# Light dimmer circuits

## Disclaimer

I disclaim everything. The contents of the articles below might be totally inaccurate, inappropriate, or misguided. There is no guarantee as to the suitability of said circuits and information for any purpose whatsoever other than as a self-training aid.

## How light dimmers work

Light dimmers work by chopping the AC voltage. When only part of the AC waveform is passed through the bulb it gets less power. Typical light dimmers are built using thyristors and the exact time when the thyristor is triggered relative to the zero crossings of the AC power is used to determine the power level. When the the thyristor is triggered it keeps conducting until the current passing though it goes to zero (exactly at the next zero crossing if the load is purely resistive, like light bulb). By changing the phase at which you trigger the triac you change the duty cycle and therefore the brightness of the light.

Here is an example of normal AC power you get from the receptable (the picture should look like sine wave):

```                  ...             ...
.   .           .   .
.     .         .     .
------------------------------------ 0V
.     .         .     .
.   .           .   .
...             ...
```
And here is what gets to the light bulb when the dimmer fires the triac on in the middle of AC phase:
```                  ...             ...
|  .            |  .
|   .           |   .
------------------------------------ 0V
|   .           |   .
|  .            |  .
...             ...
```
As you can see, by varying the turn-on point, the amount of power getting to the bulb is adjustable, and hence the light output can be controlled.

The advantage of thyristors over simple variable resistors is that they (ideally) dissipate very little power as they are either fully on or fully off. Typically thyristor causes voltage drop of 1-1.5 V when it passes the load current.

### What are thyristors and triacs

A Silicon Controlled Rectifier is one type of thyrister used where the power to be controlled is unidirectional. The Triac is a thyrister used where AC power is to be controlled.

Both types are normally off but may be triggered on by a low current pulse to an input called the gate. Once triggered on, they remain on until the current flowing through the main terminals of the device goes to zero.

Both SCRs and Triacs are 4 layer PNPN structures. The usual way an SCR is described is with an analogy to a pair of cross connected transistors - one is NPN and the other is PNP.

```                   +------+
+------+                |
|
E \|
PNP    |---+-------< IG(-)
C /|   |
|     |
|   |/ C
Gate IG(+)  >-----+---|    NPN
|\ E
|
|
- >------------------------------------------+
```
If we connect the positive terminal of a supply to say, a light bulb, and then to the emitter of the PNP transistor and its return to the emitter of the NPN transistor, no current will flow as long as the breakdown voltage ratings of the transistor are not exceeded because there is no base current to either.

However, if we provide some current to the base of the NPN (IG(+)) transistor, it will turn on and provide current to the base of the PNP transistor which will turn on providing more current to the NPN transistor. The entire structure is now in the on state and will stay that way even when the input to the NPN's base is removed until the power supply goes to 0 and the load current goes to 0.

The same scenario is true if we reverse the power supply and use the IG(-) input for the trigger.

A Triac works basically in a similar manner but the polarity of the Gate can be either + or - during either half cycle of an AC source. Typically the trigger signals used for triggering triacs are short pulses.

### Incandescent lamp physics

A typical incandescent lamp take power and uses it to heat up a filament until it will start to radiate light. In the process about 10% of the energy is converted to visible light. When the lamp is first turned on, the resistance of the cold filament can be 29 times lower than it's warm resistance. This characteristic is good in terms of quick warmup times, but it means that 20 times the steady-state current will be drawn for the first few milliseconds of operation. The semiconductors, wiring, and fusing of the dimmer must be designed with this inrush current in mind.

Because lamp filament has a finite mass, it take some time (depending on lamp size) to reach the operating temperature and give full light output. This delay is perceived as a "lag", and limtis how quicly effect lighting can be dimmed up. In theatrical application those problems are reduced using preheat (small current flows through lamp to keep it warm when it is dimmed out).

