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Author Topic: 3-bladed shutters Matthew Jaro Film Handler Posts: 74 From: Gaithersburg, MD, USA Registered: Jul 2003 posted 07-23-2004 08:45 AM                        This is just mere speculation: but do you think that the strobing problem that Don Furr aluded to could have been caused by lamp power supply ripple? 120 Hz ripple - 72 Hz shutter interruption rate = 48. Since the line frequency is not synchronized with the shutter, the effect would be strobing. Does this seem likely?  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 07-23-2004 09:28 AM                        Yes, if you have significant 60Hz current ripple in the DC being supplied to the xenon lamp, you can get a 12Hz "beat" with the 48 or 72 frequency of the shutter blades, which is very noticeable as flicker. This 12Hz beat pattern would be the same for both 2-blade and 3-blade shutters.  |  IP: Logged Matthew Jaro Film Handler Posts: 74 From: Gaithersburg, MD, USA Registered: Jul 2003 posted 07-23-2004 11:00 AM                        John -- Since full wave rectifier bridges are commonly used, wouldn't the ripple frequency be 120 Hz (one ripple for each half wave)? Rectification occurs on both the positive and negative swuings of voltage. Poor filtering due to defective filter capacitors or a shorted choke would cause this to occur. In this case, the strobe would be more noticable with a three blade shutter, since the shutter interruption speed is closer to the full wave ripple frequency.  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 07-23-2004 01:36 PM                        When I've seen it, it was closer to a 12Hz frequency. Could it be that for some reason the ripple had a strong 60Hz component? (e.g., the full-wave rectifier was not balanced?)  |  IP: Logged Edward Jurich Master Film Handler Posts: 305 From: Las Vegas USA Registered: Jul 2003 posted 07-25-2004 05:41 PM                     I would think high ripple would suggest bad filter caps. If the caps are that bad the voltage usually drops and my experience with bad caps and low voltage is the lamp is hard to strike at times. Now perhaps if one of the rectifier diodes was leaking or shorted on a 3 phase rectifier you could get some strobing.  |  IP: Logged

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