I would like to remove the n27 sockets from household light fixtures and replace them with LED lights used in the automotive industry. I would like to put a reasonable amount of LEDs in the fixture to provide commensurate light of say, a 100 watt bulb. I would like to have a light in every room (7) and run them off of a 12v battery charged by PV. Obviously wired seperate from the AC system which will be abandoned. My question is do I have to regulate amperage. If how or what do I do that with, do I need it for each light fixture? If not you're my new favorite.

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    \$\begingroup\$ Better on the DIY.SE Stack? \$\endgroup\$
    – Solar Mike
    Sep 20, 2018 at 5:48
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    \$\begingroup\$ gordonelectricsupply.com/… This is an option too but not omnidirectional unless you have reasons to change fixture, connectors wiring \$\endgroup\$ Sep 20, 2018 at 6:04
  • \$\begingroup\$ How many 12v batteries are you planning to have? Are you planning on building your own fixtures or using premade automotive parts? \$\endgroup\$
    – K H
    Sep 20, 2018 at 6:15
  • \$\begingroup\$ I agree with @SolarMike. Also the question could be improved to indicate critical tradeoffs like cost, efficacy, glare, design scope ( make or buy), illumination values (CCT,CRI, Beam pattern, dimmable, etc.) and overall effort. I once considered 20 yrs ago, a central 48Vdc wiring using obsolete telephone wire for household illumination or using existing 16AWG re-routed to 48V battery and charger like a DC UPS. But 8W dimmable ceiling PAR lamps are cheap now and I prefer 4500K tri-phosphor tubes for better CRI and 86 LPW with 30k to 50kh MTBF \$\endgroup\$ Sep 20, 2018 at 18:01
  • \$\begingroup\$ If you could stic with LED lights designed for 120V, you can get them cheap if you keep your eyes open. We just bought a box of 8 bulbs at local big box hardware store for 50 cents (USD) each because the OEM was changing the 8-box packaging, so they were getting rid of the old boxes for cheap. \$\endgroup\$
    – CrossRoads
    Sep 21, 2018 at 2:21

2 Answers 2


If the lamps are designed for 12V automotive use, then no current limiting is required. Simply connect a small cluster of them in place of each 100W lamp.

You will need to work out how many 12V lamps you need to replace each 100W lamp. Try to source decent quality ones that have a quoted lumens rating.

While you don't need a current limiter, do put a fuse in the circuit near the battery. A 12V battery can easily put out enough current to melt your house wiring if you accidentally short something out. The fuse needs to match the current rating of your house wiring and the switches used.

There is a risk that large numbers of 12V lamps will end up exceeding the current rating of your wiring. In that case, you will have to split it into several circuits, each with its own fuse.

  • \$\begingroup\$ Thank you for your time and for sharing your knowledge and experience. Both answers helped me a great deal. I'm going to stick to automotive applications, wiring and fuses. Ill simply run separate wires throughout my house,( living alone right now in old trailer, no big deal). This is a trial run for new build in the spring. Thinking dedicated 12v lighting and running the rest of the house inverted ac. Looking for 12v RV lighting that I like. \$\endgroup\$
    – Dave Z.
    Sep 20, 2018 at 20:08

I do not know what an N27 socket is but I can possibly help.

Using automotive with PV may not be a good match. In automotive efficiency is not a priority. There is plenty of 12V power to waste.

If you were to use original brand name parts like Mercedes or BMW then you would have the reliability. No name replacement parts would not be a good choice either.

100 W incandescent is about 1500 lumens.

For purposes of this suggestion, I am assuming you are going to create your own circuit board.
I assume you want something very efficient.
The most efficient LED today is the mid-powered Samsung LM301B.
You would need about 28 of them for 1500 lumens.

These will work well for a simplified fixed, non-dimable fixture.
Because their forward voltage is slightly below 3V at 2.8V,
four of them in series is efficiently (≈93%) powered by 12V and a current limiting resistor.

Wire them in sets of 7 in parallel, and wire in series 4 of the parallel sets of 7.
Wire them to 12V with a 1 Watt 1.62Ω 1% resistor in series.
This would be 71 mA per LED just slightly (6 mA) over its "test current".
Total power consumption 6W for about 1500 lumens. The cost of the LEDs for 7 fixtures (7 x 28 LEDs) would be about $45US (22¢ ea.). This can be reduced by using less LEDs. You could cut this $45 in half and double the current. 28 LEDs is conservative which leaves room for error and the generation of less heat and maximum efficiency.

