Low Voltage LED Lighting

Monday, July 13, 2015 

My kitchen has had halogen lighting for 20 years, from back when it was a slightly more efficient choice than incandescent lighting and had a pleasing, cooler (bluer, meaning the filament runs hotter) color temperature.

LEDs Installed

Progress has moved on and while fluorescent lights still have a lead in maximum luminous efficacy (lm/w), for example the GE Ecolux Watt-Miser puts out 111 lm/W, they’re less versatile than LEDs and installation is a hassle while low voltage LEDs are easy to install and look cool.

System Design

The goal of this project was to add dimmable, pleasing light to the kitchen that I found aesthetically interesting.  I wanted a decent color rendering index (CRI), ease of installation, and at reasonable cost.  I’ve always liked the look of cable lighting and the flexibility of the individual, adjustable luminaires.

I couldn’t find much information on how variable output LEDs work and what can be used to drive them.  I have a pretty good collection of high quality power supplies, which I wanted to take advantage of, but wasn’t sure if I’d be able to effectively dim the bulbs from the documentation I found. So I did some tests.

Test Configuration

I bought a few different 12V, Dimmable LEDs and set up a test configuration to verify operation and output with variable voltage and variable current.  The one bit of data I had was that using standard commercial controllers, the lowest output is typically stated to be around 70% of maximum output: that is the dimming range is pretty limited with standard (PWM/Transformer) controllers.  The results I found were much more encouraging, but revealed some quirks.

LED Test Apparatus

LED Test Apparatus

I used a laboratory-grade HP power supply with voltage and current control to drive the LEDs, decent multimeters to measure voltage and current, and an inexpensive luminance meter to measure LED output.

I measured 3 different LEDs I selected based on price and expected compatibility with the aesthetics of the project and because they looked like they’d have different internal drivers and covered a range of rated wattage.

VendorItem LinkAngleLuxRatedMeasuredPrice
Torchstar4W LED30°47404W4.29W$4.99
Torchstar5W LED36°47605W4.77W$7.99
JackyLED7.5W LED60°69507.5W5.69W$9.99
Sample LEDs

Sample LEDs

Test Results

These bulbs have internal LED controllers that do some sort of current regulation for the diodes that results in a weird voltage/current/output response.  Each bulb has a different turn-on voltage, then responds fairly predictably to increasing input voltage with increasing output, reaches the controller stabilizing voltage and runs very inefficiently until voltage gets over the rated voltage and then becomes increasingly efficient until, presumably, at some point the controller burns out.  I find that the bulbs all run more efficiently at 14V than at the rated 12V.

As a side note, to perform the data analysis, I used the excellent xongrid plugin for excel to perform Kriging interpolation (AKA Gaussian process regression) to fit the data sets to the graphing function’s capabilities.  The graphs are generated with M-Chart and the table with TablePress.

Watts v. Volts

This chart shows the wattage consumed by each of the three LEDs as a function of input voltage, clearly demonstrating both that the power consumption function is non-linear and that power consumption in watts improves when driven over the rated 12V.  Watts are calculated as the product of the measured Volts * Amps.  Because of the current inversion that happens as the controllers come fully on-line, these LEDs can’t be properly controlled near full brightness with a current-controlled power supply, though it works well to provide continuous and fairly linear dimming at low outputs, once the voltage/current function changes slope, the current limiting controller in the power supply freaks out.

Lux v. Volts

This chart shows the lux output by each of the three LEDs as a function of input voltage, revealing the effect of the internal LED driver coming on line and regulating output, which complicates controlling brightness but protects the LEDs.  The 5W LEDs have a fairly gentle response slope and start a very low voltage (2V) so are a good choice for a linear power supply.  The 4W LEDs don’t begin to light up until just over 6V, and so are a good match for low-cost switch mode supplies that don’t go to zero.

Lux/W v. Volts

This chart shows the luminous efficiency (Lux/Watt, Lumen measurement is quite complicated) by each of the three LEDs as a function of input voltage, showing that overdriving the LEDs past the rated 12V can significantly improve efficiency.  There’s some risk it will overheat the controller at some point and result in failure.  I’ll update this post if my system starts to fry LEDs, but my guess is that 14V, which cuts the power load by 20% over 12V operation with the 7.5W lamps I selected, will not significantly impact operational lifetime.

Update: This system has been running for 7 years now.  In that time two linear power supplies have failed (they were fairly inexpensive models as such things go).  The LED modules had a high infant mortality rate: 2-3 failed in the first few months, another one failed just about every 6 months for the first couple of years.  I think it has been 4 years since the last one failed.  This implies that longevity is primarily a function of build quality, which varies.

