DC to DC variable voltage supply

rdemart44rdemart44 Registered Users Posts: 7
Hi Everyone,
I want to find a DC to DC variable voltage supply that I could plug into my solar charge controller so I can generate 0 to 12 volts DC.
Thank you,
Bob D.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: DC to DC variable voltage supply

    Welcome to the forum Bob.

    Just to be clear, you will not be able to run such a thing directly from the charge controller; it will need a 12 Volt battery to power the variable power supply, with the panel & controller keeping the battery charged.
  • rdemart44rdemart44 Registered Users Posts: 7
    Re: DC to DC variable voltage supply

    Hi Carl,
    How about directly off the panel?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: DC to DC variable voltage supply
    rdemart44 wrote: »
    Hi Carl,
    How about directly off the panel?

    No, the reason being panels are a current source not a Voltage source. They will try to produce their full current rating (Imp) and allow the Voltage to go anywhere from zero to open circuit rating (Voc). This is exactly the opposite of what you want. Without a battery to maintain a steady Voltage source against varying current (in and out) you have no stable reference to base from.
  • rdemart44rdemart44 Registered Users Posts: 7
    Re: DC to DC variable voltage supply

    OK I guess I got to do more thinking. Is there any way to run low DC voltage items off panels?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: DC to DC variable voltage supply
    rdemart44 wrote: »
    OK I guess I got to do more thinking. Is there any way to run low DC voltage items off panels?

    Directly from panels? No, with a few exceptions. Most things require a stable Voltage input which panels can not provide. Some things like incandescent lights and DC motors don't care if their Voltage source varies as long as you don't care if their function varies.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: DC to DC variable voltage supply
    rdemart44 wrote: »
    OK I guess I got to do more thinking. Is there any way to run low DC voltage items off panels?

    I don't have much to add to Cariboocoot's response... well, one thing... depending on what you want to power a linear current booster might be of some value.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: DC to DC variable voltage supply

    Yes, you can run low voltage equipment off of solar panels--But, if you don't have any energy storage (usually a lead acid or other chemistry battery pack), you are forced to have a panel that is ~2+x larger than the load wattage to keep things running.

    And, what ever you are running has to be able to take interruptions without "issues" (i.e., a computer would lose data and take seconds to minutes to reboot. A radio simply would brown out and restart).

    A solar panel, on average, and an efficient controller produce around 77% of rated power at high noon. Take a bit of clouds, smog/fog/dust, and sun moving across the sky--It tends to be difficult to get "optimum" use of a solar array with unbuffered loads.

    In some cases, this is OK... For example, there are water pumps (usually with an electronic power supply front end) that work very nicely with solar power. The pump's output water flow is related to the moment by moment energy harvest by the solar array.

    But for many people, the want their system/project to run in less than full sun or even at night... Battery Storage is usually the only practical solution.

    Another issue with solar panels is they are described as "solar batteries"--But these are no battery you have worked with before. Solar panels when operating efficiently are really "current sources"--Basically the panel has an optimum voltage (around 18 volts to charge a 12 volt battery) and the available current is dependent on the amount of solar energy hitting the panel at that second in time (again, no power buffer).

    If you draw less than that current, no problem, the solar panel voltage rises a bit. If however, you draw a bit more current than the panel is able to supply, the output of the solar panel will plunge towards zero volts (constant current source--outputs 5 amps in full sun, you draw 5.5 amps or the sun goes behind a cloud and the available current is now 2.5 amps, output voltage collapses).

    Power = Voltage * Current... If voltage is near zero, power is near zero (no matter the level of current).

    This then gets into the type of voltage regulator you want to use. The "simple" analog regulator (3 Terminal or 3-T regulator as an electronic component) are very nice in that they can take a wide range of input voltage and output a very stable DC voltage. However, they are terribly inefficient. For example, if you have a 12 volt panel (really ~18 volt Vmp to 22 volt Voc) and want 5 volts out--A 3-T regulator will be:

    5 volts output / 18 volt input = 0.28 = 28% efficient

    And, all of that wasted heat, goes out the 3-T regulator's heat sink (72% of the total power).

