AC Light Dimmer - 555 Timer Based - Page 1
Forum section: electronics / projects

author: DarkPrince
author_rank: Regular Contributor
timestamp: November 28, 2012, 08:08:03 pm
content: Hey everyone. So I have a question about a 120V 60Hz light dimmer circuit we designed for a 40W incandescent bulb. It is the standard dimmer based on firing angle of a switching device SCR/Triac. The requirements is the load, as stated before, and the switching device which must be an SCR. As true (stubborn) engineers, we attempted to use the least amount of reference material as possible as this was a design project rather than just approaching with the simplest or most effective. By doing this, we ended up going with the path that we'd have a device which would delay the SCRs firing angle X amount and sync at every zero-crossing. First a microcontroller was considered, by myself, as I am highly fluent in digital design. Of course in order to distribute the work-load across multiple members of the group we used a 555 timer, and I am glad as it turned out to be quite fun actually. The fun things is, near midnight on campus and we ended up with a design that almost works.So mostly I just wanted to share the design and see if you all have any comments or anything. Unfortunately I'll probably also be laughed at but eh'. There is a design flaw which I would like to inquire about and see if you have any thoughts about. The zero-crossing circuit would normally be triggered early due to the SCR shorting out the bridge rectifier and thus the circuit would think it is zero-crossing again. This actually doesn't happen due to the voltage drop across the SCR is greater than the Vbe voltage of the zero-cross detect transistor, allowing it to remain on through the whole cycle, but only by luck. Any quick thoughts around this? Been pondering and will continue but eh, curious if you have insight.As you may have guessed, this is a group project for a class in the uni. I am not asking for design as that is nearly done, and as it stands now actually works. Just some insight into alternative ideas for the zero-crossing detection circuit. Unfortunately the project is due next week, though no concern the final prototype will be made most likely this weekend. Just seeing if there is a way to less rely on this voltage drop across the SCR. I've considered a resistor in series with the load to increase the voltage drop, but only if the circuit is deemed unstable at high voltage.Right now I have tested the first run prototype on 12V and it does work. As expected, it is sensitive to very high hold-off times causing major distortion on the output. A trip-pot is going to be used to trim out so the end-user cannot reach that limit. Also it is only possible to reach near 100% as it needs to detect the voltage before turning on. Most dimmers seem to be that way, and should be acceptable. The simulation was tested down to 1% and operated, and a switched-pot will be used to cut the SCR drive signal to create the 0% on state.Finally, I am sorry for the attached image. I remoted in to my desktop from my phone standing out in the cold, had to print to an XPS document, e-mail myself, then at work found out I had nothing supporting xps so used an online tool to convert to JPG (augh). So... horrible. If need be (and probably is) I will get a real capture tonight when I am on my home machine.Edit: I cleaned up the image a bit... should work for now. Still not at home but found that it was much higher resolution than expectedEdit 2: I forgot to note that where you see two diodes in the circuit which seemingly do nothing... they don't. That was before I realized that I could attach the input to the zero-cross detect circuit into the full-wave bridge rectifier. Hoped to reduce component count. Interestingly it didn't work and the prototype DOES have those two diodes.

author: mickpah
author_rank: Regular Contributor
timestamp: November 28, 2012, 08:36:21 pm
content: Ha, even macgyver used a 555 - Tshirt idea Dave

author: Kremmen
author_rank: Super Contributor
timestamp: November 28, 2012, 10:49:34 pm
content: When SCR's were king some decades ago, we used to detect zero crossings directly from the AC voltage. These were big industrial 3 phase 6-pulse bridges for driving large DC motors in the hundreds kW to MW range.Your circuit is smart in the sense that you manage with a single SCR, although at the expense of having a full diode bridge. The usual way this would be done is a half controlled 1 phase bridge where you replace the 2 top diodes with SCR's. Then of course you need to generate 2 ignition pulses but that is not as difficult as you might think. One pulse will do since one of the SCRs will be reverse biased and won't fire. That way you save 2 diodes at the expense of 1 extra SCR.In your circuit there are 3 junctions in series - 2 diodes + SCR. In the half controlled bridge there are 2, diode and SCR. The difference may not be significant in your application, but for a large bridge the difference in power loss dissipation can be several kilowatts.

