Strange voltage - load side of dimmer.

[MarkyMarkNC]

I got a call to check out some dimmers that were feeding stage lighting at a small bar. Six 500W PAR64's on two separate circuits controlled by two 1500W Lutron rotary dimmers. I did not remember to write down the dimmer part#, but if I had to guess, I would say they were between five to ten years old.

The lamps in the stage lights had been burning out every couple of months. Owner said they only used them on weekends for about a 4 to 6 hour stretch, which definitely seems like an abnormally short lamp life. He's been paying around $75.00 each for these lamps at the local music store, and so he wanted me to check out the wiring to see if I could find anything unusual that might be causing this.

All the wiring looked OK. The dimmers were located fairly close to the panel, so I was able to check every connection all the way back, and everything looked fine. The interior's of the fixtures looked OK - A couple of the fixtures had small signs of excess heat, but no outright browning or cracking of the fixture wire insulation. Line side of the dimmers read 121V hot to neutral, and nearly the same hot to ground. When I checked the load side, however, I read just over 90V hot to neutral, on both dimmers.

Is this normal? I'm pretty sure I've checked load side voltage on dimmers before and found normal 120V. I've always understood that dimmers work by "chopping" the voltage curve, rather than adjusting voltage.

Before I just assume the dimmers are the problem, and bill the customer for some high dollar replacement that may or may solve the problem, I figured I'd check in here to see if anyone had any thoughts.

One more thing I should mention. I had left my Fluke meter at another job, so I was checking voltage with a cheap Sperry meter. I think it is still a true RMS meter but not 100% sure.

 

[dbuckley]

Some PAR lamps do have short lives. Especially the Chinese lamps, which are half the price of GE lamps, and have a much more variable lifespan.

Check out GE's Stage and Studio catalogue which has the full monty on lamp specs.

 

[MarkyMarkNC]

The lamps they had were GE's. Website says 2000 hrs. Even if they were running them 20 hrs. a week, that should still give them close to two years of life.

 

[alfiesauce]

Dimmers will give you wierd readings off of a volt meter...

Do they have huge subwoffers in the bar and/or a dance floor?
High vibrations can shorten lamp life in a huge way.

Does the light fixture look factory or is it a homemade job- is it trapping to much heat and causing the bulb to burn out prematurely?

 

[MarkyMarkNC]

Yep, they have some pretty big speakers in the place. They are not that close to the lights though.

Lights do look a bit cheesy, but there were no blatant signs of excess heat.

 

[mxslick]

MarkyMarkNC,

First off, as already posted, you won't get a reliable voltage reading on the load side of any electronic dimmer. If the voltage going into the dimmer isn't higher than the bulbs can withstand, then the output voltage won't be higher and cause any issues.

I see two main issues:

First off, the speakers. Bass soundwaves are HUGE, in tens of feet, and the bass can easily reach high levels and be vibrating the lights to death. So even though the speakers are "not that close" they are still close enough. How are the lights mounted? Clamped to a pipe or light bar? Or are they attached solidly to the walls or ceiling? If they are rigidly mounted I would mount them with rubber isolation pads to help suppress vibration.

And about the lights themselves: you mentioned that they look "cheesy". Are they home made or is there a manufacturer's brand on them?

Did you check very carefully for bad sockets/connectors or loose connections? Those in combination with the bass from the subs will eat lamps rapidly.


Can you post pics of the lights/install?

Overall I suspect it may be a combination of the vibrations from the music and cheap luminares.

 

[pbeasley]

It is possible he has a bad batch of lamps... It might be good to try a different source and see what happens.... GE, Osram, or Sylvania are all quality products. There are also much better places to get these lamps... I would expect these to run $40 or less, under $30 when bought by the case of 6 lamps. Try a Stage & Studio lamp supplier or Theatrical Lighting dealer.

Another thought... if the lamps are in direct flow of air conditioning it can result in thermal shock and premature failure over time. Not usually an issue with PARs, but something to consider.

 

[MarkyMarkNC]

Thanks for all the good advice guys!

The lights are clamped directly to a structural beam in the ceiling, so I will try out some shock mounting for sure. The lights are not homemade, but they are definitely not top of the line fixtures - lots of cheap hardware and thin metal. I checked out the internal wiring on all of them, and the connections and wiring are all fine. The sockets, like the rest of the fixture, are not the best quality either.

I will check to see if there is air flow directly over them when I go back there this afternoon.