The ideal lamp would produce 50% light output at 50% power input. Unfortunately, incandescents aren't even close that. Most require at least 15% power to come on at all, and afterwards increase in intensity at an exponential rate.

To make thing even more complicated, the human eye perceives light intensity as a sort of inverse-log curve. The relation of the the phase control value (triac turn on delay after zero cross) and the power applied to the light bulb is very non-linear. To get around those problems, most theatrical light dimmer manufacturers incorporate proprietary intensity curves in their control circuits to attempt to make selected intensity more closely approximate perceived intensity.

## Typical 120V AC dimmer circuit

The following circuit is based on information from Repair FAQs: http://www.paranoia.com/~filipg/REPAIR/Repair.html

This is the type of common light dimmer widely available at hardware stores and home centers. The circuit is a basic model for light dimmer for 120V AC voltages.

While designed for incandescent or heating loads only, these will generally work to some extent with universal motors as well as fluorescent lamps down to about 30 to 50 percent brightness. Long term reliability is unknown for these non-supported applications.

```
Black   o-----------------+------------+-----------+
|            |           |
|         R1 \           |
|      220 K /<-+        |
|            \  |        |
|            |  |        |
|            +--+        |
|            |           |
|         R2 /           |
C1 _|_      47 K \           |
.047 uF ---           /         __|__ TH1
|            |         _\/\_ SC141B
|            +---|>|   / |   200 V
|            |   |<|---  |
|        C2 _|_   D1     |
|   .062 uF ---  Diac    |
|            |           |
Red o-----------------+---CCCCCC---+-----------+
L1
40 T #18, 2 layers
1/4" x 1" ferrite core
```
The purpose of the pot P1 and capacitor C2 in a diac/triac combination is just to delay the firing point of the diac from the zero crossing. The larger the resistance (P1+R2) feeding the capacitor C2, the longer it takes for the voltage across the capacitor to rise to the point where the diac D1 fires turning on the triac TH1. Capacitor C1 and inductor L1 make a simple radio frequency interference filter. Without it the circuit would generate quite much interference because firing of the triac in the middle of the AC phase causes fast rising current surges.

I also saw a quite similar dimmer circuit posted to sci.electronics.design newsgroup one day (posted by Sam Goldwasser). This is the type of common light dimmer (e.g., replacements for standard wall switches) widely available at hardware stores and home centers. This circuit uses slightly different component values than the previous one and does not have any radio frequency interference filtering. This one contains just about the minimal number of components to work at all!

```    Black o--------------------------------+--------+
|        |
|  |        |
R1 \  |        |
185 K /<-+        |
\  v CW     |
|         __|__ TH1
|         _\/\_ Q2008LT
+---|>|   / |   600 V
|   |<|--'  |
C1 _|_  Diac    |
.1 uF --- (part of |
S1                     |    TH1)   |
Black o------/ ---------------------+-----------+
```
S1 is part of the control assembly which includes R1. The reostat, R1, varies the amount of resistance in the RC trigger circuit. The enables the firing angle of the triac to be adjusted throughout nearly the entire length of each half cycle of the power line AC waveform. When fired early in the cycle, the light is bright; when fired late in the cycle, the light is dimmed.

Due to some unavoidable (at least for these cheap dimmers) interaction between the load and the line, there is some hysteresis with respect to the dimmest setting: It will be necessary to turn up the control a little beyond the point where it turns fully off to get the light to come back on again.