author: IanB
author_rank: Super Contributor
timestamp: November 28, 2012, 10:56:35 pm
content: The classic approach to this circuit is to use a TRIAC for switching, a DIAC for triggering, and an RC circuit for phase angle delay timing. This makes for a very simple circuit with only three or four components in it.Clearly your circuit has many more components and is much more complex. Can you articulate any particular advantages of your circuit that would justify the extra complexity, especially the use of several transistors, diodes and a 555?

author: DarkPrince
author_rank: Regular Contributor
timestamp: November 28, 2012, 11:28:43 pm
content: Quote from: IanB on November 28, 2012, 10:56:35 pmDesign requirements were stated in original problem statement (first post).Quote from: Kremmen on November 28, 2012, 10:49:34 pmYeah that is a consideration. We have already acquired the full-wave bridge rectifier though. Could be an alternative but functionally (especially with the puny 40W load and being a one-off design) it operates exactly the same (seemly just two diodes in series making one of them functionally pointless. Should be alright though for a one off if I can see if I can make the zero-cross more assuring.I will conisder removing the full-wave rectifier and it could eliaviate the zero-crossing issue... possibly. Will think about it when I get out of class.

author: IanB
author_rank: Super Contributor
timestamp: November 28, 2012, 11:55:13 pm
content: Quote from: DarkPrince on November 28, 2012, 11:28:43 pmI didn't see a stated design requirement that you had to use transistors or a 555 timer? As I read it, your main requirement was that you had to use an SCR.It still seems to me that you could have used an RC network for timing even if your SCR needed to work with rectified DC rather than the AC that a TRIAC could work with?Just take this as a constructive comment. You need to develop a thick skin in professional life, and one of the key questions people will ask about one of your designs is "Why?". Why did you design it that way? What advantages does your design have over other designs? What simpler alternatives did you consider and rule out? Did you do a cost analysis of your solution?If your tutor is any good you will get asked similar questions.

author: DarkPrince
author_rank: Regular Contributor
timestamp: November 29, 2012, 01:41:30 am
content: Quote from: IanB on November 28, 2012, 11:55:13 pmYou are right, the glue logic used to control the SCRs wasn't specified and isn't a requirement. I took the wrong impression when you stated it isn't the normal design approach including stating that it is normally TRIACs.Granted an RC circuit could have worked and it is still being considered as a general thought process. Unfortunately during the design process this approach was selected as it was at the moment the most understood way to control an SCR's firing angle. I would have liked to do the analysis of an RC circuit and implement a design but the decision making process is weighted as a group (mostly, anyway). Also time was a factor, and this yielded the least amount of time consumed going from requirements to design.This design project is a sub-set of our power electronics course. The objective was to design a light dimmer with those specified requirements earlier. Other requirements such as how cost-effective the device is, utilizing the 'best' design, and needing an advantage over previous designs was not a requirement, and thus we took what we considered a unique route. We have two semesters dedicated to our "senior design project" which is a class in itself, which all of those questions you stated are actually part of the design requirements. We will be questioned then.If we are penalized for the complexity/lack of being cost effective than the teacher is deviated from the original requirements stated. This project, as I perceive it, is to demonstrate our understanding of a specific power control topology rather than the design process itself.

