Film-Tech Forum ARCHIVE: Component Engineering LS30M Power Supply

This is topic Component Engineering LS30M Power Supply - LED dead in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Bernie Anderson Jr (Member # 410) on 08-25-2014, 10:30 PM:

This is strange and the second time its happened. I have a rack mounted CE duel power supply meant to be used with a basement DD and analog reader. Today, I turned on the left power supply (I divided the unit: left card for left machine, right card for right machine) and I had no voltage for the LED. The right side PS is fine. I swapped the cards and it is definitely the PS. There is no noticeable marks or burns on the card. It just stopped creating the 5 volts for the LED. The 12 volts works on the reader side. What would cause this? Have you heard of this happening. I have back up supplies but its odd this is now the second PS to crap out. The first one totally went dead. For my knowledge, I can use any kind of 5 volt supply for the LED and any 12 volt for the reader if needed?

Posted by Dave Macaulay (Member # 813) on 08-29-2014, 10:38 AM:

Anything electronic can die, but two in a row suggests a wiring issue. Check the wiring is OK, a decent sized wire (I think they ask for 20ga minimum size) should go directly from the PS to the LED assembly.
A huge number of these units have been removed from service in the last few years, I think someone here will have one to sell for little more than shipping cost. I didn't keep anything from the hundreds of 35 systems binned. Except a Century double shutter gearbox and a Simplex framing assembly: I just like gears, I guess.
Don't use a standard 5V supply - the LED supply has to be current regulated. The voltage is fixed by the LED's forward voltage (and 5V sounds high for an LED but I never measured it with a Dolby LED array), the current is regulated to adjust LED brightness. The LED alignment should be optimized and the current should be turned down to where your processor input gain is at about 80% for proper Dolby level; this maximizes LED life as they last much longer when well below maximum output (450mA for these LED arrays as I recall).
For a DIY supply you can make a current regulator quite easily using a three terminal voltage regulator IC, there are plenty of schematics for this on the web. You can use any 1A+ voltage regulated wall wart for power, I would use 12VDC - a higher input voltage (within reason) just means the current regulator IC gets hotter.

Posted by Bernie Anderson Jr (Member # 410) on 08-30-2014, 11:32 AM:

Thanks Dave for the reply. How about this, the power supply is in the sound rack about 10 feet away from the projector. Originally this unit was mounted in the back of the console only 2 feet away from the cell. Could the distance have something to do with it? I'm using Beldin 20 gauge wire from the cell to the power supply.

Posted by Dave Macaulay (Member # 813) on 08-30-2014, 06:50 PM:

That should not matter. I think they ask for shielded wire but it's been a while. 450mA is not much current for 20ga wire. I would recommend putting an ammeter in the circuit and checking the current, possibly you're overdriving the LEDS and since you don't use them much they are OK so far but it is causing the PS to fail?