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Arri S magazine motor


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#1 NCSProducts

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Posted 08 July 2006 - 11:01 AM

I tried running an Arri S magazine motor at 8V then at 12V, and sure 'nuff,
it does run faster at 12V.

So I decided to stick a resistor inline with the motor.

Posted Image

(Well, it's two 15 ohm resistors in parallel for 7.5 ohms resistance. The motor voltage is now about 9V.)

Curious if this is necessary, or if they can be run at 12V with no ill effects.
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#2 Tim Carroll

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Posted 08 July 2006 - 11:38 AM

Clive Tobin in the best person to answer this question.

-Tim
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#3 Luke Prendergast

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Posted 08 July 2006 - 05:35 PM

Those look like only 0.25w resistors. Dropping 3v they'll be dissipating 0.6w each. You might want to up the rating or they'll bake in pretty short order. Can't answer whether the motor's okay on 12v though, sorry.
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#4 Chris Keth

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Posted 08 July 2006 - 07:48 PM

Edit: Never mind. I had a question that I answered myself.

Edited by Christopher D. Keth, 08 July 2006 - 07:51 PM.

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#5 Charles MacDonald

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Posted 08 July 2006 - 08:35 PM

Those look like only 0.25w resistors. Dropping 3v they'll be dissipating 0.6w each. You might want to up the rating or they'll bake in pretty short order. Can't answer whether the motor's okay on 12v though, sorry.

belive it or not the newer resistors have shrunk in size, I recently got some that look a lot like those in the picture, and I think they were 1 or 2 Watts. The same physical size as the Allen bradly 1/2 watt units used to be..

(charles who dabbles in too many technologies)
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#6 NCSProducts

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Posted 08 July 2006 - 09:22 PM

Those look like only 0.25w resistors. Dropping 3v they'll be dissipating 0.6w each. You might want to up the rating or they'll bake in pretty short order. Can't answer whether the motor's okay on 12v though, sorry.

Good point. They are actually half-watt. The motor draws about 1/2 amp, so they are dissipating almost 2W. They weren't hot to the touch, so I didn't worry about it. It's not like the motor is running continously--only in short bursts.

But getting back to the original question, I'm looking for anecdotal 'evidence' along the lines of:
"I've run the magazine motor at 12V for years with no problems" or "My magazine motor burned out while running at 12V"
:unsure:
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#7 Tim Carroll

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Posted 09 July 2006 - 12:00 AM

Well I was hoping Clive would chime in here, but maybe he has not looked at the boards lately. From his web site:

The 16-S 400' magazine torque motors, however, are mostly designed for 8 volt operation. They must be converted for use on 12 volts to prevent overheating and burnout, and also to reduce electrical interference with the crystal drive motor.


And:

A 12 volt conversion of your 400 foot Arri-S magazine torque motor, required for 12 volt operation, is $49.


More info at:

Tobin Cinema Systems


Hope that helps,
-Tim
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#8 Hal Smith

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Posted 09 July 2006 - 12:24 AM

I tried running an Arri S magazine motor at 8V then at 12V, and sure 'nuff,
it does run faster at 12V.

So I decided to stick a resistor inline with the motor.

Posted Image

(Well, it's two 15 ohm resistors in parallel for 7.5 ohms resistance. The motor voltage is now about 9V.)

Curious if this is necessary, or if they can be run at 12V with no ill effects.

No opinion on 12 volts, but a LM7808N regulator or equivalent will drop anything up to about 30 volts down to a pretty well regulated 8 volts, it's rated up to 1 ampere. A small 1 uf at 15 volt or so capacitor across the output will keep motor spikes out of it. It would require a heat sink but that could be something as simple as a dab of heat sink grease and bolting it down somewhere on the magazine frame. It's a 3 terminal device, in...out...and common (ground). No support components required but the small capacitor on the output is good engineering practice.
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#9 Clive Tobin

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Posted 09 July 2006 - 07:01 PM

No opinion on 12 volts, but a LM7808N regulator or equivalent...


What I put in is 3.6 ohm 5 watts, with two specially selected strategically placed capacitors to bypass some of the wicked arcing to ground, plus a surge suppression diode of 15 volts to protect anything else in the neighborhood that is delicate, like a drive motor with solid state electronics inside.

We are likely only doing this until we are no longer selling the drive motors for the Arri S and M. The crystal motor is discontinued and we still have some variable speed ones left.

No offense but I would suspect that something as modern as a solid state voltage regulator will not last long inside an Arri S torque motor. Have you seen the arcing that goes on on the commutator and brushes? It amounts to a spark gap transmitter that wipes out AM radio reception nearby.
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#10 Tim Carroll

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Posted 09 July 2006 - 10:58 PM

It amounts to a spark gap transmitter that wipes out AM radio reception nearby.


:D
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