This is the most efficient 12V, 1500 lumen light source you could possibly build.

You could improve efficiency by lowering the 12V to 11.4V using a 0.2Ω resistor. This is a ballpark number, you would have to measure the exact forward voltage to calculate a precise resistor value and the 11.4V supply would have to be very stable.

A store bought 100 W eqv. LED light bulb would consume closer to 14 watts.

You could push them a little harder (1Ω resistor/100 mA per LED) raising their Vf and the wall watt efficiency will increase but may get warm enough to need a heatsink if pushed too hard.

Light bulbs are best for incandescent. For LED lighting, strips of LED are far superior. Cramming a bunch of LEDs in to a small space is very inefficient.

If you had a 20V supply you could use Samsung F-Series Gen3 or Bridgelux EB Series Gen 2 strips. Very inexpensive ($4US) and the most efficient off the shelf LED strips.

Note about lumens and diffusers.

While I specified that 28 LM301B LEDs would be about 1500 lumens, that is where the LEDs are not covered or diffused.

An LED light bulb must have enough LEDs to produce more than 1500 lumens because the diffusing light bulb cover is going to absorb a significant amount of light. Figure about 30% loss.

For indoor lighting, when appropriate, I bounce the light off a white ceiling or wall.

This is four high power white LEDs in series with a 2Ω resistor powered with a 12V UPS.

enter image description here

For 12V wiring throughout the house I bought this on Amazon for $25. All I wanted was the box and fuse panel. The power supply was crap. Enclosing a power supply in this box is not recommend. Very poor heat transfer characteristics.

enter image description here

  • \$\begingroup\$ Hey, just an addition along the lines of optimal diffuser efficiency, especially because you're recommending an already spread out arrangement of smaller LED's, you can probably do a lot better than 30% for diffuser losses with an AR film and something like Acrylite Satinice(diffusion accomplished by glass beads embedded in acrylic). There may be visible hotspots on the lens looking up at it, but there's a very good chance you could get away with a single 60 mil layer with 90% transmission. \$\endgroup\$
    – K H
    Sep 21, 2018 at 0:10
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    \$\begingroup\$ @KH The first link I looked at after searching for Acrylite Satinice, says, Satinice optimum light diffusion white WD008 DF, Up to 65% Light Transmission. The 90% does not have very much diffusion. Still good stuff. It is not unusual for this level of diffusion to only have a transmittance of only 30%. I have their datasheet from 2 years ago. You really have to go with a company that has the aluminum extrusions and made to fit diffusers. I like Klus. Their stuff is sold on 1000lightbulbs.com and reasonably priced. \$\endgroup\$ Sep 21, 2018 at 0:32
  • \$\begingroup\$ It doesn't have to too much diffusion, not enough to prevent hotspots on the fixture for sure, but with a 10ft ceiling and 28 LEDs to begin with, your diffusion pattern starts pretty good compared to higher power individual LEDs, I think you could do really well even with a flat sheet. What I've been eyeballing is taking something like satinice and attaching it back to back to a sheet of something like pyramid shaped lens diffuser with even higher transmissivity. \$\endgroup\$
    – K H
    Sep 21, 2018 at 0:48
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    \$\begingroup\$ @KH I do horticulture so I do not use diffused much. I had a job in a NY hi-rise condos ($5,000/sq.ft) for their indoor landscaping and that is why I was looking a diffusion a couple of years ago. The LM301B cannot be beat today. When you add in the PCB and assembly it gets pricey. The best value is the Bridgelux EB Series Gen 2 strips ($4 for an 11" strip) and Samsung F-Series Gen3. Those I drive with a Mean Well HLG CC driver. I was driving EB strips with 1400 mA (max) and no heatsink was needed maybe 55°C, at 700mA 42°C. The LM301B run cool at 100mA, they get very warm at 200mA. \$\endgroup\$ Sep 21, 2018 at 1:40
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    \$\begingroup\$ No MCPCB required. The BXEB strips are 560 mm (22"), 112 LEDs, FR4, ($7) and no heatsink required even at max current (1400 mA). The Samsung F-Series would get a little too hot a max current. A lot of cannabis is grown under these strips. I make 9 mm wide strips with 48 LEDs wired 3P16S. With no heatsink I can drive them at 300 mA (100 mA per LED) with a PCB temperature of 47°C. With a minimal heatsink I can go up to 600 mA @ 56°C. No heatsink @ 400 mA = 70°C, a little too hot but within the LED's spec. \$\endgroup\$ Sep 21, 2018 at 2:26

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