Model train nostalgia - incandescent versus LED lamps

This is a part of Model train nostalgia site.

[lamps and photoresistor]

When digitalizing old model trains, an original incandescent lamp (picture above left) is replaced with a LED lamp (picture above middle). To compare their optical characteristics and optimise the use of LED lamps, I measured their luminous intensities using CdS photoresistor (picture above right). The photoresistors are not good at providing exact absolute values of illumination, however their wavelength sensitivity is very close to the human eye wavelength sensitivity. This is very important because invisible infrared wavelengths (which are predominant in incandescent lamps) should be ruled out from the result.

Primary testing showed that currents above 50mA will damage LED lamp, as the luminous intensity will irreversibly drop. Tests were therefore performed up to 40mA. As can be seen on video above, the luminous intensity increases sharply for lower currents, while at higher currents the increase weakens. Note that the video brightness is fixed to the top brightness of the LED lamp experiment.

[LED luminous intensity characteristics] [LED current characteristics]

This observation is confirmed by measured luminous intensity characteristics shown on the figure above left. Note that the absolute values of luminous intensity are difficult to obtain, so the plotted values are relative to the reference value of approximately IV0=1cd. On the figure above right is the voltage characteristics of the LED lamp.

CONCLUSION: Because of the weaker intensity increase at higher currents, the optimal current value is about 10mA, corresponding roughly to luminous intensity of 1cd and voltage of 2.9V. For the typical model train supply voltage of 12V, the ideal resistor in series is 910Ω.

Primary testing showed that typical top model train supply voltage of 12V corresponds to approximately 40mA, so tests were performed up to that current. As can be seen on video above, the luminous intensity is negligible for currents below 16mA, while at higher currents the increase strengthens. Note that the video brightness is fixed to the top brightness of the incandescent lamp experiment.

[incandescent luminous intensity characteristics] [incandescent current characteristics]

This observation is confirmed by measured luminous intensity characteristics shown on the figure above left. Note that the absolute values of luminous intensity are difficult to obtain, so the plotted values are relative to the reference value of approximately IV0=1cd. On the figure above right is the voltage characteristics of the incandescent lamp.

CONCLUSION: At approximately 12V and 40mA incandescent lamp luminous intensity is still 20 times smaller than LED lamp luminous intensity at 10mA.

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