Lighting

Rectifier for carbon brute arc 450? (2 replies and 2 comments)

lydgate
4 days ago
lydgate 4 days ago

Hi there, I am interested in purchasing a 450 Brute arc but will need a way to play it on 3 phase AC. I haven't had any luck sourcing out a rectifier online that is readymade for film use (w/ 5-wire input). Obviously any rectifier can be made to accept camlocks, though I am curious as to whether I still need to use a grid, and if not how would I control the voltage of the rectified DC power?

I am a bit confused as to why the lamp head reads ' 225 amp at 120 VDC....since I thought that they needed a grid to step down the voltage to something like 72 VDC? However as I am not using a DC generator would my rectifier have to take it strait from 120 VAC to 72 VDC?

I am assuming that I have to have this rectifier custom built, or can I buy one somewhere that is specifically designed for brutes? 

Mike
4 days ago
Mike 4 days ago

I acknowledge your interest in these old lamps but is this the way to go. Modern lamps do the same job without all the grief!

Its been some time since I had my hands on these "things" but they certainly worked well in their day. As far as I can remember, these lamps could handle (up to) 225 amps but you need a transformer to step down the juice to a lower voltage. Here in the UK and Europe we use 240 volts where in the US is 110 volts which is why we had a transformer. The rectifier is to switch AC to DC and you probably can build your own if you can source the right crystals. By all means renovate the lamp as a collectors piece but think twice if you are going to use it as a studio lamp. They are thirsty in terms of eating carbon rods, you'll be lucky to get 30 minutes continuous use in high output mode or 45 minutes in reduced output. They need constant maintenance too, cleaning the chimneys of soot and greasing the carbon rod holder as the intense heat dries out any lubrication.

Samualson in the UK eventually modified their arc lights to take high output bulbs which eventually killed the carbon lamps but they are still used throughout the world as the main fill lamp.

Wish you luck though as it's a worthy project.

Mike
4 days ago

May have my wires crossed here. I was referring to the original lamps you maybe talking about the second generation used around the 1970/80's. I was referring to the 1940/50's.

lydgate
2 days ago

I think we are talking about basically the same light, don't think the overall look of the 450 ever changed much...but all of the Molearcs out there I have seen read 225 120 on the back.
From watching Larry Parker's videos on Youtube I have gathered that from a DC generator you would run feeder cable that is at 225 amp 120 VDC...and this goes into a grid which steps it down to 72 VDC....so then does the carbon rod diameter determine at which amperage the lamp will operate?

Mike
2 days ago
Mike 2 days ago

I visited a carbon rod factory in East London once and discovered how rods were produced. They were moulded in all shapes and sizes round, square etc. Once the carbon rods were moulded and dried in a kiln, they were then machined to exacting standards. They were then inserted in a large projector type machine and the light was measured for purity. Many were rejected, crushed and put back into the mincer.

The lamps I remember used 3/4 inch wide rods. On start up they probably needed 225 amps but then when adjusted correctly the amps reduced. It had a spring clock type mechanism which automatically adjusted the gap as the rods were being eaten away by the heat. The closer the gap the brighter the light. Our lamps were using studio mains 240 volts but stepped down by a fan cooled transformer probably to 110 volts. Mitchell's used 90-110 volts. Levers-Rich tape sound recorders were also 110 volts but had internal 18 volt transformers. Today's aircraft are also 110 volts from 24 volt inverters.

Let us know if you buy the lamp, interested to see if it works.

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