Total System Efficiency

The emitter efficiency is relatively objective, but the total system efficiency includes the power supply.  I used a Daiwa SS-330W switching power supply I happened to have in stock to drive the system, which cost less than a dimmable transformer and matching controller, and should be significantly higher quality.  The Daiwa doesn’t seem to be easily available any more, but something like this would work well for up to 5A total load and something like this would handle as many as 40 7.5W LEDs on a single control, though the minimum 9V output has to be matched to LEDs to get satisfactory dimming. It is important not to oversize the power supply too much as switch mode supplies are only really efficient as you get close to their rated output.  An oversized switchmode power supply can be extremely inefficient.

With the Daiwa, driving 13 7.5W LEDs, I measured 8.46A at 11.94V output or 101 Watts to brightly illuminate the entire kitchen, and providing far more light than 400W of total halogen lights.  I measured the input into the power supply at 0.940A at 121.3V or 114 Watts.  That means the power supply is 88.6% efficient at 12V, which is more or less as expected for a variable output supply.

Increasing the output voltage to 14.63 Volts lowered the output current to 5.35A or 78 Watts without lowering the brightness at the installation; I measured at 168 lux at both 12.0V at 14.6V. The input current at 14.63V dropped to 0.755A or 91.6 Watts, meaning the power supply is slightly less efficient at lower output currents (as is usually the case).

  • Overdriving the 12V rated LEDs to 14.63V improves plug efficiency by 20%.

At the low end, the SS-330W’s minimum output is 4.88V, which yields 12 lux at the counter or a 14x dimming ratio to 7% of maximum illumination, a far better range than is reported for standard dimmer/transformer combinations.


Raw Data:


(MS Excel file, you will need the xongrid plugin to update the data as rendered in the graphs)

product image
Author Rating
Aggregate Rating
5 based on 1 votes
Brand Name
Product Name
High Power MR16 LED 10 Pack
USD 59.99
Product Availability
Available in Stock
Posted at 02:45:36 GMT-0700

10 Responses

  1. Ted Dow says:

    Hi David… I can see your housekeeping style is unchanged! (Engineeringg genius ala carte)

  2. Reichart VW says:

    I’m currently piecing together my entire garden lighting system. My plan is to make it 100% off grid. No matter what else the world is doing, my garden will be well lighted.

    Basics are several hundred watts of PV, charge controller, a few random car and security 12v batteries (have lots of these already, the car batts can’t do deep cycle of course, but I can solve that with a threshold float), sun sensor, and of course, lots of LED MR16 and outdoor housings.

    They run 3K, I would like warmer of course. I’m planning to hit them all with an amber shellac. I’ve tested with success.

    I will scale the panels until I can keep them lighted from dusk to dawn. Although I plan to set up two sets. A primary path (walkway) set to ensure there is always light where it counts, and then a secondary decor set, that can phase out around 1am. I’ll probably use a simple sun sensor + timer combo for that.


  3. David Gessel says:

    I’ve had good luck with Morningstar solar controllers in Iraq. They handle heat well (+57C ambient once, >+50C regularly) and are robust against electrical issues like bad grounding and high static build-up.

  4. Reichart VW says:

    Agreed. I have used their products for a couple of decades. Their early models used a trick I have always been a fan of, just dunking all the circuity in resin. Assuming it can dissipate heat, it protects everything from the elements.

  5. David Gessel says:

    I’m glad to hear that – I have two years of operating time so far: decades is very comforting. They’re still solidly built, but the controllers I have are boards bolted to heat sinks with the plastic cover bolted over. The board -appears- to be conformally coated, but not potted.

  6. Reichart VW says:

    This is what I bought for my garden experiment. I expect it to fail, I also expect it to be a piece of crap, but at $22 with free shipping to my little island, it is a perfect device to test with, and then I will upgrade to something like a ProStar.

    (I will probably throw it into some plastic external housing, but rust where I live is like dust in most places, it just happens, and covers everything).


  7. David Gessel says:

    I haven’t – have you tried em out? That’s a pretty solid CRI, though I’m guessing it would take quite a few for a good video light. :-)

  8. David Gessel says:

    Nice: “Industrial-grade chips and precision components are adopted for all the controls.” I’m definitely interested in how it performs.

  9. Reichart VW says:

    In general I have the freedom to buy what ever I want, this makes buying the cheapest crap extra fun for me. You really learn, and you don’t care if you do something stupid.

    Learning “how things fail” is part of my joy of engineering. I once bought a dozen X10 modules from every manufacturer at the time (there were like 7). I tried to ensure I used each in every location. By far Leviton failed the most and the fastest. Most died in 5 years. Everyone else was about equal, with death by 10 years. All of it was pure shite.

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