author: IanB
author_rank: Super Contributor
timestamp: November 29, 2012, 02:25:45 am
content: Quote from: DarkPrince on November 29, 2012, 01:41:30 amSure, I understand how group dynamics work, especially on student projects.Here is an example of the "simplest" circuit using an SCR:The funny looking thing on the gate of the SCR is a Shockley diode.The operation of the circuit is such that the capacitor charges up through the resistor until the capacitor voltage exceeds the threshold voltage of the Shockley diode when it rapidly turns on and the conducted current turns on the SCR. Shockley diodes apparently are no longer made, but a DIAC would perform the equivalent function.The time delay of the capacitor charging up is what allows the triggering of the SCR to be delayed by a variable amount.

author: DarkPrince
author_rank: Regular Contributor
timestamp: November 29, 2012, 05:34:05 am
content: Alright, I had an idea how it'd work but I suppose I was caught up about the minimum firing angle on a configuration like this. Based on how I see it, it is just the breakdown voltage of the DIAC which determines the minimum firing angle. Based on a quick search seems like most of the breakdown voltages are significant (20V - 60V but few have low breakdown voltage which I wasn't aware of existing). Based on a 40V breakdown I calculate a minimum firing angle of 13° which is a bit high.. so a less common DIAC would be needed.. I think that is what I choked up on thus switching to this alternative design. Also I forgot to mention that it was stated it should be able to go from 0% to 100%... A pot with an inline switch took care of the 0% constraint but the step between minimum and zero has to be monitored.Line Voltage:120 Vrms => ~170 VppSolve for wt with output rising to 40V...40V = 170*sin(wt)wt = 0.23752 (13.61°)I guess this is a dilemma of ethics. In terms of trying to engineer a product I would agree with a simple and cost-effective design. Our approach on the other hand was a fun engineering experience and was an experience itself. Using that design, I would be re-purposing that circuit rather than engineering my own... effective for profit, not so much benefit in an educational point of view. Still will dabble probably with both designs but only one makes it for the assignment.

author: IanB
author_rank: Super Contributor
timestamp: November 29, 2012, 05:48:28 am
content: The example circuit I showed does indeed have limitations over the range of control available. It doesn't go all the way down to 0%, neither completely to 100%. It also has some hysteresis at low outputs causing funny on/off behavior at the low limit.This is why I asked you about advantages of your circuit. One advantage may be a more complete range of control between fully off and fully on (full coverage of 0% to 100% firing angle); another advantage may be more predictable behavior without hysteresis at the off end of the range.So I'm not suggesting you should use the exact circuit I linked to. Many people over the years have modified that circuit to make it better in various respects. I just wanted to see if you were aware of it, and if you had a rationale for using a rather more complex design.

author: DarkPrince
author_rank: Regular Contributor
timestamp: November 29, 2012, 07:02:38 am
content: Yeah that makes sense and I know what you mean. I saw a document going over all those issues with the design but... I guess it would have taken to long to discover these problems (which we already somewhat knew about but not all of them) than we had the time available for. I was going over it and became lost in the theory but I did save the document so I can learn the actual short-comings of the configuration and how they are eliminated. That is why I like our design though... something we understand right away. So yes, it can be rationalized that the limitations of the circuit you provided has too severe of limitations and an alternative design was decided rather than attempting to correct those limitations.I'm looking into a alteration of the circuit where it is a full-wave rectifier with the SCRs integrated as two of the diodes... looks interesting but will need to consider the control signals. Would solve the issue trying to detect the zero-crossing point though.Yes I see the issue with the design; it is complex compared to the initial design. I am trying to keep complexity in mind and attempt to optimize the circuit where possible while maintaining this design. If I am not paying attention suddenly there is over half a dozen diodes in the circuit rather than just the full-wave rectifier.

author: Mr J
author_rank: Regular Contributor
timestamp: November 29, 2012, 06:36:47 pm
content: We just did this in class on the cheap $5, all you need is a dimmer switch, LED, Photocell, Pen and Electrical tape.We cut the pen to make a tube about 2 inches long, put and LED on one side of the tube and the photocell in the opposite side and coved with electrical tape, this will be your opto-isoloator.The we took an off the shelf Light dimmer switch from Home Depot removed the pot and replaced it with the photocell.Hook 5 volts up to the led with a current limit resistor and wolla a 5 volt controlled 120V lamp, we hooked ours up to an Arduino and PWM it to dim. And it will fit in a standard electrical box to boot.

