Dimming Stage Lamps by Al Whale

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Theatrical Lamp Dimmers

(The original dimming of lamps was accomplished using Rheostats or Variacs)

1/ Dimming the lamps by adjusting the voltage.

Rheostats and Variacs

These dimmers required a lot of space and used large controls to operate. The dimmers had to be operated in the rack where they were housed (remote control panels where unavailable).


Rheostats work by inserting a resistor in series with the lamp. When the lamp is dimmed, some of the wattage used for the circuit is converted to heat across the rheostat. This proves to be a very inefficient.

Rheostats and Variacs

Power Calculations

Power Calculations

a\ Consider a 100 watt lamp

According to the formula,

Resistance of Lamp

b\ if the rheostat = 144 ohms

The voltage across the rheostat would now equal the voltage across the lamp

That is 60 Volts (1/2 of 120 volts)


Both the lamp and the rheostat are now using 25 watts each. The lamp's wattage is mostly converted to light, but the rheostat's wattage is all converted to wasted heat.

This is the worst case scenario for this configuration.

In the real world situation, the calculated values will be slightly different, because the resistance of the lamp will change with heat. The rheostat will still dissipate a lot of heat at mid settings.


In the earlier days, the Variac was the preferred method used to dim stage lights.
It was expensive and bulky but didn't waste as much heat as the rheostats.


The same calculations apply to the variac, except that heat is not wasted (except for the small efficency losses).

Aside from bad contacts while adjusting, these circuits do not induce noise on the power line

2/ Dimming the lamps by adjusting the on/off time.

With the advancement to solid state devices, a lot of dimmers were now being replaced by Thyristor circuits. These circuits were smaller and could easily be remote controlled. This meant the heavy wiring could be in a remote room with a local consol controlling the dimmers.

(SCR's and Triacs are members of the thyristor family)


The AC Power Line

To understand the Thyristor operation, a person must first understand the nature of the incoming AC Power Line.

Scope Presentation of the Power Line

Unless a person is familiar working with live 120 VAC,
this test should not be performed.

The scope presentation shows the voltage on the Hot Line (Purple)
in reference to the Neutral line (Black).


The sinewave shows the variation of the voltage over time.
This property is used to vary the on-time of the circuit.

A Thyristor is fully off (open circuit), until it is triggered. Once triggered, it turns fully on (closed circuit).
It will stay on until the current is removed (in this case, when crossing zero volts), at which point it will turn off.

The SCR will only operate when the anode is positive to the cathode. Two SCR's can be placed back-to-back in order to operate with positive and negative voltages. A Triac is equivalent to two SCR's back-to-back.

SCR's ∧ Triacs

To use the thyristor for dimming, a timing circuit is needed.
The timing circuit is reset and the thyristor turns off, every time the voltage reaches zero.

The duration of the timer within the half cycle of the sinewave, determines the brightness of the lamp.

Triac Circuit

Note: the duration of the trigger pulse remains constant.

Dimming at about 15%


Dimming at 50%


Dimming at about 85%


When the timer interval is short, the lamp is brighter (the AC voltage is applied to the lamp earlier).

As the timer interval increases, the lamp gets dimmer.

Bulb Life

The incandescent bulb is under severe stress when first turned on.

The heating and cooling of the filament causes it to expand and contract.

Dimmers are often set to maintain a minimum of 5-10% dimming voltage on the lamps.
This will keep the lamps preheated, thus avoiding some of the expansion and contraction. Preheat is set to keep the lamp filament at a slight glow when dimmed to the minimum.

Dimming Curve

The sinewave nature of the supply voltage will affect the linearity of the control.
As the control is adjusted from low light to full light, the adjustment will be slow at either end of the travel (low parts of the sinewave), and quicker in the center part of the travel.Often the control voltage is filtered to an "S" curve. This will modify the result for a more linear control.

Hi Frequency Noise

A SquareWave consists of the sum of the fundamental and odd harmonics of a SineWave.

For 60 Hz ... the square wave equals:

   1st Harmonic (60 Hz sinewave ... Full Level)
+ 3rd harmonic (180 Hz sinewave ... 1/3 Level)
+ 5th harmonic (300 Hz sinewave ... 1/5 Level)
+ 7th harmonic (420 Hz sinewave ... 1/7 Level)
+ 9th harmonic (540 Hz sinewave ... 1/9 Level)
Etc (into Radio Frequencies)
      Square Wave

The fast turn-on of the Dimmer Waveform produces harmonics in a similar manner.

Fast Turn-on Time

Due to the nature of the dimming, these harmonics are produced
during each half cycle of the 60Hz power (ie 120 times a second).
The harmonics can extend into the Radio Frequencies.

These Radio Frequencies can get into audio amplifiers, and will sound
like a 120hz buzz. They can also cause disturbances in video lines.

Since the amplitude of the instantaneous rise in voltage is worse
at the 50% dimming, this is where the noise will be most prevalent.

