Film-Tech Forum ARCHIVE: Connecting a Change Over

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Posted by Randy Stankey (Member # 64) on 02-07-2002, 03:58 PM:

I'm going to be adding another improvement to the projector here at the college. (New job in an old place.)

It'll be a Kelmar C/O box on top of a Simplex XL (PR-1003) Operated by the Comp. Eng. TA-10 automation.
I have virtually no concern about the mechanical hookup. It's the electrical that I have a question on:

Which set of terminals do I tap into in order to bring the power over to the relay? There is the 120v line for the automation power and another one for the projector motor power.

I was thinking that if I hooked it to the automation power feed the inductance from the solenoid coils would cause voltage drops/spikes that might cause the automation to crap-out.

I'm planning to hook up to the motor's supply but I just wanted to ask before plowing ahead. KnowwhatImean?

Posted by Pat Moore (Member # 380) on 02-07-2002, 06:21 PM:

Hi Randy; Shouldn't be a problem either way, actually. I think the TA-10 is pretty immune to problems. That said, a more "normal" practice might be to use the same AC Input that runs the motor to power the changeover.

Pat

Posted by Mark Gulbrandsen (Member # 72) on 02-07-2002, 06:29 PM:

Pat is right, use the motor circuit. Thats usually what I do. Also don't forget to fuse the C/O circuit with an inline or panel mounted 3AG type fuse holder. Even the Kelmar C/O's can burn up if the switch is held down long enough, or if the TA-10 belches for some reason or other. Also put a snubber across the C/O Switch to quench the spark that'll occur when the switch is released. This will help keep noise out of the sound system as well as prolong the life of the switches.
Mark @ GTS

Posted by Frank Angel (Member # 248) on 02-08-2002, 02:18 AM:

Does anyone know of change-over solenoids that are DC driven rather than 120vac? Even though we have gotten almost all 60hz hum thumps out of the mag change-over, the subbass drivers still give out a very low level hum for the duration of the time the foot pedel is depressed. The staff says they can barely hear it, but it drives me crazy. It all points to 60hz magnet fields radiating from the solenoids.

Someone said that instead of 120vac, I should try DC solenoids. Makes sense. Then he said that I should just use the old exciter power supply to power them with 12vdc @10-15amps. That makes NO sense. Could a coil designed to pull in at 120vac work equally as well on 12vdc? I am assuming there must be some change-over units that are made specifically for DC voltage. Or, could I just put a bridge rectifier on the coils and feed them 120vDC?

Hey, you want an embarassing moment? In my attemt to get rid of the hum, I thought I would wrap the change-over unit with a lead shield. I went to a shop and they said they had lead in sheets but only in 100lb rolls. So I got myself enough lead to cover about 1000 change-over boxes. I carefully cut the lead sheeting and wrapped the two c/o boxes. I was quite proud of my efforts and called one of my collegues from the TV studio to come over to see my handywork. He came over and saw the c/os wrapped in lead. He asked why I had done that, and I told him to block the magnetism of the coils from being picked up by the penthouse playback heads. He began to laugh and asked what made me think that would work, so I explained that encasing the coils in lead would make the magnetic fields impenetrable. He laughed even more as he told me, "Lead doesn't stop magnetic fields! It only stops Superman's Xray vision." Oh.....I knew that.

Posted by John Pytlak (Member # 331) on 02-08-2002, 06:58 AM:

Mu-metal can be used to shield magnetic fields:
http://www.thermovgscientific.com/Tecnical%20Support/mu-metal.htm
http://www.magneticasoft.com/shield/earthfield.html
http://www.magneticasoft.com/materials/magnet1.html

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Steve Guttag (Member # 268) on 02-08-2002, 09:12 AM:

Yup, Mu-Metal is the only good shield for mag heads.

What type of projectors do you have? I've seen a bunch of Century JJs that do not have ALL of their shields. The most important being the "tongue shield" below the head, then there is the one that is around the head and lastly, there is the one mounted with the changeover to stop it's EM at the source. I believe the Mu-Metal shield for the changeover for the JJ was called the SM-1556. Strong still lists it for $46.00 list.

If you wish to go the DC coil route...Essaney did offer DC coils Wolk, the current manufacturer of Essaney c/os may still offer them. Essaney used 120V changeovers.

