In series wiring

hotdawg23hotdawg23 Solar Expert Posts: 32
Almost hate to ask this question but am stumped on it. I am closing in on my prototype and got all the cells soldered together, neg to pos and ended up with 3 cells across and 13 high. All mounted on masonite. This stuff wont burn but will smolder if next to open flame.
This should produce about 20 volts.
When i finished i realized that the beginning cell had two tabs on front and two on the back that were open-ended, ie they were not connected to anything.
Sooo what do i do with those? ie, they must some how connect to each other but how?
Also, i had same situation at the end of the string...same question, what do i do now.
Now i have 3 columns and i want to connect all three in series to produce the desired 20 volts, How do i do that.
I know i need to end up with two wires, one negative and one that is positive but can not figure out how to attain that.
Please draw me some diagram pics on this cause telling me how may not do the trick.
All help is appreciated.

Comments

  • SevenSeven Solar Expert Posts: 292 ✭✭
    Re: In series wiring

    The simple series answer is to connect the tabs at the bottom of one column to the tabs at the top of the next column.
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: In series wiring

    Have you installed bypass diodes?

    You should not have more then 20 to 24 cells in series without bypassing diodes.

    If one or two cells become shaded they can become reverse biased. It is common for cells to have defects that create a relatively low resistance 'hole' leakage point on the cell. It can create a hot spot, a very hot spot. It has been known to create such a temp rise to crack the cover glass on a panel. The low resistance leakage point is minor under normal illuminated forward voltage of less then 0.65 vdc, but when the cell becomes reverse biased it can have much higher voltage pushed by the full illumination current of the unshaded cells. This creates a lot of power heating at the defect point of the cell.

    All it takes is a leaf or a bird taking a big dump as it flies by.

    Most commercial 37v mpp panels with 60 to 72 series connected cells have three bypassing diodes. They typically have six rows of cells. Down and back to top with two rows in series gives three series strings of 20 to 24 cells. These are in turn series connected at junction box with three bypassing diodes on each of the three sub-strings.
  • bsolarbsolar Solar Expert Posts: 103 ✭✭✭
    Re: In series wiring

    6X6 or half cells? .. your going to wind up 22-23v open circuit in full sun my guess .. heres a vid showing the layout and bussing of strings together:
    http://www.youtube.com/watch?v=v-XHSDh47sM
  • hotdawg23hotdawg23 Solar Expert Posts: 32
    Re: In series wiring
    RCinFLA wrote: »
    Have you installed bypass diodes?

    You should not have more then 20 to 24 cells in series without bypassing diodes.

    If one or two cells become shaded they can become reverse biased. It is common for cells to have defects that create a relatively low resistance 'hole' leakage point on the cell. It can create a hot spot, a very hot spot. It has been known to create such a temp rise to crack the cover glass on a panel. The low resistance leakage point is minor under normal illuminated forward voltage of less then 0.65 vdc, but when the cell becomes reverse biased it can have much higher voltage pushed by the full illumination current of the unshaded cells. This creates a lot of power heating at the defect point of the cell.

    All it takes is a leaf or a bird taking a big dump as it flies by.

    Most commercial 37v mpp panels with 60 to 72 series connected cells have three bypassing diodes. They typically have six rows of cells. Down and back to top with two rows in series gives three series strings of 20 to 24 cells. These are in turn series connected at junction box with three bypassing diodes on each of the three sub-strings.

    Yes i will be using diodes... inline and bypass. I have some 5 Amp 100 volt ones that i think will work??? Any thoughts on this?
    Thanks.
  • hotdawg23hotdawg23 Solar Expert Posts: 32
    Re: In series wiring
    bsolar wrote: »
    6X6 or half cells? .. your going to wind up 22-23v open circuit in full sun my guess .. heres a vid showing the layout and bussing of strings together:
    http://www.youtube.com/watch?v=v-XHSDh47sM

    Ok i looked at the video but i am still having trouble with the fact that each cell has a open ended negative. Cause i have connected the cells starting with the first cell positive side to next cell negative side and so on. When finished each cell will have the top part of the negative tabs not connected to anything... is this correct way to do it?
    i do get the busing method and have been able to apply the technique.
  • mike95490mike95490 Solar Expert Posts: 8,500 ✭✭✭✭✭
    Re: In series wiring
    hotdawg23 wrote: »
    Yes i will be using diodes... inline and bypass. I have some 5 Amp 100 volt ones that i think will work??? Any thoughts on this?
    Thanks.