author: DarkPrince
author_rank: Regular Contributor
timestamp: November 29, 2012, 08:10:05 pm
content: Quote from: Mr J on November 29, 2012, 06:36:47 pmThat'd work but wouldn't meet the requirements requested by the professor. The project is about designing a dimmer circuit, not interfacing to an already designed one. Plus components would be used that aren't allowed (TRIAC specifically). That design could work, though I'd use two opto-iso. One to control the switching device and the other going the other way for zero-cross detection. Similar to this design minus being isolated from mains.Unfortunately no solution seems to be rising with this design... I've looked into two-SCR based design but because of time limitations and such I don't think it is feasible. I'll just start to push up the power on the current design and see how it behaves.

author: T4P
author_rank: Super Contributor
timestamp: November 30, 2012, 06:03:51 pm
content: Quote from: DarkPrince on November 29, 2012, 08:10:05 pmSorry i can't take it any longer ...Why do you need a 555 for just a light dimmer?Question yourself if there's any point in reinventing the wheelThe age old way of using a triac is there for a reason, it works and it only wastes a negligible amount of power and it is the cheapest possible!But the thing is, why do you turn a cost effective light dimmer into something much more complicated just because you wanted to use a SCR?Engineering is about getting same or better performance out of something while making it cheaper - not the other way round or else we'll be stuck on tubes! (Well some people are of course)

author: IanB
author_rank: Super Contributor
timestamp: November 30, 2012, 06:06:23 pm
content: Quote from: T4P on November 30, 2012, 06:03:51 pmThey didn't "want to use an SCR". They were required to use an SCR.I think maybe the subtext of the assignment was to figure out how to use an SCR instead of a TRIAC in an AC dimming circuit to achieve the same result.

author: DarkPrince
author_rank: Regular Contributor
timestamp: November 30, 2012, 07:24:13 pm
content: Quote from: IanB on November 30, 2012, 06:06:23 pmI am thinking that, although the class material was more about the control of power systems than how the systems are controlled (we just represented the switching device as a switch, not a specific device). An extension of that, I would believe that it was to prevent the design from actually being the traditional TRIAC based circuit. He has placed a lot of emphasis on the design aspect than anything else.Quote from: T4P on November 30, 2012, 06:03:51 pmI have a different perspective on this. The control circuit is more complicated and thus, by its perspective, the TRIAC based design IS tubes. Designs can become more complicated but if it is used to correct the problem (assuming that this actually has a proper problem statement) than it can be considered to be moving in the right direction if no alternative solution (being most cost effective and can meet the requirements) is available. The TRIAC design needs to become more complicated if the requirements aren't met by the traditional design, which in this case is the minimum firing angle.

author: DarkPrince
author_rank: Regular Contributor
timestamp: December 03, 2012, 06:14:22 am
content: I have been dabbling a bit around the original design... It probably breaks nearly every rule made for HV design... honestly the most I've learned about it is all that you all posted on these forums. Anyway... just wanted some insight. I'll add this project to my list of "to-do" maybe so sometime I could do it right. Anyway, what do you think of the first-pass layout?