## 1 kW 230V AC light dimmer circuit

The following circuit is HELVAR 1 kW light dimmer dimmer circuit published at Bebek Electronics magazine. The circuit is a quite typical TRIAC based dimmer circuit with no fancy special features. The triggering circuit is a little bit improded compared to the 120V AC design. This circuit is only designed to opertate with non-inductive loads like standard light bulbs.

```         o-----LAMP--------+------------+--+------+---+--------+
|            |  |      |   |        |
|        P1  \  |  P2  \   |        |
|      500 K /<-+  1M  / <-+        |
|       LIN  \         \            |
|            |         |            |
230V                       |            +---------+            |
AC IN                      |            |                      |
|        R1  /                      |
C1   _|_      2k2  \                      | A2
150 nF ---           /  R2                __|__ TH1
400V   |            |  6k8               _\/\_ TIC226D
|            +-/\/\/---+---|>|  G / | A1
|            |         |   |<|----  |
|        C2 _|_   C3  _|_   D1      |
|     150nF ---  33nF ---  ER900/   |
|      400V  |         |   BR100-03 |
|            |         |            |
o----FUSE---------+---CCCCCC---+---------+------------+
L1
40..100 uH
```

## Safety issues on building the circuits

Because light dimmers are directly connected to mains you must make sure that no part of the circuit can be touched when it is operating. This can be best dealt by buildign the dimmer circuit to small plastic box. Remeber to use potentiometer with plastic shaft and install it so that no potentiometer metal parts are exposed to user.

Remeber to make circuit board so that the traces have enough current carrying capacity for the maximum load. Make sure that you have enough separation between PCB traces to widthstand mains voltage. Remeber to install correct size fuse for the circuit (fast acting if you want to give any protection to TRIAC). Make sure that all components can handle the voltages they face in the circuit. For 230V operation use at least 400V triac (600V better). The capacitor which is connected between the dimmer circuit mains wires should be a capacitor which is rated for this kind of applications (those are marked with letter X on the case).

Remeber to use coil type which can handle the full load current without overheating or saturating. Use capacitors with enough high voltage rating. Make sure that the TRIAC has enough ventilation so that it does not overheat at full load.

Triac based light dimmer circuits the mains sine wave is chopped, which causes fast voltage and current changes. Thost fast voltage and current changes cause high frequency interference going to mains wiring unless there are suitable reafio frequency interference (RFI) filter built into the circuit. The corners in th waveform effectively consist of 50/60Hz plus varying amounts of other frequencies that are multiples of 50/60Hz. In some cases the interference goes up to 1..10Mhz frequencies and even higher. The wiring in your house acts as an antenna and essentially broadcasts it into the air. Cheap bad quality light dimmers don't have adequate filtering and they cause easily lots of radio interference.

Dimmer circuits typically use coils that limit limit the rate of rise of current to that value which would result in acceptable EMI. The coil itself does not solve the problem because of the self-capacitance of the inductor: they typically resonate below 200 kHz and look like capacitors to disturbances above the resonance frequency. That's why therte must be also capacitors to suppress the interference at higher frequencies.

If your dimmer circuit cause interference, you can try to filter out the interference by adding a small capacitor (typically 22nF to 47 nF) in parallel with the dimmer circuit as near as possible to the electronics inside the circuit as possible. Keep in mind to use a capacitor which is rated for this kind of applications (use capacitors marked with X). Keep in mind that the filter capacitor and it's wiring make a resonance circuit with certain resonance frequency (typically around 3.6 MHz with 0.1 uF capacitor). The capacitor does not work well as filter with the frequencies higher than the resonance frequency of the circuit.

## Buzzing problems with dimmers

Each good dimmer has a filter choke inside. Those chokes hekp to filter out electrical noise that often causeshum to be picked up in sound system and musical instrument pick-ups. The chokes also help to eliminate 'lamp singing' that can cause audible noise to come from the lighting fixtures. In providing those filtering functions, the chokes themselves can generate a slight buzz. Fast current changes in the coil can make the coil wiring and core material easily vibrate which causes buzzing noise. A little bit of puzzing is normal with filtered dimmers. If the buzz from dimmer can be a problem it is recommended that the dimmer is placed in the area where this buzz will not be a problem.