    If they are not Schottky (low voltage drop) diodes, bad news.
    http://store.yahoo.com/wind-sun/blocdiod8amp.html
    These shoud mange 4 amps fine with a moderate heat sink, or the full 8 amps, with a massive heatsink. Or check Digikey or other paarts vendors for high amp Schottky diodes.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • hotdawg23hotdawg23 Solar Expert Posts: 32
    Re: In series wiring
    mike90045 wrote: »
    If they are not Schottky (low voltage drop) diodes, bad news.
    http://store.yahoo.com/wind-sun/blocdiod8amp.html
    These shoud mange 4 amps fine with a moderate heat sink, or the full 8 amps, with a massive heatsink. Or check Digikey or other paarts vendors for high amp Schottky diodes.

    Here is the description of the diodes. Maybe you can direct me on this as this is my first dealings in electronics.

    SR5100 5 Amp 100V Schottky Rectifier Diode, SB5100, x10 (250600957467)

    Bought them on Ebay and there is a big write up at the bottom of the page but is Greek to me.
  • hotdawg23hotdawg23 Solar Expert Posts: 32
    Re: In series wiring
    Seven wrote: »
    The simple series answer is to connect the tabs at the bottom of one column to the tabs at the top of the next column.

    Seven
    I understand about that . I kinda liken this to the batteries in a flashlight etc, positive to negative. However since i had 3 long strings of 13 cells i alternated the direction of the middle column so the positive to negative tabs would be traveling opposite direction of the two outside columns, but the flow would be the same as end on end. This eliminated the need to run a wire all the way to the top of the middle column and would put my contact wires at the bottom of the panel.
    Another problem i have is i can not get more than 5.78 volts from any single column, no matter how i test it. It there a different way to test cells other than positive to negative? (using a digital meter with the setting of 20 volts DC).
  • mike95490mike95490 Solar Expert Posts: 8,500 ✭✭✭✭✭
    Re: In series wiring

    What are the cell specs - I'm looking for max amps, your 5A diodes need to be at least 2x the normal amp spec for your cells, otherwise they will overheat.

    So your cells should not be speced for more than 2.5A. If they are, you need larger (more amps) diodes, or you risk them overheating and failing.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: In series wiring
    mike90045 wrote: »
    What are the cell specs - I'm looking for max amps, your 5A diodes need to be at least 2x the normal amp spec for your cells, otherwise they will overheat.

    So your cells should not be speced for more than 2.5A. If they are, you need larger (more amps) diodes, or you risk them overheating and failing.

    You can think of diode amperage rating as how big the diode die is. Like putting identical diodes in parallel. The higher amperage rating will give you a lower voltage drop across the diode at a given current.

    A Schottky diode like a SBR20U40CT will have about 0.35 vdc voltage drop at 5 amps. Power dissipation will be 1.7 watts so you should have some heat sinking. It doesn't need too much of a heatsink. Just clamping it to a metal junction box should give enough heatsinking. You need to use an insulating heatsink spacer because diode case is one of the diode contacts.

    Attached is a an IR temp imager picture of a shaded cell in a panel with bypass diodes across every 24 cells. The hot spot is caused by about 10 vdc of reverse bias due to shading on just that cell and an acceptable leakage spot defect on the cell. The hot spot is about 125 deg C (257 deg F) and getting about 4-5 watts of heating.

    One of the biggest mistakes homebrewers make is lack of bypass diodes. If a panel was built with 72 cell (37vmpp) and just one bypass diode is put across the total panel the shown defect hot spot heating would go from 4-5 watts to about 65 watts. Bad things will happen. Typical homebrew purchased cells are 'the bottom of the barrel' in terms of defect count and general quality, wafers that were rejected to the quality standards of the commercial PV panel manufacturers. Just another reason not to build your own panels.
  • hotdawg23hotdawg23 Solar Expert Posts: 32
    Re: In series wiring
    mike90045 wrote: »
    What are the cell specs - I'm looking for max amps, your 5A diodes need to be at least 2x the normal amp spec for your cells, otherwise they will overheat.

    So your cells should not be speced for more than 2.5A. If they are, you need larger (more amps) diodes, or you risk them overheating and failing.



    uh ohh. Amps are 3.98 per cell? or at least that is what it appears to be.
    It is .5 volts per cell and 1.98 watts percell.
    Sooo, back to ebay
    thanks for your help
  • mike95490mike95490 Solar Expert Posts: 8,500 ✭✭✭✭✭
    Re: In series wiring
    hotdawg23 wrote: »
    uh ohh. Amps are 3.98 per cell? or at least that is what it appears to be.
    It is .5 volts per cell and 1.98 watts percell.
    Sooo, back to ebay
    thanks for your help

    Better to find out now, before it's all potted up !
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • hotdawg23hotdawg23 Solar Expert Posts: 32
    Re: In series wiring
    RCinFLA wrote: »
    You can think of diode amperage rating as how big the diode die is. Like putting identical diodes in parallel. The higher amperage rating will give you a lower voltage drop across the diode at a given current.

