Film-Tech Forum ARCHIVE: Lamp strikes, house lights go out

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Posted by Thomas Pitt (Member # 4322) on 02-25-2010, 07:41 AM:

At my nearest theater, I've noticed that whenever the lamphouse strikes, the auditorium lighting for that screen briefly dims or even turns off - only to come on again when the bulb is working. This can also happen (to a lesser extent) if the lamphouse for the adjacent auditorium strikes.

I'm assuming this has something to do with the power surge caused by the lamphouse turning on - but is this effect 'normal' for a multiplex? Is there anything that can be done to stop or minimize the effect? In this theater in particular, all the lighting comes from sconces on the walls, and to plunge the audience into total darkness for a second or two doesn't seem very professional...

Incidentally, they used to dim the house lights a little bit when the advert reel started, then turn the lights off at the start of the trailers. Now, the lights remain at full brightness until the start of the trailers, at which point they dim and turn off.

Posted by Louis Bornwasser (Member # 3063) on 02-25-2010, 10:00 AM:

Certainly not normal. It sounds like the rf from the igniter is getting into the dimmer input somehow. Might try capacitors on the input to the igniter and supression on the dimmer input.

. . . .of course, I could just blame it on "slow" electricity. (50Hz). Louis

Posted by Jim Cassedy (Member # 4115) on 02-25-2010, 10:10 AM:

I've had two auditoriums where a slight short inside the
lamphouse during ignition was feeding back to the dimmers
through ground (I guess) and causing an effect similar to
what you describe. In my case, one the high-voltage wires
was too close to another metal part inside the lamphouse
and when the igniter fired, a small spark was jumping from
the high-voltage wire to ground. There wasn't enough leakage
to affect bulb ignition, but it did cause the auditorium
lights to momentarily dim. I also noticed it caused the
platters in one of the booths to "jump" slightly. I could
see the film tension changing during ignition.

Once I found out what was going on, it was simply a matter
of re-positioning the high-voltage wire away from other parts
in the lamphouse and the problem was solved.

Mabye you should check & see if you have a similar situation.

Good Luck!

>One more thing- - I located the short by turning off all the
booth lights and striking the lamp with the lamphouse door
open, but with the rectifier turned off, so that the bulb
would not actually ignite. The "leakage" spark was easy to
spot in the dark. You'll probably have to hold down or
bypass the safety switch on the lamphouse door if you try this.

Posted by Paul Mayer (Member # 355) on 02-25-2010, 12:25 PM:

Used to see an effect like that at an art house in the Melrose district in L.A. It was a changeover house, and every time a motor was started for a changeover, the sconce lights in the house would dim for a second or so.

Posted by Rick Raskin (Member # 1561) on 02-25-2010, 01:11 PM:

Just a thought: It sounds like the lamp strike might be momentarily loading down the power circuit and causing the dimmers to drop out. I don't know how the electrical codes in the UK would address this, but feeding the dimmers through a 1:1 transformer might serve to overcome the problem. And then that slow electricity might just can't be fixed. [Wink]

Posted by Gordon McLeod (Member # 33) on 02-25-2010, 01:14 PM:

I would suspect RF is causing the dimmer to misfire
1st disconnect the automation control leads to the dimmer and see if it still happens
If it does then I would get some better supression on the lamphouse and probably add some bypass caps and gmov's to the input of the dimmer
what make of dimmer is it

Posted by Geena Phillips (Member # 3726) on 02-26-2010, 04:21 PM:

We had a problem like this at one of our locations. The lamphouses all had AC igniters, and every time they'd strike, it would send out a pulse that scrambled the programming on the lighting automation. The only thing that ultimately solved it was replacing the igniters with DC ones.

Posted by Thomas Pitt (Member # 4322) on 02-26-2010, 05:54 PM:

It certainly looks to me like it's a direct result of a power surge from the igniter. On certain occasions, there's a little buzz/crackle from the stage speakers at the moment the lights go out.

I doubt it's the dimmer, as the lights pop off/dim and then jump back on straight away, rather than fading up slowly like they do when the film ends. It also happens in every single auditorium when the relevant lamphouse strikes (not just in one), so a design flaw may be to blame...

Posted by Caleb Johnstone-Cowan (Member # 3710) on 02-26-2010, 08:09 PM:

quote: Thomas Pitt
Incidentally, they used to dim the house lights a little bit when the advert reel started, then turn the lights off at the start of the trailers. Now, the lights remain at full brightness until the start of the trailers, at which point they dim and turn off.

