Hooking up LED lights to 12V battery

Discussion in 'OnBoard Electronics & Controls' started by Nickpontoon, Oct 5, 2012.

  1. P Flados
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    P Flados Senior Member

    LEDs can be confusing. For lighting products, most seem to fall into one of two categories.

    One category is lighting with a large number of low voltage, low current, low cost LEDs. Typically you will have one or more "banks" with all of the LEDs in one bank wired in series. The lamps start to light up at a threshold voltage (~75% of nominal???), and get to the design operating point at nominal voltage. At above nominal voltage, light output goes up a little, but power goes up a lot. With the increase in power comes lots of heat that is probably bad for element life.

    I re-wired a lamp element from one of those cheep 3 - AAA cell flashlights to use with a re-purposed small wall transformer (9 vdc output I think) as my garage nightlight. I put a couple of red LEDs robbed from some dead gadget in series with flashlight LEDs to get the voltage right. It points up to the white ceiling, and makes it safe to walk around (I exit my house through the garage before dawn). It uses next to nothing in terms of DC power.

    The other class is the high output LEDs. These are designed to push a lot of power through each LED and the ones I have seen are designed to run hotter. These are probably less efficient than the other category, but still more efficient than most other types of lighting.

    Making sure that you have the right voltage at each LED is important.
     
  2. michael pierzga
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    michael pierzga Senior Member

    Sorry about the bad picture...dark over here as I fit LEDs.

    What is the circuit on the back of this LED ? Its light output is roughly similar to a 20 watt incandescent

    The piece of paper is its packaging and description.
     

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  3. P Flados
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    P Flados Senior Member

    There is more circuitry than needed to simply drive LEDs from a proper voltage.

    The extra may be for the extra versatility of the product. I noted a wise range of input voltages, 8 -30, and I noted it was dimmable.

    The power required is 1.3 watts. This translates to 0.11 amps when supplied by 12 volts DC.
     
  4. Don H
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    Don H Junior Member

    Hi Michael, P, the components on the back of the pcb make up the led driver circuit. (Sorry Nick to go away from your original question but this may be of interest to you).
    All of the high power leds need some form of led driver, a resistor off the battery is not good enough. I did a lot of design work with these high power leds when they first came out several years ago but it’s too competitive now and I haven’t kept up with the latest chip designs.
    Basically though, current through a led is zero until the voltage reaches the led’s threshold and the led lights. This voltage is slightly different for every led. Once the led has lit, the Voltage vs. Current curve is very steep and any small increase in voltage causes a substantial increase in current. This could potentially destroy or shorten the lifespan of the led if not carefully controlled.
    The LED driver maintains a constant current to the leds no matter what the input voltage is within its 8 to 30v operating range. It does this by varying the pulse width of its output.
    Dimming is able to be done by and external control over the pulse width.
    The voltage range of 8 to 30vdc is high to allow the same unit to be marketed for 12 and 24v systems with some tolerance at each limit. Even though the driver and led light might only be connected to a 12 or 24v system a resistor or linear regulator would not be sufficient due the heating that would occur.

    Thanks Don
     
  5. michael pierzga
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    michael pierzga Senior Member

    Thanks...that makes sense to me.
     
  6. rxcomposite
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    rxcomposite Senior Member

    Yes you can but it is not a good engineering practice. LED's are current controlled device therefore a constant current source and a series LED is preferred. As always with design, less parts count, less to fail and cheaper to make.
     
  7. rxcomposite
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    rxcomposite Senior Member

    That is an electronic pulsing circuit. LED's are pulsed to obtain the highest power rating to minimize heat generation.
     
