Mod #81: 12 Volt Fluorescent Lighting on the Cheap

Mod #81: 12 Volt Fluorescent Lighting on the Cheap

Submitted on: 05/10/09

     Category: electrical, lighting
Mod Rating: 12345

(33 ratings)

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Mod Description:

This mod comes to us courtesy of RV.net senior member Gdetrailer, otherwise known as Gregg Berry, and is copyright with all rights reserved.

For boondockers and anyone who just wants more light than the typical RV incandescent lights you just can’t beat fluorescents. 12 volt RV fluorescents tend to cost an arm and leg, and for the most part look rather cheap, especially the economy versions (LOL they still cost $30+). This mod details a way to modify low cost fluorescent lighting fixtures to correctly work off your RV’s 12 volt power system.

Mod Difficulty:

While browsing around Home Depot, I found some battery powered single bulb fluorescents, 4w 6V DC ($5) and a 8W 12V DC ($9). Now you know the 6V ones will present a problem voltage-wise but most folks would think that the 12 volt models would be fine hooked up to a RV 12 volt system. Not so, this is a case were 12 volts may not be the same!

The 12 volt battery light takes 8 AA batteries. Dry cell batteries when new typically have 1.6 volts. So 8 new AA batteries would give 12.8 nominal voltage. Generally most devices will tolerate up to 10% over design voltage which would be 14.08V in this case.

Now in a RV with the advent of multistage charging your typical RV 12 volt system can see voltages as high as 15 volts. This will definitely let some smoke out of your low cost battery fluorescents.

Not to worry. I have a low cost solution to prevent damage and even be able to use the 6 volt versions. Even better yet is the cost, $1.50 per fixture (Radio Shack) or as low as $.50 per mail order. It is a voltage regulator IC, the LM78xx series, 3 pin, handles 1A of current, handles up to 35V DC on the input. This is more than enough to use with these battery powered fixtures. It is overload protected, in case of a short or overload on the output it will reduce the voltage down to 1.2V if needed.

The regulator does draw a small amount of power, typical 5ma which is a small price to pay to get low cost fixtures. I used a LM7806 for the 6V fixtures and LM7812 for the 12V fixtures.

Two possible combinations of mods, #1 easy mod, the regulator is used external to the light, basically regulator is mounted in a small box then output is plugged into the light. Note: you will need a small box to house the regulator and a coaxial power plug to fit the light. This also will allow fixture to still use batteries.

#2 Advanced mod: this is the one I have done. The regulator is mounted inside the battery compartment and hard wired to the internal inverter. This mod will prevent the use of internal batteries since the regulator is mounted in the battery compartment of the lighting fixture.

DISCLAIMER… The included photos and modifications only apply to the Amerelle brand lights I purchased at Home Depot. The design of the circuit boards is subject to change so you mileage may vary!

Warning! The inverters do generate HIGH VOLTAGES. Do not power up the light fixture while case is open.

Warning! Do not apply power to the fixture without the bulb installed, damage to the inverter may occur.

Comments: I’m adding some errata info missed from my draft of this section.

The lights used for these mods I purchased some time ago from Home Depot (6 months or more), in searching the Internet I noticed that these lights are selling about $9 for the 6” 4w ( model 73025) and $11 for the 12” 8W (model 73040) through Online stores. Still not a bad deal though.

American Tack is the import company.

It is possible to use LM7805 (might be able to still purchase at Radio Shack) for the 4W 6V light, this was what I used for my first conversions. It does work but I did notice one of the lights sometimes doesn’t get full brightness. For this reason I recommend the LM7806 (would need to special order at Radio shack or buy through mail order) which has a 6V output.

These mods could be done for other brands of lights but you would need to figure out how to connect the regulator to the inverter. In this case the simple route is to add regulator in between power jack and the inverter, no modifications to the circuit board would be needed.

You can also bypass the power jack and run the power leads out of the light body.

OK, lets get to the internal mods..

