..
His needs are simple he really wants to find very small differences between the output of cells the losses across different connections and losses across different parts of the wiring..

Example all cells read 2.65 v . ok he is happy all strings put out 5.95 amps he is happy
now if one cell putsout 2.45v he not happy as that cell if inferiour,, if the best output string puts out 5.95 amps and the worst? one puts out 5.45a then that string has some connection or wire problem he is not happy.

Julie your sentance "And besides, knowing R comes in handy sometimes. Like, if he ever has to calculate ohms per kilofoot. Or furlongs per fortnight." is nothing more than just a waste of typing... useless crap, try to be more constructive. you have a good well educated technical brain USE IT.

[greenHouse]

I know just pointing out Hall Effect has absolutely no resistance at all, and extremely easy to use. No pro uses a Current Shunt anymore, those are antiques found in museums and old textbooks.

Right, but that's besides the point. There =is= a reason to learn Ohm's Law and you've got the guy thinking it doesn't work.

And besides, knowing R comes in handy sometimes. Like, if he ever has to calculate ohms per kilofoot. Or furlongs per fortnight.

[Sunking]

A Hall Effect meter should still work -- amps is amps.

I know just pointing out Hall Effect has absolutely no resistance at all, and extremely easy to use. No pro uses a Current Shunt anymore, those are antiques found in museums and old textbooks.

[greenHouse]

True except for Hall Effect meters.

Yeah, but he's trying to grasp Ohm's Law, not the Hall Effect ...

A Hall Effect meter should still work -- amps is amps.

[Sunking]

Amp meters of that sort use very small values of "R". Close enough to 0 not to muck up the measurements, high enough to actually have a V to read.

True except for Hall Effect meters. Most all current Shunts are based on 50 millivolt full scale reading. 50 millivolts is not zero by any means, but close enough it can be ignored and considered ZERO.

So a 10 amp current shunt has a resistance of .05 volts / 10 amps = .005 ohms. Close enough to Zero for me.

[greenHouse]

sunking how silly of me, so ohms law does not apply for current sources, please enlighten me

Hint: R != 0.

Because R != 0, V = I x R != 0.

Amp meters of that sort use very small values of "R". Close enough to 0 not to muck up the measurements, high enough to actually have a V to read.

[RifRaf]

sunking how silly of me, so ohms law does not apply for current sources, please enlighten me

[Sunking]

all i want to do is measure the short circuit current, if we simplify the argument to Vs=0 then all testing is void, ohms law is broken and Is=0 because I=V/R an V and R are both 0, so there is no short circuit current, see how silly this gets and does not help any.

Until you grasp the idea a solar panel is a current source, and not a voltage source.

[RifRaf]

i don't get your point sorry, my test gear is fine for millivolts, amps and milliohms, it does not matter if i am testing a single cell, a string, 2 strings in parallel, a panel, an entire array or a chip from a cell, ohms law still applies exactly the same way.

personally if i was testing a set of cell strings and most had 0.002R calculated resistance and one had 0.008R i would be concerned, there will be a 4 fold increase of voltage drop across that string than the others, 4 times more power loss than the 0.002R string. in real testing the resistances appear to be higher than these numbers which makes it matter more.

please remember the whole point of testing is to ensure matched strings of cells to make the best functioning diy panel, am trying to find issues in order to fix them, not interested that a perfect cell has almost no voltage at maximum current, am trying to find the Not perfect ones