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  1. #1

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    LED bulbs as drop-in replacement for Philips PCS150's bulbs?

    I've got a Philips PCS130 enlarger with a PCS150 light source. This enlarger uses additive "filtration," with three Philips 13165 halogen bulbs (MR11, 14v, 35W), which pass light through separate red, green, and blue filters. The control unit controls the brightness of the light (by reducing the voltage, if I'm not mistaken).

    The trouble is that 13165 bulbs are expensive -- in the $15-$35 range, depending on where you buy. Following a post in another thread on LED safelights, I did some digging and found that LED MR11 bulbs are available in red, green, blue, and white, so my crazy (?) idea is to replace the 13165 bulbs and their associated filters with colored LED bulbs. this vendor has bulbs with claimed "dominant wavelengths" of 474.0nm (blue), 532.6nm (green), and 626.3nm (red). These all seem to be within the spectral sensitivities Kodak specifies for its color papers, but the values aren't absolutely ideal. The vendor also warns not to exceed 14v with these bulbs, so the Philips' top voltage is cutting it awfully close. The bulbs cost $7.95 apiece from this vendor, so the price is appealing compared to the Philips 13165 bulbs, particularly if the LED bulbs would last significantly longer.

    Still, I thought I'd run the idea past the folks here. I know people have discussed custom-building LED heads for enlargers, but would these bulbs be likely to work as drop-in replacements without extensive additional control circuits? (I could solder together a simple circuit for each bulb, if necessary, but I'm not exactly an electronics specialist or hobbyist, so I wouldn't want to do anything too complex.) Would the dimming function work correctly? If so, would the calibration values still be reasonable on the control unit? Would these bulbs be of appropriate brightness? If the red, green, and blue bulbs' spectral output is too far off optimum, could the white bulbs be used with the existing filters instead?

    My hunch is that this idea won't work, or at least would have serious drawbacks, but I thought I'd ask here for advice before discarding the notion.

  2. #2
    Bob F.'s Avatar
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    I would not expect the dimming function to work if the original lamps are dimmed by varying the voltage. LED brightness is controlled by the current flowing through them, not by the voltage applied across them. Varying the voltage will only give dimming over a very short range as once the voltage across it drops below the LED's forward voltage rating, it will switch off. Exceeding the maximum forward voltage by even a small amount will kill the LED (hence the 14V maximum warning).

    If the enlarger varies the exposure from each lamp by varying the time it is on (which is safer with halogen lamps as they are not 100% happy with being dimmed either, though for different reasons than LEDs) then you may be in with a chance. I've no idea if those LED lamps will have enough brightness though - I would expect the halogens to be much brighter.

    I guess there may be problems with the beam pattern of the light too but it's probably worth getting a red one to try (you can always use it as a B&W safelight if it turns out to be too dim for the enlarger ).

    Have fun, Bob.



 

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