4W 6V DC light

  • To open, push in the tab at the battery compartment. The battery door end will swing away.
  • Remove the clear lens and bulb.
  • Remove the white plastic sticker which is under the bulb.
  • Once the plastic sticker is removed the main body should split in to two parts.
  • Carefully remove the opposite end from the body.
  • Once main body is apart you will see the inverter board.

Now I decided to make my modification insert the regulator after the switch, on the 6V inverter it is a little more involved.

Parts required: LM7806 – 6v@1A 3 terminal regulator Optional but recommended: Heatsink, can be a small piece of metal or actual purpose made heatsink that fits inside battery compartment.

  • To do this I had to cut two of the circuit board traces (#1) then run a jumper to connect the trace ends that was cut (#2).
  • Connect the input of the regulator (LM7806) to the pad that was isolated (#3), this is one end of the switch.
  • Ground of the regulator is connected to the incoming 12V neg (#4).
  • The output of the regulator (#5) is connected to the same trace as #2 red jumper.
  • Alternate connection before the switch, left photo, disconnect red wire from the center of the DC jack, connect regulator input to the jack.
  • Connect regulator output to the pad where the red lead connects.
  • Connect regulator ground to the black wire on the DC jack.

You can see the detail of regulator in the photos above:

#1 is the input. Middle pin is the DC ground, mounting tab is also DC ground. #3 is regulator output.

I also mounted the regulator on a heat sink, mine is salvaged from old obsolete equipment. You can buy small aluminum heat sinks or even use a short piece of aluminum, copper, or steel. The idea is to provide a way to keep the regulator cool.

I also used a plug method for connection. This was salvaged from a old PC. You can also buy these but in a pinch you can wrap the wire around the pins and solder the wire to the pin. Use a small soldering iron. Do not use a gun. The regulator and heatsink fit into the battery compartment.

I had to cut notches in the heat sink to get past some ribs molded in the plastic. This also helps to keep the heat sink from moving around.

The light is ready to reassemble and test.

12V 8W light

  • To open, remove the screws that are on each end.
  • Remove clear cover and bulb.
  • Carefully remove the white plastic that is under the bulb.
  • With both ends removed the two halves of the main body will separate. I did have one of mine that was glued at the battery compartment seam. Using a knife lightly cut the glue.

Parts Required: LM7812 – 12V@1A three terminal regulator, heat sink. This light will draw more than .5A so a heat sink is a must. This can be a piece of metal or purpose made that fits inside the battery compartment.

  • Be careful to not lose the two metal pieces (one on each end) since these are used by the screws to hold the ends on.
  • Remove the red wire (see pictures above), create a solder bridge from the center pad (where the red wire was) to the pad that is to the left (#1 of right picture).
  • Solder regulator (LM7812) output to the pad beside the two pads you bridged (#2)
  • Remove the old red wire from the power jack and solder the regulator input wire to the power jack (#3)
  • Solder the regulator ground wire to the black wire on the power jack (#4)

I re-purposed an old 3.5” floppy drive power connector. I rewired it so input is red, ground is black, output is yellow, and removed 4th wire (unneeded).

If you don’t have a connector, just solder wires to the regulator terminals. Use a small soldering iron. You might want to wrap some black tape on the terminals. Do not use a soldering gun!

See picture above of completed modifications with regulator installed in the battery compartment.

Reassemble light. Don’t forget to install the small metal pieces at each end.

One draw back to the switch modification is the fact that the light switch function has been changed. What was OFF is now the ON position. To get around this I peeled the switch sticker off and flipped it. I had to cut off one end of the label. Light is now ready to test!

