I have 1 overhead projector that blows a new lamp right away. The correct lamp is rated 82Vac and 360W. I have 120Vac to the lamp, which makes sense why the lamp is blowing immediately. The only items in front of the lamp in the circuit are a fuse and a diode. Similar problem with my second projector except my voltage to the lamp is 24vac. I believe I have 2 bad diodes, but each diode has a different number even though they look very similar. I don't know which one could be the right one. The manufacturer can't tell me because they don't sell parts for these, they just warranty them out with full replacement. I don't qualify for direct replacement unfortunately. Does anyone know how to figure the right diode for what I need in this circuit? I'm an electrician but I don't work with electronic components. All I know is diodes allow current flow in one direction.....

  • \$\begingroup\$ There should be a fan in-line before or after that 82V bulb. That's pretty much how the 82 V bulb makes any sense. 40-odd volts goes to running the fan. \$\endgroup\$
    – Ecnerwal
    Commented Feb 4, 2017 at 2:56
  • \$\begingroup\$ Not exactly. The incoming 120v is paralleled off the switch and goes directly to the fan. The other line coming off the switch goes to a circuit board with a fuse, then the diode, then goes straight to the lamp. \$\endgroup\$
    – Mike
    Commented Feb 4, 2017 at 3:01
  • 2
    \$\begingroup\$ What part numbers do you have? They both appear to be marginal. A photo would help. \$\endgroup\$ Commented Feb 4, 2017 at 3:13
  • \$\begingroup\$ No luck with the picture but here are some numbers: Shorted diode, NTE 572, LF074G. Other diode, 506, 6A, 10. Thanks for the help! \$\endgroup\$
    – Mike
    Commented Feb 4, 2017 at 3:37

1 Answer 1


One diode has failed shorted. A diode reduces the RMS voltage by a factor of \$\sqrt 2\$ minus the diode drop, so 120VAC becomes around 84VAC.

The current is about 5A, so to be safe the diode should be rated well over that (turn-on surge) and will require a large-ish heatsink.

I am not sure what is happening with the second lamp-24V does not make a lot of sense. If it appears to be of normal brightness, that could simply be the way your meter responds to half-wave rectified AC.

I am not convinced that replacing the part with either OEM part is ideal. At the very least the diode will be likely to fail when the lamp burns out.

Edit: your diodes are 6A axial lead general purpose types rated for 1KV. They are marginal- the NTE one has probably been replaced once already- NTE parts are typically used by service technicians, not manufacturers.

You can buy 10A types in the same package also for cheap (dimes) but I suspect that heat is a big problem. What I would do is graft in a $3 35A 1kv bridge using only one of the 4 17A diodes (that's important- use it as a bridge and the lamp will blow) and screw it to the case or other heatsink. The bridge will have spade terminals, mounting hole and adequate insulation (but safety needs to be considered- earthed chassis only). I am not going to try to re-engineer your situation remotely without photos etc, but that is what I would try to do- and also avoid that supplier in the future.

  • \$\begingroup\$ I figured that I have at least one shorted diode which explains why I have 120v at the lamp on one projector. The other projector has 24v at the lamp. The diodes look the same, but have different numbers on them. For replacement purposes, I don't know which one is correct. The manufacturer is no help. What information is necessary to shop for the correct one? \$\endgroup\$
    – Mike
    Commented Feb 4, 2017 at 3:14
  • \$\begingroup\$ For the record my fuse is marked T6.3AL250V, so assuming it is rated for 6.3 amps? \$\endgroup\$
    – Mike
    Commented Feb 4, 2017 at 3:19
  • \$\begingroup\$ Yes, 6.3A 250VAC \$\endgroup\$ Commented Feb 4, 2017 at 3:20
  • \$\begingroup\$ Also, I never tried the new lamp in the one reading 24v. I was hesitant because lamp blew right away in the other one. These projectors are not mine and the lamps are expensive. I was asked to look at them by a friend. Do you honestly think that it may be possible that my meter would not read it correctly? \$\endgroup\$
    – Mike
    Commented Feb 4, 2017 at 3:24
  • \$\begingroup\$ bulbs consume 10x the power on cold start if at peak sine. @Sphero what do you think of using 5A ICL and 5A PTC with 10A diode? \$\endgroup\$ Commented Feb 4, 2017 at 4:32

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