As far as the 'bulb singing' concerned, a bulb consists of a series of supports and, essentially, fine coils of wire. When the amount of current flow abruptly changes the magnetism change can be much stronger than it is on a simple sine wave. Hence, the filaments of the bulb will tend to vibrate more with a dimmer chopping up the wave form, and when the filaments vibrate against their support posts, you will get a buzz. If you have buzzing, it's always worth trying to replace the bulb with a different brand. Some cheap bulb brands have inadequate filament support, and simply changing to a different brand may help.

Buzzing bulbs are usually a sign of a "cheap" dimmer. Dimmers are supposed to have filters in them. The filter's job is to "round off" the sharp corners in the chopped waveform, thereby reducing EMI, and the abrupt current jumps that can cause buzzing. In cheap dimmers, they've economized on the manufacturing costs by cost-reducing the filtering, making it less effective.

In very high power dimming systems the wiring going to lighting can also cause buzzing. The fast current makes the electrical wiring to vibrate a little bit and if the wire is installed so that the vibration can be transferred to some other materialk then the buzzing could be heard. The bussing caused by the vibration of the wiring is only problem in very high power systems like theatrical lighting with few kW of lights connected to the same cable. Better filtered dimmers can reduce the problem because the filter makes the current changes slower so the wires make less noise.

### Why does dimmed lighting sometimes hum, and how can it be corrected?

Because of the way all dimmers deliver power at settings other than full brightness, the filaments inside a light bulb may vibrate when lighting is dimmed. This filament vibration causes the hum. To silence the fixture, a slight change in the brightness setting will usually eliminate bulb noise. The most effective way to quiet the fixture is to replace the light bulb.

### How can I avoid the buzzing the dimmers cause to my sound system ?

There are numerous ways that dimmer noise can get into audio systems and it's largely trial and error in determining what in particular is causing your problem and hence how to fix it. The principle ways are either back up the mains or induced into your audio equipment or cables.

What you hear typically in audio system is common mode noise on the hot and neutral, the spike of turn-on of the scr. The higher the rise time of the current in the dimmer, more noise is sent to the mains wiring. So well filtered dimmer will generate less noise problems.

Reduce the possibility of it coming up the mains by taking a totally separate mains supply from the lighting, if possible get a totally separate power socket (or sockets) run in for sound from wherever the electricity board intake is. If this is not possible, then an isolation transformer stops quite much of the noise on the secondary side (better with shield between coils). So put the sound system on the isolation transformer and tie to earth (ground) almost no problems. This assume that sound wiring is correct, especially shielding is done well and ground loop are avoided.

To reduce the possibility of interference induced to the audio cables, run all non speaker level audio cables as balanced lines (or certainly all of any length). You might have to buy balancing transformers if your kit isn't balanced already. Also keep them as far away physically from any lighting cable runs as you can. Make sure that your system does hot have any harmbul ground loops. Make sure none of your audio kit is anywhere near the dimmer racks.

### Now can I dim up the lights smoothly ?

With many cheap dimmers, the lights "Pop On" rather than dim up smoothly. This problem is usually related to the construction of the dimmer electronics. One technique used in some cheap dimmers to allow dimming up smoothly is to place another potentiometer (trimmer) across the control potentiometer. That trimmer potentiometer is set so that the dimmer works smoothly:

• a)Set "Control" to Minimum light level.
• b)Adjust "Trimmer" to filaments JUST "glow"
• c)Turn off dimmer
• d)Turn on dimmer to see if filaments "glow". IF not... set trimmer up a snit.... go to c)
Continue until minimum voltage/current is supplied to lamps (filaments do not seem to glow at all). When everything is properly adjusted, the dimmer circuit will nicely dim up from the lowest setting up to maximum brightness.

Normal light dimmers are designed to only dim non-lunductive loads like light bulbs and electric heaters. Normal light dimmers are not suitable to dim inductive loads like transformers, fluorescent lamps, neon lamps, halogen lamps with transformers and electric motors. There are special dimmers available for those applications.