TipNow that you have low cost fixtures, you can dress them up if the plain white plastic is not to your liking. You can make a small wood box (or metal box painted to your favorite color) to surround the light. Then take a piece of Plexiglas (1/8”or thinner would be fine) cut to the size of the box. To further hide the light, simply take sand paper and lightly sand one side only of the plexi, install with the sanded side in towards the light. You can do this by hand or with your electric sander. The sanding will make the plexi opaque which will hide the light and provide a professional touch.
TipHave one of those Thin Lites fixures with a dead inverter/ballast? Well it may be possible to use two of the inverters from these lights (one for each bulb) to replace those expensive Thin Lites inverter/ballast ($30 for one of those). I have not tried this idea but some of the Thin Lites do use 8W bulbs. A bonus to this is for boondockers, add extra on/off switch to the fixture and you can select one or both lights depending on your needs. I would however suggest if you decide to try this that you should still use a voltage regulator to prevent high DC voltage damaging the inverters (you would need one LM7812 for each inverter). To fit in the fixture you can unsolder the board with the battery terminals, then fabricate a insulated mount for the boards. In this case for longer life of the inverter (8W version) you can also add a small heat sink to the inverter power transistor if you have the space. The tab of the transistor may not be electrically insulated so you should ensure that the transistor heat sink can not touch any metal.


ModMyRV recommends these parts for this mod:
12V Fluorescent Lights
7805 +5V Voltage Regulator TO-220

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12 Comments For This Mod

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  1. partero

    I too have discovered these low cost units (Mine have a GE label, but they are made in China) and I have 3 of the 8 watt models installed in my MH. I also discovered that these units don’t like anything above 12.5 volts for extended run times, they fry the small switching transistor that they come with. My solution was to install a larger transistor on a small piece of aluminum as a heat sink, and presto, they will run as long as you got power. My Tripp-lite 750W. UPS charger/inverter keeps my house batteries @ 13.6 DVC, and the fixture that I have in the bathroom runs all night. The down side to these cheap units, is that their inverter circuit apparently does not have any built in regulation, and my fixture burns out it’s 8 watt fluorescent tube about every 2 months. At $5 replacement costs for the lamp, I’m looking for a better fixture.

  2. Gregg

    Hi, since I am the author of the this mod I though perhaps I could address the issue of short lamp life that was posted in the comments.

    partero writes “I have 3 of the 8 watt models installed in my MH. I also discovered that these units don’t like anything above 12.5 volts for extended run times, they fry the small switching transistor that they come with.
    My solution was to install a larger transistor on a small piece of aluminum as a heat sink, and presto, they will run as long as you got power.

    The down side to these cheap units, is that their inverter circuit apparently does not have any built in regulation, and my fixture burns out it’s 8 watt fluorescent tube about every 2 months. At $5 replacement costs for the lamp, I’m looking for a better fixture.”

    End of quote.

    These lights were designed primarily to operate from AAA dry cell batteries, 8 to be exact for the the 8W models. Typical new dry cell will have a no load voltage of 1.6V, multiply by 8 and you get 12.8V. Dry cells have a higher internal resistance than say a larger 12V wet cell you will find in a car or RV. When a dry cell is presented with a light load the voltage will drop enough to safely operate the light for long periods of time.

    When connecting this light to a car battery the lamp presents a very small load VS the internal cell resistance and will not drop the voltage enough to safely operate directly for a long period of time.

    Your solution of a larger transistor will fix the inverter burnout at voltages above 12.0V but now it will over drive the flourescent lamp causing premature lamp burnouts to occur.

    The inverter in these lamps do not have any input voltage regulator, the bulb will absorb any extra current as a result of too high of an input voltage. With your mod you should check to see how much current is being drawn, 8W bulb should draw at 12V .6A (600ma), over that will be overdriving the bulb. If I remember correctly the 8W lights in my write up were drawing about .5A (500ma) or 6W which is a bit under driven (which will result in long lamp life). The 8W lights I used the inverter also has an potentiometer which allows some drive adjustment of the inverter.

    Even the $40-$80 name brand RV 12V fixures do not have any input voltage regulator. They are however designed with 12V auto/RV voltages in mind from the start. They typically are rated up to 15V DC.

    I have two of these expensive RV name brand fixtures that came in my RV, one worked and one was dead. Opened the dead one to find a burnt up inverter, it is a simple two transistor oscilator with a high wattage output transistor. I about fell over when getting a price for a replacement inverter, it was almost as much as a new fixture with bulbs!

