Stabilizing voltage for LED Lighting

GregK
GregK Registered Users Posts: 1
in Off Grid Solar & Battery Systems #1
I am looking at installing an off-grid Solar PV system with 24v battery bank for use in running all the lighting (LED based amounting to about 300W in total) and a 24v fridge/freezer (perhaps averaging 50W) in a new build beach villa in Mozambique. The house will actually have AC power from the grid which I plan to use for kitchen appliances, etc. However, the power grid in Mozambique is very unreliable and I do not want my lights going out every other night. My thoughts were to keep things really cheap, simple and reliable by not bothering with an inverter and simply running the DC system independently from the AC system. I am prepared to over spec the battery bank a bit to be robust against a few cloudy days but thought an easy backup in extremes would be to run a charger from the AC system.

My problem now is that I discovered that high quality warm white LED lights will have their operating lives severely shortening if they are subject to the typical voltage variations due to a Solar PV system's charge controller eg. 20v-29v. I had a look at the Thin-Lite products but I need warm white and I need classier looking fixtures.

So I started researching DC-DC converters that I could use to stabilize the output voltage from my battery bank. I did not find many products especially one that could handle around 300W and they are not cheap. Now I am wondering about their reliability and efficiency eg. http://www.powerstream.com/dc-buck-boost.htm

Maybe this means that I need to face facts and use an inverter (but there is no way I am going grid tied since in Mozambique that will take many years and cost a fortune to get through the bureaucracy.

I am kind of disappointed that this is getting so complicated. I thought there would be loads of standard systems and products out there at sensible prices.

Any advice gratefully received.

Greg
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Comments

  • tons001
    tons001 Solar Expert Posts: 71 ✭✭
    #2
    Re: Stabilizing voltage for LED Lighting

    I wonder if you can wire two of these 24v->12v isolated output converters in series to get a regulated 24vdc output? Maybe someone smarter than I on here can answer that question. In theory (if it worked) the two in series would give you 326watts nominal at 24vdc.

    http://www.mastervolt.com/marine/products/dc-master/dc-master-24-12-24a-isolated/
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Stabilizing voltage for LED Lighting
    tons001 wrote: »
    I wonder if you can wire two of these 24v->12v isolated output converters in series to get a regulated 24vdc output? Maybe someone smarter than I on here can answer that question. In theory (if it worked) the two in series would give you 326watts nominal at 24vdc.

    http://www.mastervolt.com/marine/products/dc-master/dc-master-24-12-24a-isolated/

    Not likely to work as the negative side is bound to be a pass-through connection, so placing the 12 Volt outputs in series would create a dead short from the negative of the first unit to its positive output via the negative output of the second unit.

    In my opinion the OP is better off forgetting about using low Voltage DC for anything and just setting up and inverter system to smooth out the unreliable utility power. It's been done many times and is a lot less hassle as all the load wiring is standard AC.
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  • tons001
    tons001 Solar Expert Posts: 71 ✭✭
    #4
    Re: Stabilizing voltage for LED Lighting

    I was putzing around on their site and found this. No clue what it costs though.
    http://www.mastervolt.com/marine/products/mac-magic/dc-dc-converter-magic-24-24-20/#specifications
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
    Re: Stabilizing voltage for LED Lighting
    Cariboocoot wrote: »
    Not likely to work as the negative side is bound to be a pass-through connection, so placing the 12 Volt outputs in series would create a dead short from the negative of the first unit to its positive output via the negative output of the second unit.
    Not necessarily. "Isolated output" would mean that both the + and the - were isolated, otherwise there would be no point in isolation. This requires that there be a transformer inside each unit.
    However it can sometimes be a little tricky in terms of regulation, etc. to put two controlled DC power supplies in series.

    You do not necessarily need a fully featured 24 volt regulator, just something which can drop the voltage to the LEDs to a safe level. (Assuming that they will still work, but at reduced output at the low end of the voltage range.
    Dropping a fixed 4 volts using a power zener diode in series, for example, would give you a voltage range to the LEDs of 16 to 25 instead of 20 to 29.
    By the way, if you drop the output voltage of your 24 volt battery bank down to only 20 volts under load, you will be destroying your batteries very quickly. So I think that the range will not be as great as you think.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Stabilizing voltage for LED Lighting
    inetdog wrote: »
    Not necessarily. "Isolated output" would mean that both the + and the - were isolated, otherwise there would be no point in isolation. This requires that there be a transformer inside each unit.
    However it can sometimes be a little tricky in terms of regulation, etc. to put two controlled DC power supplies in series.

    You do not necessarily need a fully featured 24 volt regulator, just something which can drop the voltage to the LEDs to a safe level. (Assuming that they will still work, but at reduced output at the low end of the voltage range.
    Dropping a fixed 4 volts using a power zener diode in series, for example, would give you a voltage range to the LEDs of 16 to 25 instead of 20 to 29.
    By the way, if you drop the output voltage of your 24 volt battery bank down to only 20 volts under load, you will be destroying your batteries very quickly. So I think that the range will not be as great as you think.

    DC transformer? No.
    More likely it is "isolated" by virtue of having semi-conductors and over-current protection on the positive side.

