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An Experimental Lamp Dimmer and Motor Speed Control Using A Simulated Diac
The diac is a common component used to trigger triacs. The construction of a diac is similar to that with a transistor, but with both junctions doped to similar levels so that the two junctions' revers break down characteristics are similar. This is a fun project to demonstrate the use of a pair of bipolar transistors operating in their negative resistance region to simulate a diac triggering a triac in an incandescent  lamp dimmer.  The circuit is not optimized for performance -it is only intended to demonstrate the principle.

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This project uses lethal voltages.  If you are not experienced in working with lethal voltages, read this project, but don't build it. You only have one life, and AC power can take it from you very quickly. Or leave you with terrible injuries.

When working on high voltages, remember:
• Keep one hand in you back pocket. Don't provide a path through your heart.
• Use and insulated mat or proper insulated footwear (bare feet on a tile or concrete floor doesn't make it.)
• Make sure your equipment is in good repair and is properly grounded.
• Never assume a conductor is safe to touch.
• Don't work when alone.
• When working with AC line voltages, use a ground fault interrupter.
• Never work on high voltages while under the influence of alcohol or drugs.

Do not build this project if you are not experienced in working with lethal voltages.


The Principle of Operation

The waveforms above should look like the last 90 degrees of the positive and negative half cycles of the AC line, but this is what the hot AC line actually looks like in my neighborhood. No wonder some jurisdictions require appliances to meet power factor requirements.  By the way, be careful to consider your scope probe's voltage rating when probing the AC mains, even through an isolation transformer, since the peak voltage may be higher than the probe or scope's rated voltage. I used a X100 probe rated at 1kV.

I'm not going to spend much space on this because this is not a beginners' project and the operation of this kind of lamp dimmer is covered elsewhere on the web.

The photograph above shows the voltage across an incandescent light bulb that is being driven by the triac dimmer circuit. Notice that the lamp is only being driven 1/2 the time of each half of the power line cycle. That means that the lamp only gets half the average power. The fraction of each power line half cycle applied to the lamp is determined by the time within the half cycle that the triac, which switches the voltage across the lamp, turns on. When the voltage goes to zero, the triac turns off and waits for the next trigger pulse. That's how this kind of dimmer works. But varying the phase within each half cycle at which the triac turns on.

The Dimmer Circuit

In the classic lamp dimmer circuit, a diac would be
used in place of the two parallel 2N2222 transistors.

Everything that happens within this circuit happens within one half of a power line cycle. Generally every half cycle looks about the same, at least they will if everything is working correctly.

Within each half cycle, the power line voltage charges the .068 uf capacitor through the 250K pot. The voltage across the .068 uf capacitor further charges the .047 uf capacitor through the 68k resistor, until the voltage across the .047 uf capacitor reaches about 10 volts. When the voltage across the .047 uf capacitor reaches about 10 volts, one of the two 2N2222 transistors abruptly switches from a non conducting state to a very low resistance. Being a low resistance, it places the .047 uf capacitor across the trigger terminal and main terminal 1 of the triac, turning the triac on for the rest of the half power line cycle. The lower the resistance of the 250k pot (used as a rheostat), the earlier in the cycle the voltage across the .047 uf capacitor reaches the triggering voltage. Varying the resistance of the 250k pot varies the phase of the trigger pulse with respect to the phase of the power line, thus achieving a variation in the average on time of the triac, which results in a varying duty cycle across the lamp.

Often, the function of the trigger, that provided by the 2N2222 transistors, is provided by a diac, a specially designed semiconductor with a structure similar to that of a transistor, that abruptly changes from not conducting to conducting when the voltage across it exceeds some specific value. I wanted to do this with transistors to show that, in a pinch at least, transistors can be used for this function.

Two transistors were used in place of the one diac so that one transistor would break down on one half cycle and the other transistor would break down on the other half cycle. The break-downs occurs in the reverse biased emitter-base junction, which is a much lower voltage than the reverse biased collector to base junction.

In this circuit, the 2N2222's provided pulses of about 1 microsecond with amplitudes of 100 to 200 milliamps to the gate of the triac.

Since the behavior of transistors in their negative resistance region is not specified by transistor manufacturers, at least as far as I know, it is not wise to design a product around this concept.

This circuit does not have a large dynamic range. Most likely this range can be extended by increasing the value of the .068 capacitor. This circuit can also stand some refinements, such as the addition of a resistor between the gate and main terminal 1 to reduce the chances of spurious turnoff, and possibly some transient protection.


The circuit was assembled on a piece of pre-punched vector board -yes, real Vector board made by Vector corporation back in the 1980's. You can use other kinds of board as long as they can stand off the high peak voltages.

The circuit side of the dimmer circuit - not many connections.
The plastic knob is a very good idea from a safety standpoint. I would not risk
my life on the bet that a cheap pot made for use as a tone control has good
enough insulation at 340 peak volts to prevent me from getting a shock.

  This is the component side of the dimmer circuitversion . The potentiometer is a dual potentiometer,
only because that was the only 250K Ohm pot that I had on hand. The 68k 1 watt resistors was made up of a series-parallel arrangement of four 68k 1/4 watt resistors.

