Idea for 120V light dimmer

[qa9b]

Hi all,

I'm trying to make a voltage-controlled dimmer for a typical 120v incandescent light bulb. That is, I want to build a circuit that varies the brightness of a light bulb based on some DC control signal.

I've done some reading on the use of triacs to control the duty cycle of an AC, but supposedly these circuits can generate significant interference and require a fairly large inductor to suppress this interference.

However, it occurred to me that I might be able to use a solid state relay in conjunction with a small DC PWM signal to make a far simpler dimmer control. The PWM signal would (more easily, in my mind) control the duty cycle of the AC. The relay, of course, would be rated for AC as well as the non-zero cross type.

My knowledge of the workings of triacs and SSRs is limited. I've looked at a few SSRs and a large number of them are described as having a "photo triac" "output device."

I understand that typical incandescent light bulbs have a large inrush current. How would this affect the necessary current ratings for my SSR?

Is my proposed circuit really all that different from the typical triac circuit? Would it still require an inductor to suppress interference? And, most importantly, would it work?

Thanks for any help!

 

[MikeMl]

SSRs rated for AC contain (guess what?) a Triac. Once triggered, they stay on until the next zero crossing of the AC line, which is the earliest time they will shut off. Some will not turn on in the middle of a half-sine wave, rather they will defer the turn-on until just after then next zero-crossing.

Sounds like you just want to switch on the load in the middle of a half-cycle, which is where the RFI, and need for inductors comes from.

 

[#12]

The first problem is that chopping the AC voltage with a fast relay would cause the same kind of interference. Better to chop a DC voltage with a PWM. In fact, it gets a lot simpler when you used DC. You don't need a SSR, you can just use a single transistor as the power controller.

This is why triac dimmers are sold by the millions..they work cheaper and simpler than any SSR approach.

 

[qa9b]

Thanks for the replies.

So... perhaps an SSR is not quite the ideal device for rapid switching of AC. I'm looking at a datasheet for a non-zero cross SSR and it will indeed turn on in the middle of a half-cycle but cannot turn off until the next zero cross.

Thing is, I already have a few SSRs lying around. What would the advantages be of using a triac?

Looking at this schematic from the site I linked to in my first post,

     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
                                                                  load

I'm still a little confused as to how this functions as a "dimmer" and not as a switch. Datasheet for MOC3020 Triac Driver. Do I still want to pump a PWM-type signal into the driver to accomplish the dimming?

I intend to incorporate a dimmer circuit like this into an audio "beat detector" circuit that includes a handful of opamps and a microcontroller. What types of precautions should I take to make sure that my audio signal is not distorted by the dimmer circuitry?

Thanks again for the help!

 

[MikeMl]

Do you understand the term "phase control"?

 

[qa9b]

No. Any suggested reading?

 

[qa9b]

Okay, So I've done a little searching around. I think I got the concept of phase control from that website I linked to, I just didn't know the proper term.

So my goal is to trigger the triac a certain time interval after it zero-crosses. Question now is what is the best way to do that? I have some experience with PICs and found a few code examples for dimmers. The PIC looks for a zero cross, then waits, then triggers the triac. Question is... how does the PIC monitor then AC signal? For some reason connecting mains voltage to a PIC's input sounds like a bad idea. Is there an easier way than using a PIC?

 

[MikeMl]

...Is there an easier way than using a PIC?

Yeah, go to Home Depot and spend about $10

 

[4pyros]

Dont reinvent the wheel. thare are many, many dimmers out thare. Google your app. and you will find its been done already. Lots of info for the DIY types. Look up Christmas light control and sync your lights to music. Have fun . Andy