can I use the Labophot's intenal power supply for LED etc?

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iconoclastica
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can I use the Labophot's intenal power supply for LED etc?

#1 Post by iconoclastica » Tue Nov 15, 2022 6:57 pm

When I remove the lamphouse of the Nikon Labophot, I measure the voltage at its contacts as a constant 17.2V. It does not change while turning the power wheel (except off). Does this mean that the halogen bulb is regulated by current at constant voltage, or that this is not the correct way to measure the power..?

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blekenbleu
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Using Labophot's internal power supply for LED

#2 Post by blekenbleu » Tue Nov 15, 2022 7:48 pm

The Labophot (1, don't know about 2) uses a 2N3716 transistor in series with a 6V 20W halogen lamp,
which nominally wants a constant voltage supply for brightness control.
Since grounded emitter bipolar transistors are nominally constant current devices,
Nikon implemented forward bias to that transistor by a variable resistor connected to its collector.
Lacking the relatively low resistance of its halogen lamp, that 2N3716 does not get enough bias to turn on,
so you measured peak voltage from the Labophot's unfiltered bridge rectifier.

To provide constant current for LED brightness control,
a Labophot 1K control wants rewiring, e.g.:
  • CCW terminal to 2N3716 emitter and bridge rectifier negative terminal (blue wire)
  • wiper (middle) terminal to 2N3716 base (brown wire)
  • a fixed resistor from CW terminal to bridge rectifier positive terminal (red wire)
This leaves the yellow wire, which previously connected the control to 2N3716 collector, disconnected.

For a 5W LED, fixed resistance of 680 Ohms between potentiometer CW terminal and red wire should be about right.
To reduce 50/60hz flicker, consider wiring an electrolytic capacitor in parallel with the LED,
which is wired (as was the halogen lamp) between the bridge rectifier positive terminal and the 2N3716 collector.
Metaphot, Optiphot 1, 66; AO 10, 120, EPIStar, Cycloptic

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iconoclastica
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Re: can I use the Labophot's intenal power supply for LED etc?

#3 Post by iconoclastica » Tue Nov 15, 2022 9:06 pm

Hmm, was suspecting something like that.

My project is a bit more complex than just replacing the bulb with a led. In fact, for the time being at least, the original lamp should continue to be usable. I was hoping to read the pot meter indirectly to mimic its setting with the led's brightness, and/or to tap the power to avoid yet another power source for the led, a small stepper motor, and the controlling arduino, together nowhere near the 50W the bulb can handle.

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