    If your loads (DC Current) is low--(0.1 amp @ 5 volts = 0.5 watt load and ~1.5 watts of wasted power on a ~3-6 watt minimum solar panel)--It may be perfectly OK.

    However if you want 10 amps @ 5 volts (50 watts), that would be 150 watts of wasted power.

    Then we get into the Switching Regulator option--These are able to operate at 80-95% efficiency and can work OK with solar panels.

    However, to find a "solar compatible" off the shelf regulator may take a little work (or hacking on your side). The DC to DC switching regulators will work fine until the sun goes behind a cloud. The normal thing will happen, and the solar voltage will collapse (too much power/current draw).

    The problem is when the sun comes back, the typical DC to DC regulator can hold the solar panel input near zero voltages because it is still trying to draw too much current (issue is that DC to DC power converters are constant power devices... I.e., Vpanel*Ipanel=Vload*Iload).

    And if the converter starts out with Vpanel near zero volts, it will always try to draw too much solar current until either the panel current exceeds the power supply input current rating and/or the output load is switch off for a moment to let the panel voltage rise to nominal value (hiccup mode--interrupting output load or converter switching a every second or so to see if input voltage will recover).

    Anyway--As you can see, this is a pretty complex/wide ranging subject. And what you are asking for can be done--if you are able to accept the limitations--Or if you are able to use a battery bank as an energy buffer (or possibly even a capacitor bank if your energy requirements are low).

    In general, a current source (solar panel) charging a voltage source (true chemical battery) is pretty simple and bullet resistant (batteries still have losses, still fail over time, and are not usually very rugged can can be killed by over discharging, etc.)...

    So, with solar (or any variable/expensive/non-standard power source), we always ask what the loads/needs are... Sometimes, there are simple solutions that will work for the job. And sometimes there are not.

    I will relate one neat engineering solution from a friend decades ago... He was on an early Mars lander (I think Lander) program that needed a very quiet (low electrical noise) power supply to run some sensors. And the engineering group was working very hard on a DC to DC power converter that had very low noise--And they where having problems meeting the requirements.

    My friend asked if the voltage had to be very exact or not--Their answer was the exact voltage did not matter too much, it was the noise issue. His solution, replace the power supply with a small battery and recharge the battery between uses (or have a second battery to swap with the first). Met the requirements with a pretty simple solution.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rdemart44rdemart44 Registered Users Posts: 7
    Re: DC to DC variable voltage supply

    Thanks Bill and everyone else for the good information. I'll keep working on it while I still use a 35 AH battery. I'll build (kit) a DC to DC variable power supply and hook it to the battery.
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: DC to DC variable voltage supply

    Trying to keep this explaination to minimum techno talk, a PV panel is an illumination based current source that is capped in voltage by the inherent diode within each individual cell. In other words its current output is based on sun's illumination. If no load is applied to the panel it will rise to maximum voltage, called Voc, for voltage open circuit, which is roughly 0.65 vdc times the number of cells connected in series within the panel.

    Let's say your panel is putting out 8 amps at full sunlight. Without any load on the panel the 8 amps generated will shunt back through the panel through the cells' inherent conducting diode. The voltage at no load will be Voc. If you load the panel less then 8 amps the voltage will stay close to Voc. If a cloud goes by and drops the illumination current to 4 amps and you had a 6 amp load merrily running along at full sun, the voltage will drastically collapse to very low level, near zero, and your device load will shut down. This is why you cannot power a load reliably directly from a panel.

    As long as your load current is small or you are sure the sun will not dip behind a cloud, you can use the panel as a voltage source of near Voc.

    Just for info, Vmpp or voltage at maximum power point, is the operating point where just a small percentage of illumination generated current (whatever the illumination provides) is wasted down the cells' inherent diode. You have to modulate the load on panel to keep finding this sweat spot as sun's illumination varies on the panel. Vmpp is approximately 0.52 vdc times the number of series connected cells within the panel. Actual voltage depends on current supplied and on temperature of the panel.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: DC to DC variable voltage supply
    RCinFLA wrote: »
    Trying to keep this explaination to minimum techno talk,

    ...a PV panel is an illumination based current source that is capped in voltage by the inherent diode within each individual cell....

    Too late. :D :D
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