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Topic: DC powered Change-over Zippers

Frank Angel
Film God

Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999

posted 09-10-2002 06:55 AM

Although I have been able to minimize the change-over thump caused by the zipper soleniods when I am running mag prints, it still bothers me. I was told that switching to change-over zippers which use DC coils will eliminate any 60hz hum, although I can't imagine that the DC surge wouldn't cause some kind of pulse noise in place of a hum -- though it's duration would be shorter, I would guess.

Anyone have any experience with this problem or DC coiled zippers? The heads are well shielded and what they pickup is very attenuated (when any reasonable level of audio is on the soundtrack, the hum is almost inaudible), but if DC coils would eliminate it entirely, I would like to try it.

Frank

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Richard Fowler
Film God

Posts: 2392
From: Ft. Lauderdale, FL, USA
Registered: Jun 2001

posted 09-10-2002 08:39 AM

D.C. energized coils are more common in projectors with common fire shutter / picture dowser such as in a Cinemeccanica....which have their own D.C. problems to consider. The rotary type such as used in a Ballentyne Pro rarely gave me problems...but that a long time ago.
Richard Fowler
TVP-Theatre & Video Products Inc. www.tvpmiami.com

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Pete Naples
Phenomenal Film Handler

Posts: 1565
From: Dunfermline, Scotland
Registered: Feb 2001

posted 09-11-2002 02:55 AM

During the course of this install I'm in the middle of it has become apparent that the CP650 remotes are very sensitive to noise coming from the Kelmar zippers on Centuries. The remotes are locking up and then resetting themselves. Before anyone asks yes it's all wired and shielded properly! After a bit of fiddling around and discussion with colleagues I tried running the zippers (220v coils) on a multi tap auto-transformer. I've got them down to 170v, at which they EMI splash isn't upsetting the remotes, and the zippers still work quite happily, in fact they are happier as they are not banging into their internals with such force, which hopefully will stop them from shaking themselves apart every other week.

Frank, maybe you could do something similar, as I'd have thought reduced coil voltage would mean reduced hum?

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 09-11-2002 07:24 AM

Yes, the DC coils will nix the hum in a mag head but there are some things to consider...first...if you merely rectify and filter the AC line it will be somewhat higher voltage than 120. A bigger concern is that you need to check your switch and relay ratings...most won't handle DC with any current...the contacts will arc and fuse together. Consider protecting them (with diodes or capacitors).

Steve

------------------
"Old projectionists never die, they just changeover!"

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Dave Macaulay
Film God

Posts: 2321
From: Toronto, Canada
Registered: Apr 2001

posted 09-11-2002 02:38 PM

Possibly just adding a snubber to the coil circuit would solve the problem. I've seen simple series R-C circuits and thos MOV discs used for this.

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Frank Angel
Film God

Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999

posted 09-12-2002 02:21 PM

Thanks guys.

Steve, I wasn't thinking so much of rectifying the 115vAC as looking for a unit that was actually designed with 12vDC coils (I've got to do SOMETHING with those old exciter power supplies). Seems that 12vDC solenoids are common enough....zippers are just two solenoids back to back on the same shaft. I was even thinking of looking for DC coils that would physically replace the coils in my existing units.

Then again, Pete's idea seems worth a try. As long as the voltage is enough to pull the arm, why use more? I even have a few 115v autotransformes in the shop I can test it with.

Dave, I've got some MOVs in a junk parts draw; can you give me quick rundown of how to setup the snubber circuit -- cap and resitor values?

Thanks. Frank

Frank

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Dave Macaulay
Film God

Posts: 2321
From: Toronto, Canada
Registered: Apr 2001

posted 09-12-2002 02:55 PM

Try just putting the MOV across the changeover wires, you need 2 each projector. The kickback voltage is a product of the solenoid's magnetic field collapsing into the open circuit and the voltage can be quite high. A MOV designed for 120VAC use should do fine, although they can break down after many spikes and cease working.
I didn't include the R-C values because I can't recall them... basically a resistor and cap in series with the RC time constant at about 1ms. Since the spike rise time is quite fast, it has a high frequency aspect and gets soaked up by what is a crude high pass filter.

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 09-12-2002 04:09 PM

Put a separate bridge rectifier and cap or filter choke on each changeover circuit. Then you don't have to worry about contacts welding as the switching would be done with AC.

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