  8. Don H
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    Don H Junior Member

    Hi rxcomposite, the led light shown in Michaels photo is for lighting, it does not require a constant current source in fact they allow for dimming in the fitting as well.
    Even when dimming is not required leds do have a maximum current specified but never a constant current. Each LED is different and will have a different current through it for the voltage applied. It is not bad engineering to use parallel leds. Many manufacturers do this. LEDS generally fail to an open circuit condition rather than a short condition. With a parallel system the light fitting will still operate but with a lower light output. The series system will lose all light. Large led lighting displays will often use a combination of series and parallel connection as a compromise between lost leds and the higher current requirement of a full parallel system. You can see this on large signage where a block of leds may be out rather than a whole row or column. There is no real saving on complexity or parts cost between the 2 types although a series driver does require good filtering to pass CE certification for conducted and radiated noise (CISPRR15). There is no doubt a series system is by far the most common for domestic situations. Current is limited and it’s hard to damage the leds or driver. A parallel system limits the voltage only so leds can be damaged if the feedback isn’t good enough.

    Not quite sure what you mean by ‘ That is an electronic pulsing circuit. LED's are pulsed to obtain the highest power rating to minimize heat generation ‘

    In this case it is a series circuit and the driver is designed to limit the current through the leds. Heat generation is determined by the efficiency of the LED not the driver. If dimming is used it will narrow the on time from the driver and this will lower heat but of course the led will be dimmer so I don’t quite understand what you are getting at to say leds are pulsed to obtain the highest power rating.

    Thanks Don
     
  9. rxcomposite
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    rxcomposite Senior Member

    Don H,
    I am quite confused by your answers. I realize this is not an electrical/electronic forum but we could be mixing discussion from general principles for a typical DIYer or engineering design.

    Firstly it is not recommended to parallel LEDs for the reasons described in the article attached with some math. This is to prevent DIY’s from making a mistake. On the other hand, manufacturers do parallel LEDs for specific use if it is of the same batch/bin numbers/electrical characteristics. LED’s are mass produced, tested individually, then segregated accordingly. Not for the off the shelf guys.

    Second, LEDs are current devices, meaning a small change in voltage will cause a large increase in current hence the need for load/current limiting resistors or a constant current source supply. An IC voltage regulator will not do the trick as its output is fixed at 5, 12, 18 volts, ect. Voltage will be constant whether the LED like it or not.

    There are several ways to power an LED, one is to use a battery with high internal resistance so it is current regulating, a voltage regulator with a limiting load resistor, a linear IC current regulator, or a Pulse Width Modulated power supply (PWM).
    High power LED’s are mostly pulsed, to gain efficiency and reduce heat dissipation. The article attached explains the principles but I fear is beyond the general interest of members reading this thread. In the case of Michael P’s attachment, I believe it is a PWM circuit judging by complexity and diminutive size of the PC board. In contrast, I am attaching a three legged linear current IC picture which dissipate a lot of heat. It needs only a current regulator IC, a resistor, and a bulky heat sink. To dim the LED by reducing the voltage means loading the IC to absorb the power and dissipate heat.

    The most obvious clue in Michael’s picture is it is dimmable and the number of pins of the IC, indicating a complex design consisting of an stable circuit block, a monostable circuit block, a power IC or FET, and a feedback circuit to vary the pulse width plus a host of other discreet devices. Complex, yes, but look at the small size and absence of a heat sink. LEDs can be driven at higher current (thru voltage, Vfmax) provided the duty cycle is not exceeded. A data sheet need be consulted but this beyond the DIY’s.

    Lastly, that large signage that you mentioned, which in all probability is a 7 segment display, an alpha numeric display or a dot matrix display are multiplexed and pulse driven. You just don’t see the flicker as it is pulsed so fast. If you see a row or a column not lit, in all probability, the segment driver (SDI) or the column driver (DDI) has failed. Each pixel/bar is arranged (electrically) in a horizontal and vertical array, not in series or parallel. In some cases, a pixel/bar may consist of multiple LEDs (in a button or bar), only the pixel will be dead, not the whole row or column. Technically, only one pixel is lit at a time. I doubt the circuit designer will connect the LEDs in parallel and opt for a well regulated supply voltage to around 5 volts, the supply voltage of the LED plus the voltage drop in the SDI, DDI, and current limiting resistor. Low voltage, high amperage design needs high power transistors, and thick copper wires, which is expensive. Connecting the LED in series will raise the voltage, reduce current, and need only small power transistors and small copper wires. This reduces cost.

