I have an old projector I'm converting to LED just for fun.

I've bought an 11000 lumen, 100 watt LED. I'd like to wire this directly into the housing unit for the old light, using the connectors from the original bulb with a step-up or step-down board to reach the 30V needed for the LED. However, I'm having trouble finding out the voltage that went to the original lamp.

The manual (https://www.manualslib.com/manual/720744/Sharp-Xg-Mb70x.html) for the projector (Sharp XG-MB70X) says it's a 275W bulb, but doesn't list the voltage/amperage that I've been able to find, though I've had trouble with understanding many of the diagrams in there, being new to this community and just beginning the long journey to understanding electrical diagrams.

Is there a way to find this out? I would just test it but thanks to numerous failsafe switches getting a Multimeter in there while it's actually running isn't possible without drilling holes through the housing.

Also, as a followup, I'm worried about the heat produced by the LED vs the original bulb. Any thoughts on the difference in heat between a 100W LED and 275W mercury bulb?

  • 2
    \$\begingroup\$ I have done exactly that. Boy o boy did it end up dim. You need those tens of thousands of lumens to be emitted from something very small if you are to have any chance in collimating it into the existing optics. This was about 4 years ago so LEDs are smaller and brighter, but you are in for a surprise and disappointment. Cooling is also much more challenging since the UHP mercury bulb radiates away almost all heat but the LED does not radiate IR so you need to heatsink and redesign/add airflow to allow it. \$\endgroup\$
    – winny
    Sep 8, 2020 at 13:19
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    \$\begingroup\$ Food for thought: curtpalme.com/forum/viewtopic.php?p=454526 \$\endgroup\$
    – winny
    Sep 8, 2020 at 13:21
  • \$\begingroup\$ Thank you Winny, that's good to know. \$\endgroup\$
    – John S.
    Sep 8, 2020 at 13:54

1 Answer 1


I suggest that you're going about it in slightly the wrong way.

The connections to the lamp itself in a projector come from the ballast, which provides a controlled current. In a projector (and modern, but not all older discharge lamps) it's likely to be a fairly sophisticated ballast circuit, producing a higher starting voltage until it detects the arc has been struck. Anyway the voltage will be high enough that you really want to avoid it.

Instead, you should probably integrate a suitable LED driver for your LED, driven off the input mains voltage (though there's a chance you could find a lower-voltage line that can drive enough current, it's unlikely). The LED lamp bought you've may need a fixed current, or the current may be regulated within the lamp.

Cooling should be OK. You're putting ~1/3 of the power in, but the LED lamp will be more sensitive to heat than the mercury one.

Two more concerns:

  • Optical. I haven't seen, or properly looked for, an LED source that provides such an intense spot as a typical projector lamp when focussed, though there are apparently some being sold. The total emission is less important than what it can emit from a small area in a narrow range of angles.
  • Bulb-checking circuits. Many of these projectors have a circuit to check the presence and function of a bulb. This often means that it will refuse to drive even a bulb that's electrically compatible with the drive circuit if the checking circuit isn't satisfied. There are ways round that; with a cheap direct equivalent to the original bulb, transplanting the checking circuit onto the new bulb may have been possible, but with an LED you'll need something cleverer.
  • \$\begingroup\$ So I'd basically have to find the right circuit to short for checking if a bulb is installed, then it would run as if a bulb were there, right? I have an LED driver already, so I can definitely attempt it that way. \$\endgroup\$
    – John S.
    Sep 8, 2020 at 13:51

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