    I would suggest instead of upgrading the switching transistor, in your case you should try adding the LM7812 regulator to the input. Yes, this regulator is a wasteful linear regulator but it will only be dropping 1-2V thus wasting very little power. The cost of the regulator runs from $.50 bulk (internet/mail order) to $2 retail at Radio Shack.

    Using the LM7812 will protect your 8W light up to 35V DC input and keep the voltage low enough to promote long life of the bulb.

    Alternately you could use three 3A@50V PIV (Peak Inverse Voltage) diodes in series on the 12V plus input side of the fixture. Each silicon diode will drop roughly .5V-.7V, three in series will drop 1.5V-2.1V which should also keep the input voltage well below the self destruct mode. You will however lose some benefits of the LM7812 such as voltage above 13.6 protection, over current foldback (short circuit protection) which the LM7812 does a great job at.

    Hope that helps.. Gregg (aka GDETrailer)

  3. AlexS

    Did you have to put any caps on the LM7812? I am getting ready to put a few of these in service.

  4. Gregg

    AlexS, a very good question!

    I didn’t use any decoupling capacitors on mine. My experience over the years with these regulators with my projects has been that the regulator is generally stable and the caps were not needed.

    I intentionly did not mention the addition of the decoupling caps in order to keep the write up as simple as possible for the novice DIYer.

    Something I have noticed though is I have two lights connected to common switch (both lights are the 6V style using LM7806 in each), sometimes one of the lights doesn’t get fully bright unless the other light is turned off then on. In this case I just might try adding in decoupling capacitors since it is possible that one (or both) of the inverters is causing too much RF noise which is affecting the regulator. Although in my case could just be an issue with one of the lights with a questionable inverter.

    All of my other lights so far are not affected so I am plenty satisfied with my results, since they are low cost and very small (can fit these puppies in tight spaces, my wife likes em so much she is wanting some put into the closet and pantry area).

    The manufacturers basic diagram does show the “typical” use with a .33uf cap on the input lead to ground and .1uf on the output lead to ground. These caps should improve ripple rejection and prevent stray RF noise from affecting the regulator output. Niether condition should cause any major issues with our use in this project, since our main goal is to prevent the lamp inverter seeing more than 12V (8W light). In this case a few millivolts of ripple would not be noticable.

    See page 22 of manufacturers doucument for “typical” diagram at http://www.fairchildsemi.com/ds/LM/LM7812.pdf

    If you have some caps you can certainly add them in, if not try it without and see if you run into any problems.

    If you chose to use caps with this regulator, you can use most any value you have close to the values I noted. Electrolitics (watch polarity!), ceramic, poly types, tantulam (watch polarity!) could be used.

    Gregg (Aka GDETrailer)

  5. Steve

    Okay…there is one little problem. The 78XX series regulators generally require a 3 volt head voltage (at least 3v higher than the regulated output) which means the 7812 would only regulate at 12v when it has 15 volts in. The fixture may light but I suspect you have less than 12v when running off your RV battery.

    And I never have found a 7806. I never tried a special order thru Radio Shack.

  6. Gregg

    STEVE writes “Okay…there is one little problem. The 78XX series regulators generally require a 3 volt head voltage (at least 3v higher than the regulated output) which means the 7812 would only regulate at 12v when it has 15 volts in. The fixture may light but I suspect you have less than 12v when running off your RV battery.

    And I never have found a 7806. I never tried a special order thru Radio Shack.”

    Steve, good points.

    Granted the 78xx series does require some “head room” voltage above the target output voltage which is roughly 2V in practice. Keeping in mind that the lights in question are in reality designed to be used as battery powered “flash light” type applications. Since they are designed around dry cell batteries they actually will work fine at reduced voltages (although at slightly reduced lighting levels).

    One must keep in mind with items designed around drycell applications that they are designed to work with a very large voltage swing (or range). What they don’t like though is exceding the max designed voltage.