    Cheers,
    Rx
     

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  10. rxcomposite
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    rxcomposite Senior Member

    The 8-30 DC volt supply means you can hook it up to any voltage within that range. The internal regulating circuit will take care of regulating the voltage/current to the designed LED operating design.

    Similar to an appliance that is rated 100-240 VAC, 50-60 Hz. You can plug it into the wall socket that has a voltage and frequency within the range.
     
  11. michael pierzga
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    michael pierzga Senior Member

    When I asked the supplier about the " circuit" he simply told me that it stabilizes the light output...no flickering as the power input changes ? Constant voltage to the LED.

    Interesting that LEDs are praised for their environmentally friendly energy consumption. I wonder what toxic materials are present in the electronic circuit that will be released at end of service life disposal ?
     
  12. rxcomposite
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    rxcomposite Senior Member

    It is actually constant current because the voltage to the LED is closely monitored in variation. After the "breakdown voltage" is reached, the slope becomes nearly vertical. A little change in voltage causes a large change in current. Flickering indicates that the LED is driven by pulses. If the voltage goes below the designed value or the battery voltage goes down, the astable/monostable circuit (oscillator) goes haywire or reduces the on/off state making it flicker. The principle is the same in movies. Light is flashed 24 times a second. Anything lower than that, your eyes will notice the flicker. I see two mini pack IC in your circuit. Are they marked xx555? It is an industry standard for making pulses.

    Linear driven (non pulsed) LEDs do not flicker but is not very efficient in terms of power usage. Some are lost as heat.

    There is some toxic materials present in LED. The lead used to solder the pins and the plastic encapsulation (not toxic but not enviromentally friendly). Compared to fluorescent light, which contains phosphorous LED is relatively friendly. Another plus is the long life so no quick accumulation of toxic materials.
     
  13. RSF
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    RSF New Member

    Hi, suggest you run in series a multimeter on Amp Draw mode, hook the light up a your twelve volt batt, take a reading of the amperage output consumed. Seems to me that will tell you what the draw actually is and help with fuse size identity. Certainly as others have indicated there may be a short, though it may be a package fault ie a defective OEM wiring problem.
     
  14. michael pierzga
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    michael pierzga Senior Member

    It would be interesting to know how an electrical engineer specifies the lighting control circuit on a new construction.

    Do they design the circuit capacity...wire diameter, fusees, breakers, switches...... to the load of an incandescent system, even thought they will spec LED lighting ?
     

  15. rxcomposite
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    rxcomposite Senior Member

    Michael,

    The Lumen is a measure of the amount of visible light from a source. The wattage (Watts) is the power required to drive the source. In Europe, it is now mandated to label the lighting equipment in Lumens instead of Watts. Not all countries have adopted it. In our country, lights are still in Watts. As a guide, a 60 Watt tungsten bulb is equivalent to 700-750 lumens.

    The type of the source of light differs considerably given the input power in Watts and is measured in Lumens/Watt, LED being the most efficient. Below are the comparison of each type;
    Type / Lumens/Watt
    Incandascent 14.3
    Halogen 13.6
    Fluorescent 50.8
    LED 100

    For house wiring, The local electrical code applies. In our country, there is no provision for low voltage supply. Each light socket is rated at max 220V, 125 Watts irregardles whether I install only 3 watts, so we have a power module for each of of the LED light that converts AC down to the DC that powers the LED.

    For boats that have 12 or 24 volt supply, The LED light unit wattage is used and the sum of all the LED wattage is converted to Amperes (W=VA) to size up the wire. Light unit means an LED light that has its own current limiter similar to the one you have, not the "raw LED" being discussed here.

    My basement lighting is wired with 24 VDC from a battery supply with each lights having individual linear current regulator (similar to the one I posted). LED's are the usual 3 Watt type so I am using more than the usual number of lights.

    Rx
     
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