Flying Spot Scanner Construction Journal

A build of a 32 line flying spot scanner.

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Re: Flying Spot Scanner Construction Journal

Postby Lawnboy » Wed Apr 23, 2014 7:46 am

I think I fixed the inrush issue. I wired the rectifier to power up with the main power switch rather than the lamp switch, and also removed one of the smoothing caps. It didn't take long for me to realize that something had to be changed after the switch contacts were welded closed on power up! :D
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Re: Flying Spot Scanner Construction Journal

Postby Klaas Robers » Fri Aug 15, 2014 8:15 pm

Lawnboy,

be aware of the fact that when you use a (bridge) rectifier to make DC directly from the AC powergrid, and you inserted a big (storage) capacitor behind the rectifier, that your DC voltage will be considerably higher than the AC voltage of the line. This will shorten the life time of your lamp a lot, although it gives more light.

If your AC voltage is 115 volt (effectuve value), then the peak voltage will be 1.4 x 115 = 160 volt. So the DC voltage will be close to that 160 volt. I expect that you have a multimeter, so measure it.

To correct that, you may connect a resistor in series in between the switch and the rectifier, or in between the rectifier and the storage capacitor. Then the DC voltage will get lower, but you also lower the large inrush current to an acceptable value. For the resistor you need a high power resistor. The ohmic value is difficult to calculate, but why not start with a heating element of an old fan heater or a hair dryer. Experiment with it until you measure the DC voltage to be in the neighborhood of 115 volts, say between 100 and 130 volt. Then both problems are solved at the same time. If you have 300 watts on lamps, don't be surprised if one heater element is still too high ohmic and you need to connect a second one (and/or a third one) parallel to it.
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