    With LM78xx series when the input voltage drops below the target output voltage + headroom voltage it will simply PASS voltage (minus the headroom voltage). So in practice, a typical fully charged lead acid battery will have a resting voltage of 12.8V, the regulator will pass 10.8V. A nearly discharged Lead acid battery would have terminal voltage of about 11V, the regulator will pass 9V (in this case the light might not start or if it was lit would be rather dim, in any case battery needs charged badly).

    As I stated before, typical drycells have a open voltage of 1.6Vx8=12.8V, generally with most electronic items there is a small amount of voltage you can excede the typical design voltage (I typically use 10% or less which in this case would be 1.28V + 12.8 = 14.08 which = smoke).

    Typically what I have found with drycell battery powered devices that they will operate rather well at voltages below the actual designed voltages. Case in point, Nicad or NiMh batteries often can be exchanged for drycells, typical fully charged they produce 1.4V x 8 = 11.2V and discharged typically 1.1V x 8 = 8.8V.

    As you can see, even with the LM78xx passing a lower voltage, the lights should still function as low as 9V or there abouts.

    What we are attempting to do with the LM78xx series is to prevent you from ever reaching the smoke point in voltage. We are not trying to prevent the voltage from dropping below 12V. To do this would require a boost/buck switching regulator which adds complexity and cost to the project.

    Since the LM78xx will pass voltage even when the input is less than ideal it will be still enough to operate your fixture (although perhaps slightly dimmer). With the 8 cell 12V light (at least the light I bought), there is actually a potentiometer that if you use an ammeter you can actually adjust the inverter current to compensate for the somewhat reduced input voltage.

    As far as the LM7806, it is easily obtained at most any mail order electronics type outlet. Retails like Radio Shack tend to not keep this voltage in stock but often time will special order (or now days you can do special orders over the internet). Keep in mind that if you doing more than one or two you can easily justfiy mail ordering since these can be had for $.50-$1 each plus shipping, at RS you will easily pay $2-$3 for one.

    When searching internet try dropping the LM, often this helps since some places stock using the numerical part.

    Here is from several online/mail order places I tend to buy from
    MCM Electronics
    http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-/7806

    All Ellectronics
    http://www.allelectronics.com/make-a-store/item/7806T/6V-POS-REG-1-AMP/-/1.html

    Just make sure you have enough items or quantities to help “cover” the cost of shipping (wouldn’t make sense to pay $7 on shipping just for one $.50 part).

    Alternately you can use a LM7805 (Pos 5V regulator) which should be readily available at RS instead of the LM7806 although with a little reduced light. A neat way to bump the output voltage is to “lift” the ground terminal by adding in a poteniometer between the ground and the ground terminal of the regulator (NOTE: in doing this you must ensure the heatsink is not touching ground or could short to ground). This makes the output voltage adjustable above the 5V.

    Another IC that also could be used is an adjustable voltage regulator LM317 which requires additional resistor and potentiometer (for adjusting the output). I purposely did not use this for my article since it does require additional parts but if you don’t mind experimenting some, it will work also.

    See the manufacturer website below
    http://www.national.com/mpf/LM/LM317.html

  7. TXsavage

    I just bought a 12 in 12vdc lamp that looks just like the one on your web page.I planned to put a voltage regulator in it per your instruction. This one is an Amerelle, Utility Lite (www.amertac.com) and at first glance it looks the same as your pictures do. But there are some differences such as the screw is missing on the end cap. The end cap snaps in place and is locked in place by a small rib on each side near the edge of the cap. The cost of this unit is $10.00 at Home Depot.

    Everything else is about the same as you describe until you get to the PCB. It is still a 2 piece board soldered together in the shape of a T. Next to the crossbar of the T is what looks like a minature transformer and next to that is a component with 3 legs soldered to the board and a square molded body that is less than 1/8 th inch thick. One side of the square is a flat metal surface that is molded in place and there is a hole through the center of the square for mounting it to a heat sink. I believe this is a voltage regulator.There is no heat sink attached. There are 3 other CAP and I think 4 resisters. The printing on the board next to the VR appears to be LX001.