If you connect inductive loads to the dimmer the dimmer might not work as expected (for example does not dim that load properly) and can even be damaged by the voltage surges generated by the inductive load when current changed radiply. Another problem is the phase shift between the voltage and current cause by the inductance. If you use a normal simple light dimmer which is just in series with the wire going to the load, this will cause that the dimmer circuit will not wirk properly with highly inductive loads. Special dimmers which have a separate controlling electronics connected to both live and neutral wire and then the triac which controls the current to the load usually work much bettter with inductive loads.

Often when inductive loads cause problems on normal dimmers, you can eliminate said problems by patching an incandescent "ballast" load in parallel with the inductive load. Usually 100W is enough for many inductive loads. Remeber that indictive loads can hum quite noticably when dimmed and the transformers can heat more because of increased harmonics content in the power coming to them.

### Dimming lights with built-in transformers

Fully loaded halogen transformers usually dim quite well. If you are planning to dim halogen light transformers, try only dim traditional transformes, because toroidal core transformer do not usully dim well. Most of the cheap halogen light transformers belong to this category as well as the transformer in for example PAR36 pinspot lights. When dimming transformers it is a good idea to put a fuse in sereis with the transformer primary so that it will blow when transfromer tries to get too much power from the line. This will protect the transformer from overhating which might be caused because of transformer core saturation (which might be caused by small DC bias caused by not very well operating dimmer). A proper fuse will save transformers from burning out.

If your halogen light system uses an electronic transformer then you must very carefully check if it can be dimmed. Some of the electronic transformers are made dimmable and work well with traditional light dimmers. The ones which are not ment to be dimmed can be damaged by the dimming.

### Dimming fluorescent lights

If you try to dim fluorescent light on normal dimmer you have to turn the dimmer full on to make the light to turn on and you can only dim it down only down to 30-50% brightness. For anythign less than this you will need a special dimmers and special fluorescent fitting.

### Dimming electric motors

Typical dimmer packs will supply power to motors and make them run, but the dimmers aren't designed for it. Some dimmers can be damaged by connecting inductive loads to them. And when the triac fails half-wave it takes the motor out too. A good idea to protect motor failures is to use a fure sized for the motor load in series with the motor. This fuse will propably burn before motor is damaged if it is sized correctly.

## How touch dimmers work ?

The basic dimmer operation principle is the same as in dimmers above. The only difference is how the dimemr is controlled. The rouch controlling is done using a special control IC and touchable metal plate. The dimmer usually has a metal plate which is coupled to the circuit via a high value resistor (>1Meg Ohm). Your body acts a little like an antenna and couples 50Hz mains signal (or 60 Hz depending on country) into the circuitry. The AC signal is fed to a shaping circuit(converted to a square wave) and then usually into a dimmer IC.

A typical touch dimmer has following circuit parts:

• A special timing circuit which senses if the contact on the touch plate was long or brief. In operation, a momentary touch of the sensor plate with the fingers (50 - 400 ms) will toggle the light ON or OFF depending on its previous state.
• A memory circuit which stores the intensity level of the lights.
• A circuit which generates the pulses necessary to vary the light intensity
Touch dimmers which typically control the TRIAC in a 45°C to 152°C conductivity region of the mains half period while the IC draws its power from the remaining power up to the 180°C of the half period.

Siemens is one of the companies who supply these IC's (for example SLB-0586). The IC itself will function differently depending how long you touch the plate for.

Lighting dimmers use phase-control - you switch on at a point on the supply voltage waveform after the zero-crossing, so that the total energy input to the lamp is reduced. The time between zero crossing and switching is controlled by external control interface which is most often 0-10V DC control voltage or digital DMX512 interface.