The quality of the DC doesn't have to be high for them to work. You can simply rectify the AC line and filter it...however, your voltage will be substantially higher than 120 volts (closer to 170 no-load). By using some high power resistors, you can taylor the force of the changeover and not over drive it.

For the Kelmar style of changeover, you can merely replace the coils with DC units and obtain a suitable off-the-shelf supply for them based on what voltage you chose.

Remember that dealing with DC is not the same as AC. Relays and switches often have a MUCH lower DC rating and the contacts will pit and arc more. Make sure you use components rated for the kind of voltage/current you are using in DC. Even better protect the contacts with diodes.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Gordon McLeod (Member # 33) on 02-08-2002, 09:42 AM:

Also with DC coils a snubing diode will be required to reduce arcing on contacts when the field in the solinoid colapses

Posted by Bill Purdy (Member # 202) on 02-08-2002, 11:56 AM:

Randy, you are asking all the right questions and your logic is good. You can use the motor source for your 120 V, but please see that the feed wires are of good size. When you push the "Start" button on the TA-10 not only is the motor started, but the Douser Close coil is given an insurance shot just to make sure nobody has left it open. This means that you have not only the high current douser coil and the motor, but you also have the start winding of the motor all calling for current at the same time. This is why I suggest big wire on the feed side.

I have a quick comment on using DC. One good way to do this is by charging up large capacitors through some resistance and then when you call for the changeover the coils get a major pulse which makes them operate rapidly but because of the resistor there is not much sustanined current through the coils and therefore they are protected. I've never tried it, but I don't see why some young Tom Edison can't work out a way to do this using the existing AC coils.

------------------
Bill Purdy
Component Engineering

Posted by John Pytlak (Member # 331) on 02-08-2002, 12:11 PM:

Bill Purdy said: "...but I don't see why some young Tom Edison can't work out a way to do this using the existing AC coils."

Calling Josh Jones!!!

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 585-477-5325 Cell: 716-781-4036 Fax: 585-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Bernie Anderson Jr (Member # 410) on 02-10-2002, 10:28 PM:

Hey Frank Angel,

We're running 12 volt DC changeovers at the Loews Next time you come by I'll show you

Posted by Randy Stankey (Member # 64) on 02-10-2002, 11:43 PM:

Quencharc?

Are we talking about those things that look like an oversized mylar capacitor but really contain a cap and a resistor... Making it, in reality, a "Resist-O-Pacitor"?

I'm going to need two of these, right? Hook them in parallel with the coil?

What's a good source and a ballpark price?

As to feed wires:
All wiring in the system is 12 AWG stranded.
There is NOoooo problem with power feed in this booth! There is a dedicated transformer just for the projector. It's a 150KVA, if I'm not mistaken!

Posted by Gordon McLeod (Member # 33) on 02-11-2002, 10:25 AM:

No put the quench arc across the relay contact and a reversediode across the relay coil

Posted by John Pytlak (Member # 331) on 02-11-2002, 10:43 AM:

A similar question on another forum:
http://webforum.philipsmcu.com/mcu/support/forum/200101/0302.html

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 585-477-5325 Cell: 716-781-4036 Fax: 585-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Frank Angel (Member # 248) on 02-13-2002, 01:21 AM:

Thanks you guys. Very helpful. We have Simplex mag penthouses and change-over units of an unknown orgin (other than they are old) sitting on XLs. If I could wrap the solenoid box itself with mu-metal, that would be a big help. Thing is, I don't want to loose hair over it (that's a joke) because it is barely audible, except to those of us who barely is too much.

And Bernie, I will take you up on that for sure! I can't get back to the Loews Jersey soon enough. BTW, anyone who is anywhere near Jersey City NJ and hasn't stopped in to see the work Bernie and Bob are doing on this magnificent theatre and the AWESOME booth, are missing a real treat.

And Gordon, now you've got to explain to me what a snubbing diode is -- how do I get one and how do I get it to go into snub mode?

Posted by Gordon McLeod (Member # 33) on 02-13-2002, 09:49 AM:

All it is is a diode placed across a relay or solinoid coil reversed so it will effectivly short any back emf that the collapsing field in the coil generates when it becomes denergized
Usually it is 400vPIV rated often the common 1N4004 is used