    Therein lies the problem; one man's "isolated" is another man's party. :p

    You're quite right about trying to regulate the output. Basically the circuit is not designed to be 'stacked' so there's no reason to expect it to work that way. This can be done with charge controllers on battery banks because the battery provides the Voltage stability. With loads only on the output the results could be unpredictable.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #7
    Re: Stabilizing voltage for LED Lighting

    Well depends. Most leds are 12v nominal, but as a rule have a constant current driver inside that is actually very tolerant of input voltage. Dimmable ones would be the exception. If you really need 24VDC regulated from a 24V nominal bank thats going to be harder, id be with coot, get an inverter.

    However for 12v leds, if you can be bothered mucking around, get a handful of these http://dx.com/p/mini-dc-dc-voltage-stabilizer-regulator-module-red-126106 (good for 2amps) and install one on each cluster of leds.

    On ebay you can these type of things as well:
    http://www.ebay.com/itm/NEW-waterproof-DC-DC-Converter-Regulator-24V-Step-down-to-12V-120W-10A-/300955866182?pt=US_Power_Inverters_&hash=item46125e1446

    good for 10amps. Theres hundreds of variations, look around.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #8
    Re: Stabilizing voltage for LED Lighting
    Cariboocoot wrote: »
    DC transformer? No.
    Actually, DC to DC converter, since the control element for regulation will typically use pulse width modulation anyway, and so will need either an inductor or a transformer.
    If you want to call the high frequency AC isolator inside some DC/DC converters something other than a transformer, you are free to do so.
    SMA SB 3000, old BP panels.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #9
    Re: Stabilizing voltage for LED Lighting

    You do occasionally see true isolated dc dc converters. But i spent a good deal of time looking and they are all expensive, ie several hundred dollars.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #10
    Re: Stabilizing voltage for LED Lighting
    zoneblue wrote: »
    You do occasionally see true isolated dc dc converters. But i spent a good deal of time looking and they are all expensive, ie several hundred dollars.
    In which case, I have no good idea what "isolated output" might mean in the units that tons001 posted. They use the term insulated and isolated seemingly interchangeably, but also may be referring to galvanic isolation, in which the input and output grounds (and negatives) are allowed to vary by at least a few diode drops to prevent electrolytic corrosion when operating from shore power. However, I know of no way to do that without having actually isolated input and output windings in the converter.
    PS: They do have an expensive look to them, and the wiring diagram shows separate input and output negative terminals.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Stabilizing voltage for LED Lighting

    It's called "marketing": the practice of using any terminology any way you like in order to achieve higher sales. You see it a lot on "MPPT" charge controllers from China.

    Another word for it is "lying".
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #12
    Re: Stabilizing voltage for LED Lighting
    GregK wrote: »
    My problem now is that I discovered that high quality warm white LED lights will have their operating lives severely shortening if they are subject to the typical voltage variations due to a Solar PV system's charge controller eg. 20v-29v.

    First off the simplest way to drive LED's is via a DC voltage through a ballast resistor. Voltage drop of the LED is relatively constant, so a constant drive voltage gives a constant voltage across the resistor. Some power is lost across the resistor but resistors are very very cheap.

    So option #1 - simple linear regulators. They are very cheap so you can put one at every fixture. Regulate everything down to 20 volts. (Or regulate down to 24 volts which is a more common minimum in a 24V system. More efficient.)

    Option #2 - nonisolated buck converter. Buck everything down to 20 volts (or whatever works best with your LED string + ballast resistor.) Since these are very efficient you have more freedom to choose a better voltage for your lamps.

    Option #3 - isolated converter. These are more expensive but a lot more available, and can also go up or down in voltage. For example 20-29Vin, 24Vout.

    Option #4 (ideal) - LED driver. These are constant current switchmode buck converters that always supply the correct current. Keep in mind that LED's care about current, not voltage, and thus by regulating the current you do away with the need for the ballast resistor and achieve best stability in light output - as well as longest life. Your system would probably use 5-6 LED's per LED driver.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #13
    Re: Stabilizing voltage for LED Lighting

    In general yes, especially in the old days. But for modern high current leds the rule of thumb is: always use current limiting regulators. If you are pushing, as i do, 20W into a led the size of a grain of rice, you cant afford to toy with thermal runaway.

    As i said the exception is where you arent pushing the rated capacity of the led array, and require dimmable. In which case a ballast resister and voltage regulator are on the cards. Just make sure the resister value is large enough to avoid thermal runaway.

    I suspect that some commerical dimmable led products have some smarts in them that converts input voltage to output current up to to a point then limits by current alone. Otherwise youd have to control the switching regulater directly to achieve dimming.

    I dont generally bother with dimming though, just use more leds and and switchs to manage ambient levels.

    If you want to build some really cheap bright leds yourself checkout this: http://zoneblue.org/cms/page.php?view=led-lighting
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #14
    Re: Stabilizing voltage for LED Lighting
    zoneblue wrote: »
    I suspect that some commerical dimmable led products have some smarts in them that converts input voltage to output current up to to a point then limits by current alone. Otherwise youd have to control the switching regulater directly to achieve dimming.

    Actually, the smarter commercial dimmable products, designed to be used with "ordinary" dimmers, actually sense the starting phase angle of the voltage they see at their input as produced by an SCR type dimmer. The really smart ones can then dim the LED at the roughly same rate as an incandescent lamp would given the same input waveform.
    If you feed them from a variable voltage pure sine wave waveform (e.g. a variac) they do not dim at all until they fall out of regulation.

    The problem is that it is very hard, looking at the data sheets, to tell exactly now a particular fixture or lamp will react to different kinds of dimming.

    If you are talking about feeding variable voltage DC to an LED fixture, you have a different problem.
    SMA SB 3000, old BP panels.
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