### Simple voltage controlled dimmer

```  230V AC o---FUSE----LAMP--------------+-----------+---------------+
INPUT        2A                       |           |               |
\ R2        |               |
/ 2.2K      |               |
R1                  \           |        R4     |
2.2 kohm             /           |       220 ohm /
+ o--/\/\------+          |           |        1W     \
CONTROL           __|_  ---->  / R3        |               /
VOLTAGE      LED  _\/_  ---->  \ LDR       |               |
|          /         __|__ TH1         |
- o------------+          |         _\/\_ BTA04/600T  |
+---|>|   / |               |
|   |<|--'  |               |
C1 _|_  Diac    |          C2  _|_
100 nF ---          |       100 nF ---
|           |       250VAC  |
NEUTRAL  o-----------------------------+-----------+---------------+
```
This circuit can control loads up to 2A (460VA). The circuit is basically a normal light dimmer circuit, but the potentiometer is replaced with LDR resistor which changes it's resistance depending on the light level. In this circuit a LED powerred form control voltage source is used for shining variable intensity light to the LDR, so you must make sure that LDR does not receive light from other sources.

This circuit is basically very simple and not very sensitive on what LDR is used as R2. The disadvantage of this circuit is that the control is not very linear and the different dimmers built around this circuit can have quite varying characteristics (depending mainly on the LED and LDR characteristics). The control voltage is optically isolated from the dimmer circuit connected to mains. If you need a safety solation then remeber to have enough distace between the LED and LDR or use a transparent isolator between them to guarantee good electrical isolation. If the dimmer sensitivity is not suitable with the circuit described above, then you can adjust the value of R1 to get the control voltage range you want.

This circuit is a part of an automatic light dimmer circuit published in Elektor Electronics Magazine July/August 1998 issue pages 75-76.

### Professional voltage controlled dimmers

Remotely controlled light dimmers in theatrical and architechtural applications typically use 0-10V control signal for controlling the lamp brightness. In this case 0V means that the lamp is on and 10V signal means that the lamp in fully on. A voltage between those values adjust the phase when the TRIAC will fire. Here is a typical control circuit schematic:

```                         Comparator

| \       Resistor
0-10V input >-------------|+  \
|    >-----/\/\/\------+
+---|-  /                  |
|   | /                   optocoupler to TRIAC circuit
|                          |
Ramp signal                   Ground
goes from 10V to 0V
in one mains half cycle
(10 ms at 50 Hz mains frequnecy)

```
The circuit works so that the comparator output in low when the input voltage is higher than the ramp voltage. When the ramp signal voltage gets lower than the input voltage the comparator output goes high which causes that curresnbt sarts to flow through resistor to optocoupler which causes the triac to connect. Because the ramp signal starts at every zero crossing from 10V and goes linearly to 0V at the time of one half cycle the input voltage controls the time when the triac is triggered after every zero crossing (so the voltage controls the ignition phase. The necessary linear ramp signal can be generated by a circuit which discharges a capacitor at constant current and charger it quickly at every zero crossing of mains voltage.

You can use your own circuit for triggering the TRIAC or you can use a ready made semicondictor relay for this (it comes in compact package and provides optoisolation in same package with TRIAC). If you plan to usre ready made solid state relay you need an SSR WITHOUT zero-crossing switching. You need an inductor in series with the SSR to prevent di/dt problems and help to cut down emission of r.f. noise. Values vary typicallt from 2 to 6 mH: they are usually specified in terms of the rise-time of the switch-on edge, but it is only rough because the inductors used are non- linear: the inductance varies with load current.

The optocoupled TRIAC triggering circuit can be for example constructed using MOC3020 optodiac and some other component. Here is one example circuit (part of dimmer circuit from Elektor Electronics 302 circuits book):

```     R1                       R2
180                      1K
+---/\/\/\----------+   +----/\/\/-------------+------------+-----------> 230V
1|   |6                     |            |             Hot
+=====+ IC1                 | MT1        |
| MOC | TRIAC              +-+           |
| 3020| Driver           G | | TRIAC     |
+=====+                   /| | TIC226D   |
2|   |4                  / +-+           |
+-------------------+   |                   |  | MT2        |
+-------------------+  |            |
|  |            |
\  |            |
R4    /  |            |  C1
1K    \  |           --- 100 nF
/  |           --- 400V
|  |            |
|   )           |
|  (   L1       |
|   ) 50..100   |
|  (    uH      |
|  |            |              Neutral
+--+------------+----o    o--> 230V
```

Most professional stage-ligting dimmers do use solid state relays. They have more in them than you would expect, usually including opto-isolation of the control input. The exact contents are commercially confidential but the operation of voltage controlled version is very similar to the idea described above.