It makes it easier to seat a screen that way when it's busy and my guess is you go to a vue and that's their company standard anyway. What you describe with the sconce lighting didn't happen at the vue I worked for or the ones I visited so my guess is it's a site problem.

Posted by Eric Robinson (Member # 2966) on 03-16-2010, 01:34 AM:

You could use a scope to monitor the AC waveform supplying the dimmer. Make sure to set the time scale to "slow power" [Wink]

You would most likely see some spiking during the ignition phase if this is where the problem is.

Posted by David E. Nedrow (Member # 4981) on 03-16-2010, 12:05 PM:

Maybe they have old Xetrol IVs sharing a ground. The manual for those talks about flickering, instability, and flashing. They have a section on it that calls for installing a "suppression network field". I only know this because I picked up two Xetrols recently, hoping to replace a non-functioning dimmer. Each of the Xetrols will control a different set of lights in the same auditorium. That, and I love to browse manuals. [Wink]

-David

Posted by Louis Bornwasser (Member # 3063) on 03-16-2010, 08:42 PM:

The problem is logically in the one lamphouse, since only it affects all dimmers.

Look around the igniter and swap capacitors that are connected to the mains power input and the one DC wire that has a capacitor on it (not the one connected to the bulb.) The other lamphouses shoudl be the same. If necessary, swap igniters. Swapping is free. Then you can purchase what is needed. Louis

Posted by Tony Bandiera Jr (Member # 2365) on 03-17-2010, 02:30 PM:

quote:
You could use a scope to monitor the AC waveform supplying the dimmer. Make sure to set the time scale to "slow power" [Wink]

Eric, I cringe every time I see someone suggest using a scope on AC power.

It is EXTREMELY DANGEROUS if you aren't 100% sure of how to do it safely!! Your scope inputs MUST NOT have ANY reference to ground, and you MUST isolate yourself and the scope to avoid getting a severe shock.

If Thomas is not an experienced technician who knows every precaution necessary to do that safely my advice is DON'T TRY IT!!

And I will not give out the procedure online as I will not be liable for someone getting hurt.

Posted by Gordon McLeod (Member # 33) on 03-17-2010, 03:13 PM:

There is no issue with the scope probes being ground referenced since the measurements for noise are in refference to ground
the normal calibration procedure of most dimmer cards is a dual scope one on the firing drive the other on the output
Again it would be nice to know what dimmers are involved

Posted by Tony Bandiera Jr (Member # 2365) on 03-17-2010, 08:57 PM:

Gordon, I don't know if scopes or electricity behaves differently up there, but past experience and a few mishaps has shown me that your scope MUST be isolated from the AC power source's neutral, ground and hot or you risk getting bit or blowing up the scope's input circuitry.

Using a battery powered scope is generally safe and the preferred method, or else connecting an AC powered scope with an isolation transformer.

Looking at my post again I can see where the confusion arises, I should have said "the AC power source" instead of the "ground". And also should be "scope" not "scope inputs" as duh, you'd be using the inputs to look at the power..

I shouldn't post before eating dinner.. [Big Grin]

Forum would not permit me to edit that....

But the signal you're looking at, (unless you're looking for noise on the ground) should be referenced to neutral (Or the other phase of the AC line, in which case a fully isolated scope setup is mandatory.)

For someone of your experience it would be safe to do, but it is not something that anyone should attempt.

Posted by Gordon McLeod (Member # 33) on 03-17-2010, 09:09 PM:

one needs to llok at noise relative to ground on both the phase to ground and neutral to ground
almost all scope probes are rated for 200v that I have come across
The only time one has big issues if on wants to measure the rise time of a dimmer for mainly filtering which is a current issue as a rule so one needs to use a non inductive shunt and measure across it and then yes the chassis of the scope would be live
If one is not confident with a piece of test gear or the safe procedures for working with it they should not attempt any troubleshooting of any electical device

Again we know where the source of the problem is the lamphouse supression but finding the magnitude is the problem

Posted by Eric Robinson (Member # 2966) on 03-18-2010, 12:22 AM:

quote: Tony Bandiera Jr
It is EXTREMELY DANGEROUS if you aren't 100% sure of how to do it safely!! Your scope inputs MUST NOT have ANY reference to ground, and you MUST isolate yourself and the scope to avoid getting a severe shock.