    If I am correct in my thinking that they have added a VR in their redesign, then it seems to me this model does not need anything except an extension cord with a 12 Vdc plug to plug into the 12V coax jack and a 12V plug to plug into power point or wire into a 12V circuit.

  8. Gregg

    TXsavage writes “One side of the square is a flat metal surface that is molded in place and there is a hole through the center of the square for mounting it to a heat sink. I believe this is a voltage regulator.There is no heat sink attached. There are 3 other CAP and I think 4 resisters. The printing on the board next to the VR appears to be LX001.”

    This actually is the output power transistor, it actually does several things at the same time. What you are looking at on this circuit board is actually a free running oscillator. The transistor works like an amplifier but is driven by feedback to oscillate (works like getting a microphone too close to a speaker which creates a loud squealing sound in the speaker). The frequency is determined by the resistor/capacitor network. The output of the transistor is goes to the transformer which boosts the voltage high enough to strike an arc in the fluorescent tube.

    Once the tube has lit the extra current flowing through the lamp causes the output voltage of the transformer to drop to a safe voltage (and current) for the tube (The strike voltage required is much higher than the actual voltage need to maintain the arc). If the voltage doesn’t drop enough the tube will stay lit but it will drastically reduce the life of the lamp, or burn up the output transistor.

    Disclaimer, I have not tried the following info, I had seen this mentioned on another website (fieldlines.com)for this same light. You should also see an adjustable resistor, this can be used to adjust the current draw of the light (I am not sure how it makes the adjustment which could be changing the frequency or limiting the max current draw through the transistor). Do not attempt to adjust this unless you have a volt meter with DC current capability! The max current should be kept below 800ma which is 8W@12V.

    The differences you have noticed may be due to changes in the actual manufacturer of the light which may occur since often times the designer/seller will put a design out for bids. The lowest bid wins the job.

    Price still seems fair when considering the cheapest RV fluorescent fixture I have seen will set you back $40+ (may be more now days).

    Most likely it will be best to add in the regulator mod when being used directly with a RV 12 volt system to reduce the chance of charging voltages exceeding the light fixtures target of 9-12V.

    Good luck with your mod, so far mine have held up well, one light we have used the most over 2 years is showing some blackening on one end of the bulb but is still working!

  9. rtfrench

    This is usually the very first mod my wife does to the cabinets. She installs these very lights just inside the doors for ease of seeing what’s inside the cupboards, under the sinks, and inside the closets. I tap into the power where available. (the overhead light in the bedroom for the closets, for example), We boondock about 50% of the time and these are a real time saver at night when you are looking for a jacket and haven’t found the flashlight yet!

  10. Joshua

    I went ahead a put together 4 of these 8w (12″) lights that I purchased from Walmart for $6/ea. I decided to go the simple route that excludes the caps. I wired my battery pos to the 7812 input leg & the output leg to my load. This seems to work just fine for the short period I have tested it. Two things I have noticed, 1) the 7812 gets really hot (to the point I cant touch it after about 5-10 mins w/o getting burned) and 2) the light puts out this high pitch noise that subtle yet annoying. I have heat sinked each 7812 but wonder how I can get rid of that noise (my wife thinks im crazy as she can’t hear it).
    BTW, my benchtop power source is a 12vdc SLA battery. The measured voltage across the lamp is 9v & current draw is 310mA which seem really low to me. Im a self proclaimed novice so what do I know?

  11. Greg

    GO LED Strips!!!!!!!

    Home Depot has a pkg with 4 LED strips ea abt 1 foot long(lots of light).for under cabnet lighting. (29.00) -.they come with a 110 to 12 Vdc xfmr block and each strip just runs off 12 v from this power block.
    Just rewire them and they work great up to 15 volts- if desired, with such low current, install a resistor to drop the voltage a bit if you are worried about long life. This works out to abt 7.50$ a strip.! and very long life with no voltage worries.

    greg

  12. Mark

    In another project involving solar panels and shop lights, I discovered flourescent bulbs will light just fine off 12V. I had to remove the ballast and starter from each fixture, but this worked just fine with no ill effects. I did the same trick for a few more lights in the house.

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