    A Schottky diode like a SBR20U40CT will have about 0.35 vdc voltage drop at 5 amps. Power dissipation will be 1.7 watts so you should have some heat sinking. It doesn't need too much of a heatsink. Just clamping it to a metal junction box should give enough heatsinking. You need to use an insulating heatsink spacer because diode case is one of the diode contacts.

    Attached is a an IR temp imager picture of a shaded cell in a panel with bypass diodes across every 24 cells. The hot spot is caused by about 10 vdc of reverse bias due to shading on just that cell and an acceptable leakage spot defect on the cell. The hot spot is about 125 deg C (257 deg F) and getting about 4-5 watts of heating.

    One of the biggest mistakes homebrewers make is lack of bypass diodes. If a panel was built with 72 cell (37vmpp) and just one bypass diode is put across the total panel the shown defect hot spot heating would go from 4-5 watts to about 65 watts. Bad things will happen. Typical homebrew purchased cells are 'the bottom of the barrel' in terms of defect count and general quality, wafers that were rejected to the quality standards of the commercial PV panel manufacturers. Just another reason not to build your own panels.

    Good information to have and thanks for that.
    Are you saying that the 5Amp diodes i have can be used in parallel to produce 10 Amp protection. Problem is, these are round and cylinder shaped and i do not think a heat sink can be used on them.
    I have three columns of 13 3x6 cells and my plans were to put a bypass diode on each one ... will that work??
    Also, my panel will be inside a sealed glass unit with aluminum frame. Do the diodes go on the outside of the frame or inside with panel. (where all the heat will be)
  • mike95490mike95490 Solar Expert Posts: 8,500 ✭✭✭✭✭
    Re: In series wiring
    hotdawg23 wrote: »
    Good information to have and thanks for that.
    Are you saying that the 5Amp diodes i have can be used in parallel to produce 10 Amp protection. Problem is, these are round and cylinder shaped and i do not think a heat sink can be used on them.
    I have three columns of 13 3x6 cells and my plans were to put a bypass diode on each one ... will that work??
    Also, my panel will be inside a sealed glass unit with aluminum frame. Do the diodes go on the outside of the frame or inside with panel. (where all the heat will be)


    Diodes in parallel need a small resistor, to balance the loads. Resistance in solar is power wasted. Heat sinking is provided by soldering to large copper heat spreaders or wires that can conduct heat away. Sealed inside makes it hard to dissipate the heat. Best is not to have shadows that activate the bypass diodes.

    Higher wattage diodes come in the TO-220 power tab package, but something still needs to dissipate the heat.

    Slide show at http://www.naturalstudies.org/Photo_galleries/LED_driver/ go to about slide #17, where I use surface mount power resistors, and use the PCB traces as a way to carry heat away.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: In series wiring
    hotdawg23 wrote: »
    Good information to have and thanks for that.
    Are you saying that the 5Amp diodes i have can be used in parallel to produce 10 Amp protection. Problem is, these are round and cylinder shaped and i do not think a heat sink can be used on them.
    I have three columns of 13 3x6 cells and my plans were to put a bypass diode on each one ... will that work??
    Also, my panel will be inside a sealed glass unit with aluminum frame. Do the diodes go on the outside of the frame or inside with panel. (where all the heat will be)


    Underline the word 'identical'. Diodes, even of the same part number, have production variance that give them a little different voltage drop versus current. Also there is temperature difference between two separate diode that can make a lot of difference between the two.

    The diode with slightly lower voltage drop will hog most of the current. It will heat up more making its voltage drop even lower, making it hog even more current, making it hotter, etc, etc.

    Placement of diodes can be in the panel but it is much easier to put them in a junction box. That is why commercial panels array the cells in down and back rows to make the bypass diode connections in the junction box.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: In series wiring
    RCinFLA wrote: »
    The diode with slightly lower voltage drop will hog most of the current. It will heat up more making its voltage drop even lower, making it hog even more current, making it hotter, etc, etc.
    I don't remember a whole lot about the materials science of diodes, but with most materials resistance goes up with temperature. Why would a diode conduct more current (lower voltage drop) when hot?
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: In series wiring
    ggunn wrote: »
    I don't remember a whole lot about the materials science of diodes, but with most materials resistance goes up with temperature. Why would a diode conduct more current (lower voltage drop) when hot?

    It's silicon semiconductor. Same thing happens with silicon PV cells. That is why maximum power point voltage (and power output) goes down as they get hot.

    Silicon diodes have about a -2 mvdc/deg C temp coefficient. Schottky diodes have less, they can actually be positive depending on doping profile. Schottky diode resistance goes up with temp. Their conduction knee goes down in voltage but their resistance slope in conduction goes up so depending on current their voltage drop can be higher at high temp for the same amount of current. Schottky diodes can be put in parallel without as much current mismatch as regular silicon diodes.
Sign In or Register to comment.