author: Kremmen
author_rank: Super Contributor
timestamp: December 03, 2012, 04:24:13 pm
content: Not too bad i would say. Personally i always strive to minimize the track lengths, whether that is strictly needed or not. Creates a useful habit however. I haven't checked the schematic nor done any analysis of power dissipations so some of this may not be really relevant. Anyway, a couple of observations:- you might want to position large components that might dissipate appreciable heat a little apart. In this case the big resistor would be a candidate. Heating sensitive components may cause unnecessary drift in operating points, although not here maybe.- The main circuit tracks between the triac and the terminal strip could easily be shorter. Why not make it so in order to avoid unnecessary voltage drop in the tracks. Probably not significant in any way in this particular circuit and with a couple amps of current, but just to get in the habit of doing so. Also, these tracks have plenty of room to be wider, why not make them so when it costs nothing?Just as a general rule, not directly pertinent here; it is generally useless to minimize track widths and clearances when there is no need to do so. The easier your layout is for the manufacturer, the better yield you will get. Especially significant in cases where the mfg does not do electrical checking or you don't want to pay the extra. In your layout the widths look reasonable and should be foolproof in this sense.

author: DarkPrince
author_rank: Regular Contributor
timestamp: December 03, 2012, 04:53:12 pm
content: I'd like to note I just noticed an air-wire in the screenshot. I'll need to look into why it passed the ERC/DRC steps. Thought something like that would have been caught. Oh well, just my ground plane is broken up and need to link them together somewhere in there.Quote from: Kremmen on December 03, 2012, 04:24:13 pmYeah the HV side of the circuit I was trying to keep the creepage distance significant (nearly twice the minimum, 100mil but saw the recommended is 40mil for an untreated outside facing copper traces). You can see where the DC side of the system shrunk down substantially and is much more compact. I probably could have squished this down a bit.Quote from: Kremmen on December 03, 2012, 04:24:13 pmYes this has been a concern and I did put an effort to isolate heat-generating parts vs temperature sensitive parts. Ex: timing cap is located at right edge of board, significant distance from the power resistor. Wasn't too happy about where the main power capacitor is on the board but was trying to conserve some room. All together the two heat dissipating devices (mainly the resistor and SCR) are on the left edge and (somewhat) critical components on the right. Zener is a bit close though.Quote from: Kremmen on December 03, 2012, 04:24:13 pmThe position of the SCR is based on a heatsink that I had laying around. Granted probably overkill (20W rated) but it is either that or a puny heatsink instead. So that is why the SCR is seemingly out in the middle of no-where. Yeah I forgot to look into track thickness honestly. I just doubled the width as a placeholder and completely forgot to actually verify that.... Just looked at an online calculator... with a 10°C rise in temperature for a 1-amp draw it is recommending 30mil trace width, where as I used 32mil. Also the RMS current draw for 40W should be approximately 0.333A so well rounded up. I may increase width to reduce copper-less area (see below for reasoning).Quote from: Kremmen on December 03, 2012, 04:24:13 pmThe track widths and spacing between them and the ground pour has been expanded in order to compensate for the fact that this board is going to be home etched. Just to allow a little more comfort room I expanded the isolation distance on the pour; my etching abilities have been on standby for a while and want to try to get it in one pass to waste little resource.Speaking of which, it concerns me how much copper I will have to eat away in the etching process. I don't want to leave the heatsink unsupported though. It should be okay as it stands now. Just never tried such a large surface area.

author: DarkPrince
author_rank: Regular Contributor
timestamp: December 05, 2012, 06:58:36 pm
content: Looks like we may be switching design after all which is very unfortunate. The SCR is refusing to turn off at all. Not completely sure what is causing the trigger, whether being leakage current through various devices or what not. I've tried to pull-down the gate in reference to the SCR's cathode by using a 1.5 AAA battery through 1kOhm... didn't exactly work right. It is being very strange and not sure what is going on. Due date is... tomorrow, awesome. So going to work on it tonight but also wire up the back-up plan of just a full-wave rectifier, scr, diac, cap, and resistor.It is a shame too, the PCB came out very well. At least I have a 12V dimmer. Yes, it works fine at 12V but the next step up, around 42V, the behavior changes. Stumped at the moment. The transistor's base was completely disconnected (by the switch) and SCR still triggering right away. May propose in the report this design and it's potential but didn't succeed. Hope the simpler version cooperates tonight.<--- I see they started adding the new emots... funny stuff. Need to find that thread that asked.