### Phase controlling using microprocessor

If you want a digital control of light dimmer you can use a simple microcontroller to do the phase controlling. The microcontroller has to first read the dimmer setting value through some interface (commercial digital dimmers use DMX512 interface). typically the control value is 8 bit number where 0 means light off and 255 that light is fully on.

The microcontroller can easily generate the necessary trigger signal using following algorith:

• Convert the light value to software loop count number
• First wait for a zero crossing
• Run a software loop which waits the necessary time till it time to trigger the TRIAC
• Send a pulse to the TRIAC circuit to trigger the TRIAC to conduct
Software loop is quite simple method and useful when you know how long time it takes to execute each microprocessor command. Another possibility is to utilize microcontroller timers:
• You can generate an interrupt at every zero crossings and every timer count.
• At every zero crossing the microcontroller loads the delay value to the timer ands starts it counting.
• When the counter time has elapset it generates an interrupt. The timer interrupt routine sends a trigger pulse to the TRIAC circuit.

## Reverse phase control

Reverse phase control is a new way to do light dimming. The idea in reverse phase controlling is to turn on then switching component to conduct at at every zero crossing point and turn it off at the adjustable position in the middle of the AC current phase. Tming of the turn-off point then controls the power to the load. The waveform is exact reverse of that is used in traditional light dimmers.

```                  ...             ...
.  |            .  |
.   |           .   |
------------------------------------ 0V
.   |           .   |
.  |            .  |
...             ...
```
Because the switching component must be turned off at the middle of the AC phase, traditional thyristors and TRIACs are not suitable components. Possible components for this kind of controlling would be transistors, FETs and GTO-thyristors. Power MOSFETs are quite auitable components for this and they have been used in some example dimmer circuits.

Reverse phase controlling has some advantages over traditional drimmers in many dimmer applications. Because turning on point is always exact at the zero phase there are no huge current spikes and EMI caused by turn on. Using power MOSFETs it is possible to make the turn-off rate relatively slot to achieve quite operations in terms of EMI and acoustical or incandescent lamp filament noise.

## Variable transformer as dimmer

One old approach for dimming of lights is do it by using variable transformer (Variac or similar brand) as a dimmer. Some of these are made specifically for this application - they'll fit into a double-size wall box (maybe even into a single-size wall box if you get a small one) and will handle several hundred watts. They're heavy and mechanically "stiff" (compared to a triac dimmer) and not cheap - but they put out a nice, clean 60 Hz sinewave (or very near to it) at all voltages, and don't add switching noise.

## Other not so good ideas for dimming

Zero cross switching will minimize noise in switchign and dimming. Unfortunately that appriach is not very practical for lampi dimming. At 60 Hz line frequency, you'd be limited to turning the lamp on and off at discrete 120 Hz intervals. You'd easily end up with a rather nasty 15-20 Hz flickering, unless the dimmer-driver can do some sort of dithering to spread out the flicker spectrum. I've never seen a dimmer of this sort being used.

In some occasions a single diode can be to dim a light bulb when wired in series with the lamp. The diode then passes only the positive or negative half of the mains voltage to the light bulb. If you put a witch in parallel with the diode, you end up having a dimmer wich has two settings: full on and dimmed. Diode will indeed work on small loads, but with larger loads the DC component this diode causes is not good for the distribution transformers in the electrical distribution system (will cause them them to heat up more than in normal use).

Tomi Engdahl <Tomi.Engdahl@iki.fi>