Film-Tech Forum ARCHIVE: Dimming Tivoli lights

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Posted by Tim Reed (Member # 137) on 08-20-2007, 12:04 PM:

QUESTION: What's the commonly-accepted method of dimming Tivoli lights?

I received a call from a location where the transformers in 2 auditoriums burned up within a couple days of the other. The electrician replaced them only to have them go out again in a day. I suggested it was something shorted in the floor, the electrician thinks it's the dimmer.

Now he says he's replaced all the lights in one auditorium and it's still overheating. I had him feed it directly (with the dimmer not even in the circuit) and it's still heating (although, since the xnsfmr may have already been damaged I can't be sure the lights are pulling it down at this point).

I figured it would be a good idea to find out how they should be hooked up, so I'm thinking right. Right now, it sounds like the dimmer is in the secondary of the Tivoli xnsfmr (but if so, even if the dimmer was shorted it wouldn't take out the transformer - the lights just wouldn't dim).

FWIW, the new transformers are exact replacements. The dimmer is an older Lehigh unit with Triacs. Everything has been working fine for years until now.

Posted by Stephen Furley (Member # 1277) on 08-20-2007, 02:31 PM:

Tim, What are 'Tivoli lights'?

Posted by Jeremy Weigel (Member # 4208) on 08-20-2007, 02:58 PM:

Hey Stephen,

Tivoli makes track(aisle) lighting for auditoriums along with other special use application.

http://www.tivolilighting.com/

Posted by Rick Raskin (Member # 1561) on 08-20-2007, 03:12 PM:

The electrician needs to measure the current to determine if it is within limits for the circuit. He should have a clip on ammeter in his toobox. FWIW, are all the lights lit? If not, maybe one or more is shorted. For all you know someone may have plugged a socket with a chewing gum wrapper.

Posted by Tim Reed (Member # 137) on 08-20-2007, 06:10 PM:

Rick, that's what I was thinking. Especially since both auditoriums went out at essentially the same time. But he says they laid down all new lights.

I asked the electrician how much current it was drawing and all he would say is it's overheating the transformer. I never got a straight answer to that question. But at least I got him to disconnect it from the dimmer and it gave the same symptom, so he can't blame me, the "camera" guy (unless that transformer had already shorted from before, that is).

So, where should the dimmer in this circuit be?

Posted by Mark Gulbrandsen (Member # 72) on 08-20-2007, 06:42 PM:

Tim,
I'm assumming that you are using a buck boost transformer to power these. Tivoli can supply the correct type of dimmer to use on the primary of a buck boost if you need. If you use the wrong type of dimmer... soimething like a standard wall dimmer you will probably cause the Buck Boost transformer to explode. In the past I used a 3 amp or larger variac to keep the circuit as "quiet" as possible. Dimmers are inherently noisy beasts.

Mark

Posted by Gordon McLeod (Member # 33) on 08-20-2007, 06:53 PM:

A transformer per se should not be bothered by the chopped wave form of a dimmer but some dimmers may be bothered by the inductive load and requireing a change to the snubbing resistor/cap part of the optocoupled output to the triac or SCR
Often I find that the transformer is too small for the load
and the electrician need to remember that the load must be no more than 80%of the VA rating of the transformer.

Posted by Tim Reed (Member # 137) on 08-23-2007, 04:48 PM:

Thanks, Gordon. I thought it was that way (the dimmer cares but not the transformer). Also, the dimmer should be feeding the primary of the Tivoli transformer, right? The electrician says it's in series with the two secondarys right now (if I understood him correctly), at the point that would normally be shorted to get 24 volts on the output.

Mark, it is a Lehigh dimmer, with triacs.

Posted by Gordon McLeod (Member # 33) on 08-23-2007, 07:38 PM:

The dimmer should be connected to the primary of the transformer and the secondary be tapped according to the need voltage

Posted by Mark Gulbrandsen (Member # 72) on 08-23-2007, 08:56 PM:

quote: Tim Reed
Mark, it is a Lehigh dimmer, with triacs.

LeHigh made a dimmer module specifically for dimming Buck Boost trannys. So if this is that module you should be ok. I would follow Rick's suggestions here... should lead you to the trouble. All it takes is a gooey coke spill at a splice in the lights to kill em.

Posted by Gordon McLeod (Member # 33) on 08-25-2007, 08:16 AM:

Leigh dimmers use a pulse transformer drive for the triac so they would probably require an external snubber to make the triac fire properly

Posted by Mark Gulbrandsen (Member # 72) on 08-25-2007, 10:36 AM:

I can't remember how they did it exactly but the Buck Boost dimmer module didn't resemble the standard dimmer module in the least.

Mark

Posted by John Hawkinson (Member # 1135) on 08-25-2007, 01:30 PM:

Shouldn't there be a fuse or breaker wired in series with the transformer secondary if this kind of failure is common?

--jhawk

Posted by Steve Guttag (Member # 268) on 08-25-2007, 11:27 PM:

A buck-boost doesn't have a secondary...it is merely an autoformer that typically comes in 24 or 36V of buck or boost.

Posted by John Hawkinson (Member # 1135) on 08-26-2007, 02:25 AM:

Picky picky...and not the point.

A buck/boost transformer is actually an insulating transformer, it is just normally wired as an autotransformer, but it doesn't have to be; it's got separate H1-H4 taps and X1-X4 taps, you just generally interconnect them.

That said, I can't quite tell from the thread if we're discussing buck/boosts or some other arrangement, and I suppose it doesn't really matter if you fuse the primary or the secondary...the point is, shouldn't it be fused so you don't blow your transformer when something goes wonky with the load?

--jhawk

Posted by Steve Guttag (Member # 268) on 08-26-2007, 12:44 PM:

It is always wise to protect the device and wiring from damaging loads/shorts. However, that is to the design and installer. I know that for Simplex projectors, we always fuse the framing lamp transformers but I've noted that it isn't common amongst other installers.

Posted by Tim Reed (Member # 137) on 09-08-2007, 01:51 PM:

Thanks for all the input, fellas.

I think I found the cause of the trouble. I didn't think it was the dimmer (as I kept telling the electrician at this location). Come to find out, the aisle lights aren't even connected to the dimmer!

The aisle lighting in each auditorium is connected to a shade-tree-mechanic arrangement that is fired from the open and close c/o pulses. On c/o open, the output of the tivoli xnsfmr is wired to provide 12 volts (by virtue of the connections between a pair of latching relays). On c/o close, the taps are rewired to output 24 volts. One of the latching relay coils must be open, so when the other one changes logic states, it shorts the transformer winding.

I didn't think it sounded right that "the dimmer" was connected to the secondary of these transformers!

Posted by Rick Raskin (Member # 1561) on 09-08-2007, 06:21 PM:

Sounds like fusing the transformer secondary is in order.