Film-Tech Forum ARCHIVE: Bulb anode, and bulb anode cable short or no short?

This is topic Bulb anode, and bulb anode cable short or no short? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Eric Robinson (Member # 2966) on 04-16-2006, 01:13 PM:

I have some Strong Super Highlight consoles which have plastic or delrin rings that slide onto the anode tip of the bulb to isolate it electrically from the anode bulb support.

It would seem to me that if the plastic ring were missing that the positive voltage would short right to ground and you would get a big nasty welding mark, but I have seen these lamps strike and run in this condition without any arcing or sparking. How could this be?

Additionally I have seen lamps run that have the anode cable touching multiple metal objects in the lamp housing interior.
[Confused]

Posted by Tommie Evans (Member # 2674) on 04-16-2006, 01:38 PM:

Assuming that all the bulb/rectifier connections are good could it not be that the current is just taking the easiest route through it's circuit? In a similar way that lightning will always hit the highest building/tree in any vicinity first?

I strongly recommend though that you insulate or isolate any shorts within the lamphouse a.s.a.p. [Eek!]

Posted by Monte L Fullmer (Member # 2797) on 04-16-2006, 09:41 PM:

quote: Eric Robinson
but I have seen these lamps strike and run in this condition without any arcing or sparking. How could this be?

...do you know that DC current ACTUALLY flows from negative to positive?

Thus, why no sparking due to that you are actually grounding that line to chassis ground instead of going back to the rectifier..which is still not good for the rectifer..

Be the same why when you work on your car's electrical system, you disconnect the negative cable.

-True...still better isolate that anode pin...get a piece of fibreglass cloth and cut out a disc using a couple of layers to do the same as the teflon disc..this is a good temp for isolation until you get the teflon disc.

-Monte

Posted by Steve Guttag (Member # 268) on 04-16-2006, 11:11 PM:

In truth, the reason why there is no problem with the anode cable touching various grounded portions of the lamphouse is that in a Strong system, there is no ground reference for the DC portion of the circuit. The output of the rectifier has no ground reference and neither does the portion in the lamphouse. Thus if you do have a short with the anode lead, so what it has no affect on the circuit. You are likely to increase the noise of the system however and once grounded run the potential to get other arcs since now there are many reference points within the lamphouse.

Contrast this to Christie that does ground the Negative portion of their circuit. For that, they tend to impress more noise on the ground system of the building during lamp ignitiion as well as have more severe failures if a ground reference becomes loose. I've also seen many an ammeter cooked in a Christie because upon lamp ignition, the meter's mounting screw provided the least resistance for the arc to go through (well at least once in the meter's short life).

Steve

Posted by Monte L Fullmer (Member # 2797) on 04-17-2006, 04:20 AM:

In this, Steve (thx for the corrective info) would this possibly take out the ignitor as well? Why I ask is that I had to do both - a rectifer and a igniter changeout in an X-90 console due to the anode pin was laying in the cradle - no teflon insulators present - in the console..

thx-Monte

Posted by Steve Guttag (Member # 268) on 04-17-2006, 06:27 AM:

Monte...just the anode being grounded was not enough to cause damage...it required the anode to be grounded AND something else to allow the circuit to have an issue. The rectifier could certainly care less unless it saw ground on both + and -.

Now some switching rectifiers have been known to have one of their outputs get a short to ground from shipping...if that occurs AND you have a ground in the lamphouse, god knows what will happen.

I hear if it happens on a Highlight with digital meters...that board will blow out.

Posted by Mark Gulbrandsen (Member # 72) on 04-17-2006, 08:26 AM:

quote: Steve Guttag
if that occurs AND you have a ground in the lamphouse, god knows what will happen.

Thats easy! You will hear a loud bang as the plastic TO-247P rectifiers, sometimes accompanied by the MOSFET switching tranaistors, explode into tiny bits, sometimes accompanied by a very short fireworks display [Big Grin]

.

Mark

Posted by Eric Robinson (Member # 2966) on 04-17-2006, 03:09 PM:

I brought this topic up because I just serviced a Strong lamphouse with a switching power supply. The projection staff had just changed the bulb recently and they very elegantly draped the anode cable downward from the bulb touching at least three different metal surfaces of the lamphouse before the termination. I got a call because the switcher was no longer providing voltage to the lamp. After I checked everything I over, I noted that the switching power supply had a buss fuse which was blown. I replaced the buss fuse, redressed the anode cable and its been running fine since.

It sounds though from previous posts that the anode cable probably did not cause the switcher buss fuse to blow because the DC path from the switcher is not associated with ground. I wonder what may have happened???