Noise Filter

To reduce the high frequency generated, the dimmers use chokes on each channel.
The choke will resist instantaneous changes in current, and will thus lessen
the steepness of the "Fast Rise Time".

Filter Chokes

When energized, Inductors will generate a magnetic field. This
field (or lack of field) will prevent sudden current changes.

LR Circuit

When the square wave changes from 0 volts to +10 Volts, the inductor will momentarily act like an open circuit (the full voltage will be across the inductor). The current will then increase to the maximum in an exponential curve
(the voltage will decrease exponentially to zero across the inductor).
The inductor will now act like a short circuit.

The Value of the Inductor is measured in henries (h).
The time for the inductor current to reach 63% of full value
equals 1 time constant
t = h / r
(t = time, h = henries, r = ohms)
An inductor is considered to have reached the
maximum voltage after 5 time constants

Similarily, when the squarewave changes from +10 Volts to 0 Volts, the collapsing magnetic field of the inductor will generate a negative voltage across the inductor. The current will then decrease to the minimum in an exponential curve (the voltage across the inductor will again decrease exponentially to zero).

The remaining voltage will be across the resistor (R).

Modified Dimmer Waveform

In a similar manner, the Inductor will smooth the Dimmer Waveform, thus reducing the magnitude of the harmonics generated.


The early thyristor Dimmers used 0-10 volts to control each dimmer. A 60 hz sawtooth waveform is generated. The voltage (0-10) from the control is compared to the sawtooth, and fires the thyristor when they are equal.

0 to 10 volt Control

This type of dimmer required one wire per dimmer plus 2 wires for the power supply (ground and +10 volts).

Serial Control:

The cost and size of multiconductor wire to control dimmers,
prompted the development of new controls.

Serial controls are replacing most of the discrete 0-10 volt controls.
Two of the systems used are Micro-Plex and DMX-512.


Dimmer Controller

Conversion Boards

Printed Circuit modules are available to retrofit the old 0-10 volt dimmers for serial control

Retrofit Boards

Micro-Plex (MPX) Control:

The Micro-Plex uses an unbalanced line to ground for control.

A 3 pin XLR connector is used for connection between the Dimmers and the Control Panel.

Pin #1: Ground (Shield)
Pin #2: +12 Volt
Pin #3: Serial Data
      Male XLR

MPX will support 64 channels (some versions support up to 128 channels)
over a maximum distance of 125 feet. Microphone cable is often used.

DMX 512 Control:

The DMX 512 control uses a dual balanced line for control.

A 5 pin XLR connector is usually used for conection between the Dimmers and the Control Panel.

Pin #1: Ground (Shield)
Pin #2: Serial Data #1-
Pin #3: Serial Data #1+
Pin #4: Serial Data #2-
Pin #5: Serial Data #2+
      Male XLR

The second Serial line is unspecified.

Some DMX 512 systems will only use a 3 pin XLR, ignoring pins #5 and 6.

Male XLR

The DMX512 system uses the RS485 standard. The cable used should be 120 ohms low capacity. The Beldon 9841 (or Alpha 5271 ) is suggested or if using both pairs, Beldon 9842 (or Alpha 5272) will work. Microphone cable is not recommended, however Cat 5 cable can be substituted (in some conditions).

DMX512 data rate is 250 bits/second and can travel 1000 feet without amplification. 512 channels can be used, however a maximum of 32 devices (including the controller) can be daisy chained together. The last dimmer must be terminated in 120 ohms.

The Future.

Although the thyristor circuits create a lot of RF (Radio Frequency) noise, Audio and Video equipment should be able to avoid interference with careful attention to grounding, balanced circuits, and correct levels.

Different circuits have been developed to reduce this noise. The "Reverse Phase Control" will slightly reduce the interference. It will turn on the conduction at the zero voltage crossing , and will turn off the conduction part way through the cycle (ie exactly reverse of the previous description).


The Insulated Gate Bipolar Transistor (IGBT)
is often used when the device is
required to turn off in mid cycle.
Insulated Gate Bipolar Transistor
... combination of MOSFet Drive
and Power Transistor.

Sinewave Dimming (or Silent Dimming)

A new method has been developed using Pulse Width Modulation (PWM).

This method will chop the incomming sinewave into small pieces (one manufacturer divides the sinewave into 255 pieces). The on/off duration of the chopped waveform is then varied to change the brightness.

PWM at 50 Percent

PWM at 50 Percent

PWM at 50 Percent

For demonstration purposes, I have shown the effects with the sinewave
only chopped 20 times (in actual circuits it would be much more).

Inductor at Different Frequencies

As the Input Frequency Increases, the Inductor has time to only partly
charge/discharge (still using the same curve). With still higher frequencies,
the choke will average the variations, resulting in a straight line.

This filtering will have a similar effect when used
with a chopped sinewave, resulting in a pure sinewave output.

Other means of filtering may be used by
manufacturers with the same effect.

By removing the sharp voltage changes, this circuit will eliminate the RF Noise.

These circuits are built today, but require
more circuitry and therefore are more expensive.

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