2013-03-23 LED Brightness Comparison

I did learn something from the Grease Spot Photometer Experiment.  I found that the grease spot really does work.  I can compare the brightness of two LEDs to find out the current in each LED.  But then I realized that I don’t really need two LEDs (I didn’t use two with my luxmeter measurements).  All I have to do is use a single LED, and switch it between  the Joule Thief and a known adjustable current source.  When I adjust the current source to give the same LED brightness then I can read the actual current from the source.  This is the system that I built.


A few years ago I built up a measurement jig to use my then new luxmeter as the instrument to compare the LED brightnesses.  I cut a small hole in the end of a small cardboard box and I fastened a LED into it so that the LED’s light beam was hitting the far side.  I then cut out a large hole in the far side to allow the luxmeter’s sensor to be placed there to measure the LED’s light output.  I used this not for comparison, but  for actual LED light output.  But I really don’t need to have an absolute measurement of the LED’s light output.  All I need is a comparison of the LED light output from the JT and then from a current source.  The important part is that both use the same LED and photocell, so the measurements will be the same.

The Box

I will use a single LED, so I need to switch it between the JT and the current source.  I used a SPDT switch, and I put a 5.6V zener diode across the wires from the JT so that when the LED was disconnected from the JT, the excess voltage would be dissipated in the zener diode.

I didn’t want to use the luxmeter, which eliminated the cost and difficulty in obtaining it.  Instead I wanted to use a cheap DMM,  And to make it easy, I used a CdS photocell from Radio Shack (276-1657). To keep ambient light away from the photocell and skewing the measurements, I chose a cardboard box

I drilled two 5mm holes for the white LED in the end of a cardboard box[1], and on the other end I drilled two small holes for the leads of the photocell, passed the leads through to the outside and soldered a short flexible pair of wires to it for the meter clip leads.  I wanted the LED to fit through the hole without moving around, so I drilled two 5mm holes side by side in a small piece of thin plywood and glued it to the end of the box, over the two holes.  This gave the LED a firmer fit than just the cardboard.  I wanted the whole jig to be stable so that measurements would not change due to position changes or movement.  I left the CdS photocell leads free so I could clip on my DMM.  I should have added a second switch to switch the DMM leads, but for now it’s working okay the way it is.

I soldered  the switch to the LED, a 100 (actually 98) ohm resistor, and a 5.6V zener diode.  I used a Joule Thief circuit I had laying around, removed the LED and soldered the switch leads to the where the LED used to be.


I connected the 1.5V supply to the Joule Thief circuit.  With the switch set to connect the LED to the JT, the DMM measured 7.4 millivolts across the 0.47 ohm resistor that was already in the JT.  7.4 divided by 0.47 gives 15.74 milliamps through the resistor.  That’s about what should be expected from a conventional JT.  I measured the photocell and it was 1902 ohms. I then switched the LED to the power supply and adjusted it until the DMM read 1894 ohms (it was very touchy and hard to adjust).  The LED was now putting out the same amount of light as it was from the JT.  I measured the voltage across the 98 ohm resistor and got 1.29 volts.  That, divided by 98, gave 13.16 milliamps through the resistor and LED.


The LED driven by the JT measured 15.74 milliamps across the 0.47 ohm resistor.  The same LED putting out the same amount of light measured 13,16 mA on the power supply.  The JT reading across the resistor in series with the cathode was 19.6 or about 20 percent higher than reading the LED driven to the same brightness by the DC from the power supply.

Others questioned the accuracy of using a current sensing resistor in series with the LED, and a DMM to measure the LED current.  They said that the DMM could not accurately measure the pulses of current through the LED.  I agreed with their assertion, but I said that the measurement gave a good relative reading of how the circuit was performing.  Now I can say that the DMM measures about 20 percent higher than it should, and if further measurements with other JTs confirm this, it could be applied in general for any JT with a DMM.

Another One – Mar 23

I put another JT on the jig and made measurements.  This JT used a PN2222A transistor, a 1k base bias resistor, and 5 turns 24 AWG wire bifilar wound on a 3/8 inch (9mm) high mu core.  The 1 ohm LED resistor measured 15.7 mV, which is 15.7 mA.  The photocell resistance was 2091 ohms, and it took 1.128 volts across the 98 ohm resistor or 11.5 mA to get the same photocell resistance.  The 15.7 mA divided by 11.5 mA gave 1.1365 or 36.5 percent higher for the 1 ohm current.  That was quite a bit more than the 20 percent earlier.

Mar 24 – #3

This one was a Supercharged Joule Thief and I used a trimpot in series with the 1k resistor.  This allowed me to adjust the V drop across the 1 ohm resistor to 20 mV, which I later measured at 20.1 mV, same as 20.1 mA.  I completed the other steps and the result was 14.17 mA DC.  Then 20.1 / 14.17 gave 1.418 or 41.8 percent higher.  This is the highest so far.  I do not know if being a Supercharged Joule Thief had anything to do with the results.

More to come.

[1]] The cardboard box measured 11 inches (275mm) long by 6.4 inches (163mm) wide by 2.75 inches (70mm) high.


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