The .o47 uf capacitor is a polyester film capacitor, but that's only because I have a lot of them. Its not really critical. The .068 uf capacitor is critical. Since it has hundreds of volts across it with some settings of the 250k pot, it is rated at 275 volts AC. It is also an "X" capacitor - one this was designed for use as a filter capacitors across the power line. These capacitors are designed to survive high voltages and even transients that cause temporary shorts, without catching fire. I suggest only using capacitors marked for use as "X" capacitors or "Y" capacitors -even better because they are designed to go from line to ground - in this sort of application.

The Motor Speed Contrl Circuit

As with some of my other projects, this one started out as an experiment to prove something to myself or just for fun. As it turnede out, bought a new weed trimmer - the kind that spins a length of plastic fishing line at high speed, and this cuts right through most weeds and grasses. The motor turns at such a high rate of speed, it makes a whole lot of noise. I had leared many years ago, to rapidly pulse the motor on and off so it spun at a lower rate of speed. This had a lot of advatages. For one, it was not so noisey, and it was much less likely to disturb my neighbors. Another advantage was that the plastic fishing line didn't break nearly as often, making the job go a whole lot faster. When cutting heavy growth, I needed to hold the switch closed so the motor could run at its maximum power and speed.

Sometime after finishing the lamp dinner experiment, I realized that if I could also use this as a motor speed control, I might be able to run the motor on my weed trimmer a little slower and get the benefits of less noise and longer finsihg line life at the same time, just as when I pulsed the power switched on and off. After this idea floating into and out of my conciousness over a period of months, I finally got around to trying it. It worked fine.

The only difference between the lamp dimmer circuit and the motor speed control version is thte .01 uf capacitor and the 1.2k resistor I added as a snubber circuit. The snubber limits how quickly the voltage changes across the triac's main termainals when the triac switches off. If the voltage were to rise too quickly, it could result in destruction of the triac.

One hitcht that I started with a single 820 Ohm 1/2 watt resistor in the snubber circuit. After a short test run, I opened the plastic enclsoure and smelled what we called "ode to Allen Bradley," the smell of burned resistor. The snubber resistor had become noticeably more brown. I changed it to the six resitor comibation shown in the schematic, and the resistor seems to be fine. The value of the damping components depend on the needs of the triac and the motor charactaristics. In this case, I just picked some good sounding numbers and gave it a try. After all, I have several spare triacs.

The remaining hitch was that after a short test run, the triac had become quite warm. I know that semiconductors can run with pretty high junction tempeartures, but I always liked it when it wasn't painful to touch a running semiconductor. In truth, I don't know how hot it got because I unplugged the circuit before opening the case and feeliing the triac.

I used some scap alminum to make a quick heatsink. After edging 20 meters of lawn, the triac seemd to be just fine. 

If you decide to try to use this as a motor speed control, be prepared to make adjustments to the snubber circuit, and maybe add a heatsink to the triac itself. By the way, the triac that I used is one of those with a completely insulated tab, making an insulated washer unnecessary. I like those.

The motor speed control version includes a resistor and capacitor
that are absent in the lamp dimmer version.

The Test Setup

I really dislike projects that involve AC line voltage. They are dangerous, difficult to troubleshoot, and sometimes painful. Here is the setup I used on this project (image above). The ground fault interrupter is there in case of accidental contact with the power line or in case of a failure of the insulation in the isolation transformer. The isolation transformer allows me to ground one side of the circuit safely and observe the circuit with an oscilloscope. For the motor speed control, the circuit was first tested with an incandescant lamp and this isolation circuit.  After the circuit worked, I plugged in the weed trimmer without the isolation transformer,and I did not probe the circuit without the isolation transformer.

In my test setup, I am first and foremost, careful to not get shocked. Decades of being shocked, ever since I was a young child, have conditioned me to dislike the sensation immensely. I use an ground fault interrupter (Also called a "GFIC"), to shut off the current within a few tens of milliseconds, just in case I come in contact with the mains voltage. Since the house I live in does not have Ground Fault Interrupter  on all outlets, I bought a Ground Fault Interrupter circuit breaker at the hardware store and but it in a plastic box, along with male and female AC power connectors. Ground Fault Interrupters are sold in this configuration in some places. I could not find one already assembled in Thailand, so I had to build my own.

The Ground Fault Interrupter is also a circuit breaker, so it offers some protection against gross overloads.

By the way, I have a second ground fault interrupter like the own shown above. I use it outdoors with the electric lawnmower, the weed trimmer, and my electric drill.

I made my own isolation transformer by connecting two rectifier transformers back-to-back, as shown in the schematic above. These were 24 volt center tapped transformers that I picked  up a Amorn, a surplus dealer that has outlets around Thailand. Since I don't have a lot of faith in the integrity of the insulation of surplus store transformers bought in Thailand, the addition of the ground fault interrupter gives me some peace of mind.

The transformers are pretty small, only about 1 VA, and this is a good thing, because it limits power to the circuit under test.

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Contents ©2007 Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.org

First posted in September, 2007. Revised May, 2008.

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