ac dimmer

[spearmint]

i am interested in taking AC wall power (120/60) and attenuating it from 0dB to 0Vpp. i could go buy a light fixture dimmer but i'd like to do this from scratch. i have heard that triacs work well for this purpose and i've learned about how they work, but i am not sure how to use one. can anyone give me some pointers?
cheers
sm<p>[ January 08, 2002: Message edited by: spearmint ]</p>

[Chris Smith]

Do you want to learn everything about them, or just build one? I have many diagrams that you can copy. The heart of the dimmer is called a Diac, bi directional diode. It controlls the triac through r/c variables. However, Zero volts may not be possible because most standard controls start around the 30 volt trigger point, but in theory it is possibe to start at Zero.

[spearmint]

i am interested in anything you can tell me! the more info the better. i am a beginner in analog design-- i've only just taken a first year lab class. any diagrams are great. do you know if there is a tutorial of some kind on this?
cheers,
matt

[Chris Smith]

The only tutorials are in my head? Id be happy to send you several diagrams. I'll pick out a few.<p>N&V this month has a whole bunch of SCR alternative plans....also. The principal is simalar, but its half wave in nature, sort of. Read the article for more insight and feel free to ask any further questions you need here. There is always sombody here to answer your question.<p>[ January 08, 2002: Message edited by: Chris Smith ]</p>

[Ron H]

Keep in mind that you can control power delivered to a load using a triac, but you can't truly attenuate the AC line voltage using triacs,while preserving the sinusoidal waveform, without jumping through some big hoops. A triac changes the average voltage delivered to the load by duty cycle modulation. The simplest way to attenuate the AC line voltage while preserving the sinusoidal waveform is with an autotransformer (variac), but this requires manually turning a shaft, or turning it with a servoed motor.<p>If all you want to do is vary power to a load, then triacs are a good way to go.<p>Ron H

[Chris Smith]

Ron, the easiest way to get a smooth AC back out is a One to One transformer that takes chopper signals in, and returns it back to sine.
The absorption/reaction in the transformer "transforms " it back to sine.

[russlk]

If you buy a Radio Shack triac, it comes with a schematic on the back of the package. Or maybe you can just go look at one!

[Chris Smith]

I have some old diagrams for you, a little more complete than the standard simple circuit.

[spearmint]

i found the triac at radio schack but didn't get it bc you need the diac as well, which they don't carry. i did find the schematic on the back though, thanks! too bad there isn't an easy way to attenuate ac; i imagine the way a triac chops the waveform introduces all kinds of harmonics...
m<p>[ January 11, 2002: Message edited by: spearmint ]</p>

[russlk]

Look up magnetic amplifiers as another approach. The basic mechanism is a magnetic circuit with DC bias to attenuate the AC. Amplification comes through turns ratio.

[MrAl]

Hi there,<p>A really nice way to drive the triac is with a zero crossing
detector, delay circuit, and one shot. At the zero crossing,
the delay circuit starts timing though 1/2 of the AC sine wave
cycle. At the point you set it to trigger, the one shot applies
a pulse to the triac gate which turns it on. The triac then
stays on untill the end of the 1/2 cycle, at which point the process
starts all over again. The nice thing about
doing it this way is you can control the
duty cycle all the way down to zero.
I have built and used several of these
circuits, one which controlled a large grinder (brush motors only)
using a common LM358 op amp.
You do have to be careful what you try to control with the
triac though, you cant use it for everything.<p>If you want more details, let me know.<p>Good luck with it,
Al<p>[ January 13, 2002: Message edited by: MrAl ]</p>

[Chris Smith]

Did you get the message last week? I have three older designs but cant send them on this forum with out a hassle.

[L. Daniel Rosa]

I don't know how full your junk bin is, but a DIAC can be improvised. If you have a 2n506x SCR (in a TO-92) and an assortment of Zener diodes then you can kludge togteher a SIDAC. Connect the anode of the SCR and the cathode of the Zener together, and connect the Zener anode to the SCR gate. A resistor between the gate and SCR cathode may be advisable. Next build a bridge rectifier around it (from whatever diodes are handy) with the SCR/Zener as the DC load and the AC side as the DIAC terminals. It'll be ugly, but you can pick whatever trigger voltage you want, about three to five volts higher than the Zener voltage. Also to it's credit is that it'll have a voltage drop of about three volts when activated, which will make triggering late in the cycle a bit more stable. If you don't mind recycled parts, I could send you some in the mail.<p>[ January 16, 2002: Message edited by: L. Daniel Rosa ]</p>