Yes your caution is wise. I think we have talked about this in the past, but could you refresh me on the danger by giving an example of an unsafe situation.

Posted by Tony Bandiera Jr (Member # 2365) on 03-18-2010, 01:20 PM:

Eric, one example would be in the case of needing to monitor a 208-240 volt DOMESTIC AC line (220-240 nominal overseas is a Line-to-Neutral voltage and doesn't apply to this example.), which has BOTH sides of the line HOT in reference to ground. If you connected the ground lead of your scope probe to one of those hot legs and your scope is either grounded through it's AC power connection (or if it's a battery powered scope and either you or it makes contact with a grounded object) you WILL cause an arc flash, burn things up or get a severe shock.

Some older 120 volt AC equipment (TVs and radios were prime examples) also used to have one side of chassis connected directly to one side of the incoming AC power. If you happened to plug the cord in the wrong way you'd have the 120 volt HOT side connected directly to the chassis. Result was a hefty shock or for techs, destruction of test gear. Incidents like that are the prime reason the "polarized" AC plug was developed. But even that won't protect you against a miswired receptacle.

In our cinema world, on the 120 volt side, there can be equipment where either old age, miswiring or a fault can put voltage on the chassis or points we would expect to be at or near ground potential.

There are too many risks involved with 'scoping AC power, so that is why I always discourage it unless someone is throughly trained and has the proper equipment.

Posted by Eric Robinson (Member # 2966) on 03-18-2010, 10:34 PM:

Hi Tony,

Yes that sounds like a bad situation. Thanks for the heads up. So far I have been lucky. I guess you could call it safe if you measure the voltage with a voltmeter between ground(the test point you consider ground) and your scope leads ground to make sure you have zero volts before you commit to the measurement. This sounds safe doesn't it, or is there another catch?

Posted by Gordon McLeod (Member # 33) on 03-18-2010, 11:15 PM:

If your scope has a 3 wire power cord the shield of the probe is ground and will not be connected to the ac

Posted by Tony Bandiera Jr (Member # 2365) on 03-18-2010, 11:53 PM:

Eric, Yes taking a voltage measurement first wouold be a good safety precaution.

quote: Gordon McLeod
If your scope has a 3 wire power cord the shield of the probe is ground and will not be connected to the ac

Gordon, why don't you take a wire from that ground and touch it to a hot AC line and tell us what happens. [Roll Eyes]

You are normally smarter that this, I hope you mistyped something or made that post without thinking first.

With extremely rare exceptions, ALL AC power systems in buildings have a direct reference to ground (in the case of an average cinema, the supply transformer is mid-tapped to give the 120 volt to neutral voltage.)

Bottom line is you gotta be super careful when trying to 'scope an AC power source.

Posted by Gordon McLeod (Member # 33) on 03-19-2010, 07:36 AM:

If your scope has a 3 wire power cord the shield which is the ground lead of the probe is tied to the chassis ground of the BNC connector and the ground pin of the AC cord
the centre probe is not tied to ground and has about 1Meg or greater impedance to ground

Posted by Tony Bandiera Jr (Member # 2365) on 03-20-2010, 02:06 PM:

quote: Gordon McLeod
If your scope has a 3 wire power cord the shield which is the ground lead of the probe is tied to the chassis ground of the BNC connector and the ground pin of the AC cord....

And since the AC power source is referenced to ground you have the shielded side of the probe connected to the AC ground, and thus you have a connection to the AC side.

If you need to look at the 240 volt source line-to-line as you would be doing here in the US, you will need to have the probe's shielded side connected to one of the hot legs...thus the danger and need for total isolation.

It can ONLY be done IF there is TOTAL ISOLATION from Ground.

Some folks use a 1:1 isolation transformer to do this, but the transformer itself can either hide or suppress noise (being essentially a choke coil) or introduce false noise (acting like and inductor) so sometimes direct connection is the only way to get a meaningful reading.

quote: Gordon McLeod
......the centre probe is not tied to ground and has about 1Meg or greater impedance to ground

That is 100% correct.

Again bottom line is it is not something that should be done without proper equipment and training. I have both but do this only as a last resort when other methods of getting the problem solved fail.

Back on topic,:

Thomas, do as Louis suggested and let us know what happens. His suggestion makes sense.

Another thing to check (I don't recall it being mentioned in this thread) is to make sure the lamphouse and rectifier are properly grounded and that there is a ground wire bonding the two together.