TechnicalRunner said:
hmm that's interesting...thanks! well that will be my pre-show mission tomorrow
just out of curiousity, how do the dimmers interfere with the headsets?
Some of the answer is what you would study at college
level electronics. I think I can give a basic explanation with the help of the link below. First I have to make sure you know how the basic
dimmer works from the electronics
point of view.
The link below refers to how light dimmers, you would find in your home, work. But the principle is the same for
Theatre dimmers , they just use higher rated components and a
bit more control circuitry.
http://home.howstuffworks.com/dimmer-switch2.htm
If you look at the diagram. It shows what an AC mains supply signal looks like. Been AC there is a positive and negative cycle. The
point at which there is no
voltage is called, naturaly enough, the zero crossing
point. As shown in the diagram at this
point there is no
power flowing to the lights.
So how the
dimmer works, is by detecting the zero crossing
point on each half cycle. The time delay from the zero crossing
point until the
TRIAC is turned on to let
power flow varies the amount of
power, hence the light
level, that goes to the light.
If you turn the light on just after the zero crossing
point you will get most of the
power available in the half cycle going to the light
= Actors saying " It's to bright and it is shining in my eyes" :
roll:. Turn it on half way through the cycle and you will get a lot less
power going to the light, so it will be much
dimmer = Parent saying "I couldn't see my little Suzzie" :
roll:
Ok now you know how the
dimmer works we can talk about how it causes interference.
The main cause of interference is that the rate of dv/dt is to high. Don't worry I wouldn't expect you to understand that. It just means that the
voltage does not rise slowly but is suddenly switched on at a
level above zero volts. Sometimes at home when you turn a light on you will hear an arcing sound in the
switch as the
current jumps the contacts in the
switch before it is fully closed. If there is a radio nearby you may hear interfence in it.
This similar in the
dimmer packs although there won't be a spark but that sudden inrush of energy causes interference to be radiated out. If you do music you will know about
harmonics. The interference comes from the
TRIAC turning on 120 times a second (two half cycles of AC) but there will be interference at other frequencies which are
harmonics of the 120 eg 240, 360
etc. So this interference can reach high enough frequencies to be picked up by radios, audio gear
etc. Some of this interference will also travel back down the
power lines from the
dimmer packs along the mains supply into a piece of audio equipment such as a
mixer. Here the amplifying circuits will see it as just another signal to amplify. Then amplify it along with the wanted audio signal.
This is why you hear people say never put the sound equipment on the same
phase as the dimmers. Not always possible to do. In some places they actualy use a separate
transformer to provide the
power for the sound
system and then run this
power to multiple parts of the building eg back
stage , sound booth
etc. This way the sound gear is all on the same
phase. This also helps to minimise mains hum inteference caused by different earth potentials, but that is a story for another day.
You will know you have
dimmer based inteference when it varies as the
level of the lights vary. At about the 50%
level the interfernce is the worst. At higher levels the lights are turned on early and the
voltage that is suddenly switched on is lower so less interfernce.
Even if the dimmers aren't causing interference in your audio systems you can notice the effects in two places.
1) The
dimmer packs. In the
dimmer packs, to help minimise this inteference they put a coil of
wire in series with the lamp to slow down the rush of
current. As you know when a
current flows through a
wire it causes a magnetic field and this magnetic field causes this the coil to move slightly. Since the
current is being turned on and off 120 times a second this causes a buzz to be heard from the coil. If you are standing next to the
dimmer when the levels are being changed you will haer the noise from the packs change.
This applies to most of the older type
dimmer packs a lot of the schools have. There are some other ways to dim the lights that don't cause as much interference. But these are newer packs, say installed in the last two years. They still cost more then the older style but will probably eventually replace the older style.
2) The lights themselves will buzz. This applies to the
incandescent type of light that has a
filament. The
filament is a
bit like a coil and the magnetic field that is created then collapses each time the
current is switched on, will shake it. It is in fact this shaking that can cause a
bulb to blow before the
filament burns out. You can hear this shaking as a buzzing. It will be more noticable at lower light levels because the
voltage is higher when the light is turned causing a bigger magnetic field.
While I am on the topic of bulbs blowing this is why sometimes you will hear the term pre-heat, in connection with the
dimmer packs. When a light
bulb is cold it's resistance is lower so when a
voltage is applied more
current flows and this bigger surge may be enough to shake the
filament to bits. When the
bulb is warmer the resistance is higher so less
current flows causing a smaller magnetic field -> less shaking.
When you blow a light
bulb at home it quite often blows as soon as you turn it on because of this cold
current inrush. Pre-heat on dimmers does exactly what is says. A small
voltage is feed to the
bulb all the time to keep the
filament warm. Pre-heat shouldn't be set so high that you can't get a
blackout. You may notice a small red glow if you look right into the light.
A
bit of a long story but hopefuly that answers your question.
)! i have a question though...what does 
